RNA Nonenveloped RNA Nonenveloped VirusesViruses

PICORNAVIRUSESPICORNAVIRUSES

ENTEROVIRUSES AND GENERAL ENTEROVIRUSES AND GENERAL

FEATURES OF PICORNAVIRUSESFEATURES OF PICORNAVIRUSES

1- PICORNAVIRUSES1- PICORNAVIRUSES small (20-30 nm) nonenveloped, icosahedral, +ve small (20-30 nm) nonenveloped, icosahedral, +ve

ssRNA virusesssRNA viruses

Replicate in the Replicate in the cytoplasmcytoplasm

resist lipid solvents (e.g. ether) resist lipid solvents (e.g. ether) because they do because they do

not have an envelopenot have an envelope

The family includes 2 groups:The family includes 2 groups:

1- Enteroviruses1- Enteroviruses

2- Rhinoviruses2- Rhinoviruses

Among the major enteroviruses are Among the major enteroviruses are poliovirus, poliovirus,

coxsackieviruses, echoviruses, and hepatitis A viruscoxsackieviruses, echoviruses, and hepatitis A virus

.. Enteroviruses infect primarily the enteric tract,Enteroviruses infect primarily the enteric tract, whereas rhinoviruses are found in the nose and whereas rhinoviruses are found in the nose and

throat (rhino = nose).throat (rhino = nose).

IntroductionIntroduction Picornaviruses are among the most Picornaviruses are among the most diverse (more than 200 serotypes) and diverse (more than 200 serotypes) and 'oldest' known viruses (temple record 'oldest' known viruses (temple record from Egypt ca. 1400 B.C.). from Egypt ca. 1400 B.C.). FMDV (foot and mouth disease virus) FMDV (foot and mouth disease virus) was one of the first viruses to be was one of the first viruses to be recognised - Loeffler and Frosch 1898.recognised - Loeffler and Frosch 1898. Poliomyelitis as a viral disease was Poliomyelitis as a viral disease was first recognised by Landsteiner and first recognised by Landsteiner and Popper, 1909 (though the virus was not Popper, 1909 (though the virus was not isolated until the 1930's.isolated until the 1930's.Name: 'Pico Name: 'Pico (Greek very small )(Greek very small ) RNA RNA Viruses'. Viruses'.

ClassificationClassification Originally based on physical properties Originally based on physical properties

(particle density & pH-sensitivity) & (particle density & pH-sensitivity) & serological relatedness, serological relatedness,

more recently based on nucleotide more recently based on nucleotide sequence. There are there are sequence. There are there are nine generanine genera within thewithin the Picornaviridae Picornaviridae. Five of these . Five of these infect humans: infect humans:

Enteroviruses Enteroviruses Rhinoviruses Rhinoviruses Hepatoviruses Hepatoviruses Parechoviruses Parechoviruses Kobuviruses Kobuviruses

Parechoviruses were formerly Parechoviruses were formerly classified among the classified among the EchoviruseEchoviruses and s and cause gastrointestinal and cause gastrointestinal and respiratory tract infections, and respiratory tract infections, and occasionally cases of encephalitis occasionally cases of encephalitis and flaccid paralysis. and flaccid paralysis.

Kobuviruses also cause Kobuviruses also cause gastroenteritis.gastroenteritis.  

ENTEROVIRUSESENTEROVIRUSESEnterovirus infections are common in Enterovirus infections are common in humans; seasonal peak in autumn; humans; seasonal peak in autumn;

frequently undiagnosed: frequently undiagnosed:    Enteroviruses

Virus family SerotypesPolio 1 - 3

Coxsackie A 1 - 22, 24Coxsackie B 1 - 6

Echovirus 1 - 9, 11 - 27, 29 - 34Hepatitis A Enterovirus 72

Other Enteroviruses 68 - 71

    Genera of Picornaviruses

Genera that infect humans

Enterovirus PolioCoxsackie A and BEchoOther enteroviruses

Diseases of the human (and other) alimentary tract (e.g. polio virus)

Rhinovirus(HRV)Disease of the nasopharyngeal region (e.g. common cold virus)

HepatovirusParechovirus

Human hepatitis virus A

Parechovirus Formerly echoviruses 22 and 23. Disease of

respiratory tract

Kobuvirus Aichi virus is the type species

alimentary  and

Genera that infect other animalsGenera that infect other animals Cardiovirus Mainly found in rodents Mainly found in rodents

Murine encephalomyocarditis, Murine encephalomyocarditis, Theiler's murine Theiler's murine

encephalomyelitisencephalomyelitis virusvirus Aphthovirus Foot and mouth disease in (FMDV ) Aphthovirus Foot and mouth disease in (FMDV ) cloven footed animals cloven footed animals

ErbovirusErbovirus

Teschovirus Teschovirus

   Properties of Rhino- and Entero-viruses

pH sensitivity

Optimum growth temperature

Detergent sensitivity

SerotypesTransmission

Site of primary infection

Rhino viruses

labile to acid pH

33 degrees C (approx)

>100 aerosolupper respiratory tract

Entero viruses

resistant to acid pH

37 degrees C (approx)

Resistant 72 oro-fecal gut

Genome:Genome: The genome consists of one s/s (+)sense The genome consists of one s/s (+)sense

RNA molecule of between 7.2kb (HRV14) to RNA molecule of between 7.2kb (HRV14) to 8.5kb (FMDV). 8.5kb (FMDV).

A number of features are conserved in all A number of features are conserved in all PicornavirusesPicornaviruses: :

Genomic RNA is Genomic RNA is infectiousinfectious (~1x106-fold less (~1x106-fold less infectious than intact particles, although infectious than intact particles, although infectivity is increased if the RNA is infectivity is increased if the RNA is introduced into cells by transfection) - introduced into cells by transfection) - CHARACTERISTIC OF (+)SENSE RNA CHARACTERISTIC OF (+)SENSE RNA VIRUSES !!! VIRUSES !!!

There is a long (600-1200 base) There is a long (600-1200 base) untranslated region at the 5' end (important untranslated region at the 5' end (important in in translationtranslation, , virulencevirulence and possibly and possibly encapsidationencapsidation

and a shorter 3' untranslated region (50-100 and a shorter 3' untranslated region (50-100 bases) - important in bases) - important in (-)strand synthesis. (-)strand synthesis.

The genome RNA has The genome RNA has positive polaritypositive polarity; i.e., on ; i.e., on entering the cell, it functions as the entering the cell, it functions as the viral mRNAviral mRNA..

The single strand of positive-sense RNA The single strand of positive-sense RNA

(messenger RNA sense) can act as a messenger (messenger RNA sense) can act as a messenger RNA once it enters the cytoplasm and RNA once it enters the cytoplasm and uncoatinguncoating has occurred.has occurred.

The genome RNA is unusual because it has a The genome RNA is unusual because it has a

protein on the 5' end that serves as a primer for protein on the 5' end that serves as a primer for transcription by RNA polymerase.transcription by RNA polymerase.(VPg)(VPg)

The open reading frame then The open reading frame then extends to near the 3' end. extends to near the 3' end.

After the open reading frame of 7000 After the open reading frame of 7000 bases, there is a short sequence bases, there is a short sequence before the poly A tract. before the poly A tract.

The poly A tract of The poly A tract of poliopolio RNA is RNA is encoded in the genome,encoded in the genome, unlike the unlike the situation with situation with cellularcellular mRNAs where mRNAs where it is added post-transcriptionally. it is added post-transcriptionally.

There is another way in which There is another way in which picornavirus RNA picornavirus RNA differsdiffers from a typical from a typical mRNA. mRNA.

The latter have a The latter have a methylated methylated cap cap structure at the 5' end, whereas structure at the 5' end, whereas picornaviruses have a viral protein picornaviruses have a viral protein called called VPGVPG. .

The large 5' leader sequence has The large 5' leader sequence has considerable considerable secondary structuresecondary structure that that comes about by intramolecular base comes about by intramolecular base pairing and one of these structures is pairing and one of these structures is the internal ribosome entry site the internal ribosome entry site (IRES(IRES) ) which allows this RNA to bind to which allows this RNA to bind to cytoplasmic ribosomes. cytoplasmic ribosomes.

. . The 5' UTR contains a 'clover-leaf' secondary structure The 5' UTR contains a 'clover-leaf' secondary structure known as the known as the IRES: Internal Ribosome Entry SiteIRES: Internal Ribosome Entry Site . . . The rest of the genome encodes a single 'polyprotein' of . The rest of the genome encodes a single 'polyprotein' of between 2100-2400 aa's. between 2100-2400 aa's. . . Both endsBoth ends of the genome are of the genome are modifiedmodified,, . the 5' end by a covalently attached small, basic protein . the 5' end by a covalently attached small, basic protein VPgVPg (~23 aa's), (~23 aa's), VPgVPg is a protein attached to the 5' end of is a protein attached to the 5' end of RNARNA during during RNA synthesisRNA synthesis in a wide variety of viruses in a wide variety of viruses including including PicornaviridaePicornaviridae such as such as Foot-and-mouth diseaseFoot-and-mouth disease and and polioviruspoliovirus . . .VPg stands for "viral protein genome-linked". .VPg stands for "viral protein genome-linked". . the 3' end by . the 3' end by polyadenylation polyadenylation

Structure:Structure:

The The capsidcapsid is an arrangement of 60 protomers in a tightly is an arrangement of 60 protomers in a tightly packed packed IcosahedralIcosahedral structure. structure.

Each protomer consists of 4 Each protomer consists of 4 polypeptidespolypeptides known as VP (viral known as VP (viral protein)1, 2, 3 and 4.protein)1, 2, 3 and 4.

VP2 and VP4 polypeptides originate from one protomer VP2 and VP4 polypeptides originate from one protomer known as VP0 that is known as VP0 that is cleavedcleaved to give the different capsid to give the different capsid components. components.

The The IcosahedralIcosahedral is said to have a triangulation number of is said to have a triangulation number of 33, , this means that in the this means that in the icosahedralicosahedral structure each of the structure each of the 60 60 trianglestriangles that make up the capsid are split into 3 little that make up the capsid are split into 3 little triangles with a subunit on the corner. There are triangles with a subunit on the corner. There are 60 60 identical subunitsidentical subunits (vertices) which contain (vertices) which contain five protomersfive protomers..

Each protomer is made up of Each protomer is made up of one copy of four proteinsone copy of four proteins, , named VP1, VP2, VP3 and VP4.named VP1, VP2, VP3 and VP4.

These proteins are made as a single polypeptide These proteins are made as a single polypeptide

(polyprotein)(polyprotein) which is cleaved by which is cleaved by cellular proteasescellular proteases

Icosahedron Icosahedron

BLUEBLUE subunits around 5-fold axes are subunits around 5-fold axes are VP1VP1; ; REDRED and and GREENGREEN are are VP3VP3 and and VP2VP2 respectively; respectively; YELLOWYELLOW subunits (seen only internally) are subunits (seen only internally) are VP4VP4.   The VP4 subunits are .   The VP4 subunits are formed by autocatalytic cleavage of formed by autocatalytic cleavage of VP0VP0 (into (into VP2VP2 and and VP4VP4) upon binding of ) upon binding of a "a "procapsidprocapsid" with " with viral genomic ssRNA.viral genomic ssRNA.   

VP1: is in blue VP1: is in blue VP2: is in green VP2: is in green VP3: is in redVP3: is in redVP4: is in yellow (only visible on the inside of the particle) VP4: is in yellow (only visible on the inside of the particle) Rhinovirus14,colorcodedby protein, as solved by X-ray Rhinovirus14,colorcodedby protein, as solved by X-ray

crystallographycrystallography

A computer generated image of a picornavirus capsid. This image is A computer generated image of a picornavirus capsid. This image is based on the real atomic co-ordinates of rhinovirus 16 and shows a based on the real atomic co-ordinates of rhinovirus 16 and shows a view inside the capsid. view inside the capsid. BLUEBLUE subunits around 5-fold axes are subunits around 5-fold axes are VP1VP1;; REDRED and and GREENGREEN are are VP3VP3 and and VP2VP2 respectively; respectively; YELLOWYELLOW subunits (seen only internally) are subunits (seen only internally) are VP4VP4.   .   The VP4 subunits are formed by autocatalytic cleavage of The VP4 subunits are formed by autocatalytic cleavage of VP0VP0 (into (into VP2VP2 and and VP4VP4) upon binding of a ") upon binding of a "procapsidprocapsid" with " with viral genomic viral genomic ssRNA.ssRNA.   

Depending on the type and degree of Depending on the type and degree of dehydration the viral particle is around dehydration the viral particle is around 27-30 nm in diameter. 27-30 nm in diameter.

The viral genome is around 2500 nm in The viral genome is around 2500 nm in length so we can therefore conclude length so we can therefore conclude that it must be tightly packaged within that it must be tightly packaged within the capsid along with substances such the capsid along with substances such as as sodiumsodium ions in order to cancel out ions in order to cancel out the negative charges on the RNA the negative charges on the RNA caused by the caused by the phosphatephosphate groups. groups.

To view an electron micrograph of To view an electron micrograph of negatively-stained picornavirus negatively-stained picornavirus particles, particles,

Transmission electron micrograph of Transmission electron micrograph of poliovirus type 1. CDC/Dr. Joseph J. Esposito  poliovirus type 1. CDC/Dr. Joseph J. Esposito 

jje1@cdc.govjje1@cdc.gov   

ReplicationReplication The viral particle binds to cell surface receptors. The viral particle binds to cell surface receptors. This causes a conformational change in the This causes a conformational change in the

viral capsid proteins, and viral capsid proteins, and myristicmyristic acids acids are are released. released.

These acids form a pore in the cell membrane These acids form a pore in the cell membrane through which RNA is injected. through which RNA is injected.

Once inside the cell, the RNA uncoats and the Once inside the cell, the RNA uncoats and the (+) strand RNA genome is replicated through a (+) strand RNA genome is replicated through a double-stranded RNA intermediate that is double-stranded RNA intermediate that is formed using formed using viral RDRPviral RDRP (RNA-Dependent RNA (RNA-Dependent RNA polymerase). polymerase).

Translation by host cell ribosomes is not Translation by host cell ribosomes is not initiated by a initiated by a 5' G cap5' G cap as usual, but as usual, but ratherrather is is initiated by an initiated by an IRESIRES (Internal Ribosome Entry (Internal Ribosome Entry Site). Site).

The viral lifecycle is very rapid with the The viral lifecycle is very rapid with the whole process of replication being whole process of replication being completed on average within 8 hours. completed on average within 8 hours.

However as little as However as little as 30 minutes30 minutes after initial after initial infection, cell protein synthesis declines to infection, cell protein synthesis declines to almost zero output – essentially the almost zero output – essentially the macromolecular synthesis of cell proteins macromolecular synthesis of cell proteins is “is “shut offshut off”. ”.

Over the next Over the next 1–2 hours1–2 hours there is a loss of there is a loss of margination of chromatin and homogeneity margination of chromatin and homogeneity in the nucleus, before the viral proteins in the nucleus, before the viral proteins start to be synthesized and a vacuole start to be synthesized and a vacuole appears in the cytoplasm close to the appears in the cytoplasm close to the nucleus that gradually starts to spread as nucleus that gradually starts to spread as the time after infection reaches around the time after infection reaches around 3 3 hourshours. .

After this time the cell plasma membrane After this time the cell plasma membrane becomes permeable, at becomes permeable, at 4–6 hours4–6 hours the the virus particles virus particles assembleassemble, and can , and can sometimes be seen in the cytoplasm.sometimes be seen in the cytoplasm.

At around At around 8 hours8 hours the cell is effectively the cell is effectively dead and dead and lyses to releaselyses to release the viral the viral particles. particles.

The mechanisms and factors involved in The mechanisms and factors involved in the replication of positive stranded RNA the replication of positive stranded RNA viruses are still unclearviruses are still unclear

Using Using polioviruspoliovirus as a model, we show that as a model, we show that a long-range interaction between a long-range interaction between ribonucleoprotein ribonucleoprotein (RNP)(RNP) complexes formed complexes formed at the ends of the viral genome is at the ends of the viral genome is necessary for RNA replication.necessary for RNA replication.

InitiationInitiation of negative strand RNA synthesis of negative strand RNA synthesis requires a requires a 3' poly(A) tail3' poly(A) tail..

Strikingly, it also requires a cloverleaf-like RNA Strikingly, it also requires a cloverleaf-like RNA (IRES)(IRES) structure located at the other end of the structure located at the other end of the genome.genome.

An An RNPRNP complex formed around the complex formed around the 5'5' cloverleaf cloverleaf RNA structure interacts with the poly(A) binding RNA structure interacts with the poly(A) binding protein bound to the protein bound to the 3'3' poly(A) tail, thus linking poly(A) tail, thus linking the ends of the viral RNA and effectively the ends of the viral RNA and effectively circularizing itcircularizing it..

Formation of this Formation of this circular RNPcircular RNP complex is complex is required for required for initiation of negative strand RNAinitiation of negative strand RNA synthesis.synthesis.

RNA circularizationRNA circularization may be a general replication may be a general replication mechanism for mechanism for positive stranded RNApositive stranded RNA viruses. viruses.

Replication of a positive strand RNA Replication of a positive strand RNA

virusvirus

RNA REPLICATIONRNA REPLICATION This is quite simple compared to some This is quite simple compared to some

other RNA viruses. other RNA viruses. Since picornaviruses spend all of their Since picornaviruses spend all of their

time in the cytoplasm, they must time in the cytoplasm, they must encode a encode a polymerase (replicase)polymerase (replicase) that is that is made from the sense strand of the made from the sense strand of the infecting virus. infecting virus.

The polymerase copies the sense strand The polymerase copies the sense strand to anti-sense which is then copied back to anti-sense which is then copied back to the sense strand that is packaged to the sense strand that is packaged into the virus (previous figure). into the virus (previous figure).

RNA replication seems to occur on the RNA replication seems to occur on the cytoplasmic surface of cytoplasmic surface of membrane membrane vesiclesvesicles to which the RNA polymerase to which the RNA polymerase binds. binds.

These appear to come from the These appear to come from the endoplasmic reticulumendoplasmic reticulum, as do vesicles , as do vesicles in the uninfected cells that in the uninfected cells that transporttransport secreted and membrane proteins to secreted and membrane proteins to the the Golgi bodyGolgi body. .

However, when the cell is infected by However, when the cell is infected by the picornavirus, the vesicles the picornavirus, the vesicles do not do not fusefuse with the with the cis face of the Golgi bodycis face of the Golgi body as the transport vesicles do (figure 2). as the transport vesicles do (figure 2).

Fig.2 When the cell is infected by a Fig.2 When the cell is infected by a picornavirus, the endoplasmic reticulum to picornavirus, the endoplasmic reticulum to Golgi body transport vesicles do not fuse Golgi body transport vesicles do not fuse

with the cis face of the Golgi bodywith the cis face of the Golgi body

These vesicles have specific targeting These vesicles have specific targeting proteins on their cytoplasmic surfaces proteins on their cytoplasmic surfaces (called (called COP proteins)COP proteins) and it is possible that and it is possible that they are involved in the response to the they are involved in the response to the viral infection.viral infection.

It should be remembered that It should be remembered that picornaviruses do not have a lipid envelope picornaviruses do not have a lipid envelope and do not have a surface glycoprotein. and do not have a surface glycoprotein.

Therefore, the production of virus is Therefore, the production of virus is not not inhibitedinhibited by compromising by compromising Golgi Body Golgi Body functionfunction, as would be the case with an , as would be the case with an enveloped virusenveloped virus..

We do not know why there is this We do not know why there is this membrane associationmembrane association of RNA replication of of RNA replication of picornaviruses but it may concentrate picornaviruses but it may concentrate various substrates in the vicinity of the various substrates in the vicinity of the polymerase (remember that in bacteria, polymerase (remember that in bacteria, DNA replication is membrane-associated). DNA replication is membrane-associated).

The RNA of picornaviruses is polyadenylated The RNA of picornaviruses is polyadenylated at the at the 3’ end3’ end, as are , as are cellular messenger cellular messenger RNAsRNAs but this polyadenylation occurs in a but this polyadenylation occurs in a different waydifferent way..

In host cell mRNA synthesis, the poly A In host cell mRNA synthesis, the poly A sequence is sequence is not coded in the DNA copynot coded in the DNA copy of of the gene but is the gene but is addedadded by an enzyme called by an enzyme called poly A polymerasepoly A polymerase using ATP as a substrate. using ATP as a substrate.

In the case of In the case of picornaviruspicornavirus RNA, however, RNA, however, the the poly Apoly A sequence in the sense strand is sequence in the sense strand is copiedcopied to a to a poly Upoly U sequence at the sequence at the 5’ end of 5’ end of the negative strand. the negative strand.

This is copied This is copied back to 3’ poly Aback to 3’ poly A, again by the , again by the replicase.replicase.   

Receptor bindingReceptor binding Different picornaviruses have different Different picornaviruses have different

receptors, among which are some receptors, among which are some intercellular cell surface adhesion intercellular cell surface adhesion molecules (molecules (ICAMsICAMs). ).

The expression of these molecules The expression of these molecules determine tissue tropism. determine tissue tropism.

CoxsackievirusCoxsackievirus (a type of enterovirus) (a type of enterovirus) and most and most rhinovirusesrhinoviruses bind to ICAM-1, bind to ICAM-1, an an adhesion glycoprotein expressed adhesion glycoprotein expressed on the surfaces of a variety of cells on the surfaces of a variety of cells (epithelial, endothelial, fibroblasts). (epithelial, endothelial, fibroblasts).

Polio virus binds to another cell Polio virus binds to another cell surface glycoprotein known as surface glycoprotein known as CD155CD155 (the poliovirus receptor). (the poliovirus receptor).

When the virus binds to its receptor, When the virus binds to its receptor, the the VP4 protein is releasedVP4 protein is released from the from the protomer. This allows the protomer. This allows the escape of escape of the viral RNAthe viral RNA from the nucleocapsid from the nucleocapsid when the virus is internalized into the when the virus is internalized into the endocytic pathway.endocytic pathway.

In the endosome, the nucleocapsid In the endosome, the nucleocapsid disassembles in the acid environment. disassembles in the acid environment. Protein synthesis is detectable with 15 Protein synthesis is detectable with 15 minutes of infection. minutes of infection.

Virus: Virus: # Serotypes: # Serotypes: Receptor: Receptor: Description: Description:

Human Human Rhinovirus Rhinovirus

91 91 ICAM-1 ICAM-1 (Intracellular (Intracellular Adhesion Adhesion Molecule 1) Molecule 1)

Immunoglobulin-Immunoglobulin-like molecule; 5 like molecule; 5

domainsdomains

Human Human Rhinovirus Rhinovirus

1010 LDLR (Low LDLR (Low Density Density Lipoprotein Lipoprotein receptor)receptor)

Poliovirus Poliovirus 33 CD155 CD155 Immunoglobulin-Immunoglobulin-like molecule; 3 like molecule; 3 domains domains

Coxsackie A Coxsackie A 33 ICAM-1 ICAM-1

Echo Echo 22 VLA-2 (very late VLA-2 (very late activation activation antigen)antigen)

Integrin-like Integrin-like

moleculemolecule

Echo Echo 66 DAF (Decay DAF (Decay Accelerating Accelerating Factor) Factor)

??? ???

EMCV EMCV 11 VCAM-1 VCAM-1 (Vascular Cell (Vascular Cell Adhesion Adhesion Molecule) Molecule)

??????

Uncoating:Uncoating: After adherence to the receptor, the virus can be eluted After adherence to the receptor, the virus can be eluted

again, but if this happens, the particle undergos again, but if this happens, the particle undergos conformational changes due to the loss of VP4 and conformational changes due to the loss of VP4 and infectivity is lost - this is also the first stage in uncoating: infectivity is lost - this is also the first stage in uncoating:

VIRAL MESSENGER RNAVIRAL MESSENGER RNA An RNA virus needs to make an RNA that can serve An RNA virus needs to make an RNA that can serve

as a as a messenger RNAmessenger RNA for protein synthesis in a host for protein synthesis in a host cell.cell.

In the case of the positive strand RNA viruses In the case of the positive strand RNA viruses (whose genome, by definition, is the same sense (whose genome, by definition, is the same sense as mRNA), the genomic RNA can serve as the as mRNA), the genomic RNA can serve as the message (figure 2). message (figure 2).

The virus capsid serves as the delivery vehicle to The virus capsid serves as the delivery vehicle to the cytoplasm. the cytoplasm.

Since the genome is Since the genome is RNARNA and copied by an and copied by an RNA RNA polymerase,polymerase, there is there is no needno need for a typical promoter for a typical promoter ((TATA box, CAT boxTATA box, CAT box etc) upstream of the protein etc) upstream of the protein encoding genes (as found in encoding genes (as found in DNA virusesDNA viruses or or retroviruses);retroviruses);

moreover, moreover, positive strandpositive strand viruses viruses do not needdo not need to to make make new proteinsnew proteins before making mRNA as their before making mRNA as their RNA can serve directly as a message. RNA can serve directly as a message.

Translation and protein processingTranslation and protein processing

The picornavirus The picornavirus RNARNA binds to binds to ribosomesribosomes and makes a and makes a single polypeptidesingle polypeptide, therefore , therefore the virus has just one gene. the virus has just one gene.

This polyprotein has regions that have This polyprotein has regions that have proteolytic activityproteolytic activity (they are cysteine (they are cysteine proteases) that cleave the polyprotein to proteases) that cleave the polyprotein to three precursorthree precursor proteins (P1, P2, P3). proteins (P1, P2, P3).

P1 is cleaved to a VP0, VP1 and Vp3 plus P1 is cleaved to a VP0, VP1 and Vp3 plus a leader peptide of unknown function. a leader peptide of unknown function.

VP0 gives rise to VP2 and VP4. VP0 gives rise to VP2 and VP4. P2 and P3 do not give rise to viral P2 and P3 do not give rise to viral

structural proteins. structural proteins.

One of the proteins that comes from One of the proteins that comes from P3P3 is the is the VPGVPG that is found at the 5' that is found at the 5' end of the viral RNA while other end of the viral RNA while other proteins from this precursor are the proteins from this precursor are the viral viral replicase and enzymesreplicase and enzymes that that modify the behavior of the host cell. modify the behavior of the host cell.

P2 is also cleaved to give other cell-P2 is also cleaved to give other cell-modifying proteins. Details of some modifying proteins. Details of some of the cleavages are still vague.  of the cleavages are still vague. 

Once the various viral proteins have been Once the various viral proteins have been made in the infected cell, the made in the infected cell, the replicase (also replicase (also call a transcriptase or protein 3Dpol)call a transcriptase or protein 3Dpol) copies copies the viral plus sense RNA to negative sense the viral plus sense RNA to negative sense RNA. RNA.

Other viral proteins are also involved in this Other viral proteins are also involved in this process. process.

As new positive strand RNAs are made, they As new positive strand RNAs are made, they can also be translated into more viral protein. can also be translated into more viral protein.

There may be as many as half a million There may be as many as half a million copies of viral RNA per cell.copies of viral RNA per cell.

Some of the Some of the proteolytic eventsproteolytic events outlined outlined above take place as the above take place as the nucleocapsid is nucleocapsid is assembled. assembled.

This is especially the case with the This is especially the case with the VP0 VP0 cleavage to VP2 and VP4. cleavage to VP2 and VP4.

P1 protein is the precursor that gives rise P1 protein is the precursor that gives rise to the four structural proteins of the to the four structural proteins of the nucleocapsid. nucleocapsid.

Five copies of P1Five copies of P1 first associate to form a first associate to form a pentamer. Endoproteolysis then occurs to pentamer. Endoproteolysis then occurs to form VP0, VP1 and VP3. form VP0, VP1 and VP3.

Twelve of these pentamersTwelve of these pentamers than associate than associate to form an empty capsid (procapsid). to form an empty capsid (procapsid).

The order of formation of the individual The order of formation of the individual viral proteins is viral proteins is importantimportant in the assembly in the assembly of the virus. of the virus.

The viral RNA now associates with the The viral RNA now associates with the capsid and at the same time, VP0 is capsid and at the same time, VP0 is cleaved. cleaved.

Release is by lysis of the host cell. Release is by lysis of the host cell.

To replicate the viral genome, RNA-dependent RNA polymerase To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy the (+) RNA genome producing ss (-) RNA. RNA-enzymes copy the (+) RNA genome producing ss (-) RNA. RNA-

dependent RNA polymerase enzymes then copy the (-) RNA strands dependent RNA polymerase enzymes then copy the (-) RNA strands producing ss (+) RNA viral genome. producing ss (+) RNA viral genome.

To produce viral mRNA molecules. RNA-dependent RNA polymerase To produce viral mRNA molecules. RNA-dependent RNA polymerase enzymes copy the (-) RNA strand into (+) viral mRNA. The (+) viral enzymes copy the (-) RNA strand into (+) viral mRNA. The (+) viral

mRNA can then be transtated into viral proteins by host cell mRNA can then be transtated into viral proteins by host cell

ribosomes.ribosomes.

FIGURE 1FIGURE 1: The picornavirus binds to a receptor on : The picornavirus binds to a receptor on the cell surface (A). the cell surface (A).

RNA is translated into RNA is translated into one primary translationone primary translation product (B) product (B)

which is then cleaved (C). which is then cleaved (C). The positive strand genomic RNA also associates The positive strand genomic RNA also associates

with an with an RNA polymeraseRNA polymerase that is bound to the that is bound to the cytoplasmic surface of vesicles, probably from the cytoplasmic surface of vesicles, probably from the endoplasmic reticulum, and is copied to endoplasmic reticulum, and is copied to negative negative stand RNA.stand RNA. VPg is at the 5' end of the negative VPg is at the 5' end of the negative strand (strand (the poly U endthe poly U end) (D). ) (D).

The negative strand is The negative strand is copiedcopied to genomic to genomic positive positive strand RNAstrand RNA (E) (E)

which associates with the which associates with the procapsidprocapsid to form a to form a 150S virus150S virus (G) (G)

that is released on cell lysis that is released on cell lysis

Genomic structure of poliovirus type 1 (Mahoney) [PV1(M)] The PV genome Genomic structure of poliovirus type 1 (Mahoney) [PV1(M)] The PV genome consists of a single-stranded, positive-sense polarity RNA molecule, which consists of a single-stranded, positive-sense polarity RNA molecule, which

encodes a single polyprotein. The 5' non-translated region (NTR) harbors two encodes a single polyprotein. The 5' non-translated region (NTR) harbors two functional domains, the cloverleaf and the internal ribosome entry site functional domains, the cloverleaf and the internal ribosome entry site

(IRES),and iscovalentlylinkedtotheviralprotein VPg. The 3'NTR(IRES),and iscovalentlylinkedtotheviralprotein VPg. The 3'NTR

is polyadenylated. is polyadenylated.

VpG is at the 5' end of the viral genomic VpG is at the 5' end of the viral genomic sense (+ve) RNA but is lost before sense (+ve) RNA but is lost before

translationtranslation

At the same time as viral protein synthesis At the same time as viral protein synthesis is occurring, host cell protein synthesis is is occurring, host cell protein synthesis is shut off. shut off.

The host cell mRNAs however remain fully The host cell mRNAs however remain fully functional when assayed in an functional when assayed in an experimental system, so selective experimental system, so selective degradation of cell mRNAs is not the degradation of cell mRNAs is not the reason for protein synthesis inhibitionreason for protein synthesis inhibition..

1- 1- One way host cell protein synthesisOne way host cell protein synthesis occurs is via the occurs is via the cleavage of initiation cleavage of initiation factor eIF-4factor eIF-4, one of the cap binding , one of the cap binding proteins of the host cell's ribosomes so proteins of the host cell's ribosomes so that that cellular mRNAs cannot bind to the cellular mRNAs cannot bind to the ribosomes. ribosomes.

Association with cap-binding proteins is a Association with cap-binding proteins is a prerequisite for the translation of most prerequisite for the translation of most cellular RNAs. cellular RNAs.

Thus, only Thus, only uncapped messages such as uncapped messages such as that of the picornavirus are translatedthat of the picornavirus are translated..

Note that most viruses express Note that most viruses express capped capped RNAsRNAs similar to normal mRNA and so this similar to normal mRNA and so this mechanism of mechanism of shutting downshutting down host protein host protein synthesis is synthesis is not available to them. not available to them.

2- The viral proteins also change the 2- The viral proteins also change the permeability of the host cell, altering the permeability of the host cell, altering the ionic composition of the cell and inhibiting ionic composition of the cell and inhibiting cell mRNA association with ribosomes. cell mRNA association with ribosomes.

3- Moreover, the large number of copies of 3- Moreover, the large number of copies of viral RNA simply out-compete the cell's viral RNA simply out-compete the cell's mRNAs. mRNAs.

Picornaviruses also have a Picornaviruses also have a protease protease activity that can cut one of the activity that can cut one of the proteins of the initiation complex, proteins of the initiation complex, eIF4GeIF4G (figure 3), and this (figure 3), and this seriously seriously affects the cell’s ability to translates affects the cell’s ability to translates normal capped messagesnormal capped messages but but does does not affect translation from the IRESnot affect translation from the IRES; ; thus the virus can suppress host cell thus the virus can suppress host cell translation while leaving the translation while leaving the translation of its own RNA translation of its own RNA unaffected.unaffected.

Fig.3 Picornaviruses also have a protease Fig.3 Picornaviruses also have a protease activity that can cut one of the proteins of activity that can cut one of the proteins of

the initiation complex, eIF4Gthe initiation complex, eIF4G

In the picornavirus RNA there is a region of In the picornavirus RNA there is a region of secondary structure called an internal secondary structure called an internal

ribosome entry site (IRES)ribosome entry site (IRES)

The fact that there is only one IRES The fact that there is only one IRES means that there is only one primary means that there is only one primary translation product; translation product;

that is there can only be that is there can only be one large one large protein madeprotein made. This protein will . This protein will eventually be cut up to make several eventually be cut up to make several proteins in the mature virus and thus proteins in the mature virus and thus we call this we call this primary product a primary product a polycistronic protein since it is encoded polycistronic protein since it is encoded by more than one gene (cistron). by more than one gene (cistron).

  The proteases that cut up the original The proteases that cut up the original polyprotein are polyprotein are encoded in the virus encoded in the virus genomegenome and the proteolytic process is and the proteolytic process is ordered as shown in figure 4.ordered as shown in figure 4.

Fig.4 The proteases that cut up the original Fig.4 The proteases that cut up the original polyprotein are encoded in the virus genome polyprotein are encoded in the virus genome

and the proteolytic process is orderedand the proteolytic process is ordered    

A single RNA can code for more than one A single RNA can code for more than one protein in protein in eukaryotes eukaryotes but this is done by but this is done by splicing out parts of the original transcript to splicing out parts of the original transcript to make another mRNA that serves as a make another mRNA that serves as a monocistronic message. monocistronic message.

Splicing enzymes that carry out this process Splicing enzymes that carry out this process are found in the nucleus (since this is where are found in the nucleus (since this is where mRNAs are made).mRNAs are made).

Since RNA viruses are normally cytoplasmic, Since RNA viruses are normally cytoplasmic, they cannot take advantage of splicing they cannot take advantage of splicing enzymes. enzymes.

Thus, RNA viruses that have Thus, RNA viruses that have only one mRNAonly one mRNA ought only to be able to make ought only to be able to make one large one large proteinprotein - but they have - but they have developed a number developed a number of tricksof tricks to overcome this (figure 5) and do, to overcome this (figure 5) and do, indeed, make more than one protein. indeed, make more than one protein.

Some can take advantage of the Some can take advantage of the alternative splicing enzymes of the alternative splicing enzymes of the host cells (and therefore must have a host cells (and therefore must have a nuclear stage).nuclear stage).

Others make a single large protein Others make a single large protein which has a protease activity; this which has a protease activity; this cuts up the large precursor to a cuts up the large precursor to a series of smaller proteins. series of smaller proteins.

Others, such as the picornaviruses, Others, such as the picornaviruses, have found ways to make a single have found ways to make a single mRNA function in a polycistronic mRNA function in a polycistronic fashion even though they are in a fashion even though they are in a eukaryotic cell. eukaryotic cell.

Fig.5Fig.5

The kinetics of Picornavirus replication The kinetics of Picornavirus replication

are rapid, the cycle being completed are rapid, the cycle being completed in from 5-10 (typically 8) hours. in from 5-10 (typically 8) hours.

Genomic RNA is translated directly by Genomic RNA is translated directly by polysomes, but ~30 min after polysomes, but ~30 min after infection, cellular protein synthesis infection, cellular protein synthesis declines sharply, almost to zero, this declines sharply, almost to zero, this is called is called 'SHUTOFF''SHUTOFF' - the primary - the primary cause of c.p.e(cytopathogenic effect): cause of c.p.e(cytopathogenic effect):

Time after Infection:Time after Infection: Event:Event:

~1-2h~1-2h Sharp decrease in cellular macromolecular synthesis; Sharp decrease in cellular macromolecular synthesis; margination of chromatin (loss of homogeneous margination of chromatin (loss of homogeneous appearance of nucleus) appearance of nucleus)

~2.5-3h ~2.5-3h Start of viral protein synthesis; vaculoation of Start of viral protein synthesis; vaculoation of cytoplasm, beginning close to nucleus & spreading cytoplasm, beginning close to nucleus & spreading

outwardsoutwards

~3-4h ~3-4h Permeabilization of plasma membrane Permeabilization of plasma membrane

~4-6h ~4-6h Virus assembly in cytoplasm (crystals sometimes Virus assembly in cytoplasm (crystals sometimes

visible)visible)

~6-10h ~6-10h Cell lysis; release of virus particlesCell lysis; release of virus particles

ASSEMBLYASSEMBLY the virus can be assembled as the various the virus can be assembled as the various

proteins are cleaved from the polyprotein.proteins are cleaved from the polyprotein. The polyprotein is first cleaved to three The polyprotein is first cleaved to three

proteins (P1, P2, P3). proteins (P1, P2, P3). P1 is then cleaved into three proteins (VP0, P1 is then cleaved into three proteins (VP0,

VP1, VP3) that make up the subunit of the VP1, VP3) that make up the subunit of the virus coatvirus coat..

This is done by This is done by virus-specified proteasesvirus-specified proteases that that are part of the polyprotein and are catalytic are part of the polyprotein and are catalytic as part of the single primary translation as part of the single primary translation product. product.

VP0, VP1 and VP3 assemble into the 5S VP0, VP1 and VP3 assemble into the 5S structural subunit (protomer).structural subunit (protomer).

VP0is only cleaved to VP2 and VP4 when the VP0is only cleaved to VP2 and VP4 when the virus has assembled. virus has assembled.

FiveFive of these protomers assemble into a of these protomers assemble into a 14S 14S pentamerpentamer and and twelvetwelve pentamers form the pentamers form the procapsid.procapsid.

RNA is encapsulated into the RNA is encapsulated into the procapsidprocapsid to form a to form a provirion. At this stage provirion. At this stage VP0 is cleaved to VP2 and VP0 is cleaved to VP2 and VP4VP4 and the virion is a mature, infectious virus and the virion is a mature, infectious virus particle. particle.

The other two parts of the primary translation The other two parts of the primary translation product (P2 and P3) are cleaved to form a product (P2 and P3) are cleaved to form a number of non-structural proteins (i.e. proteins number of non-structural proteins (i.e. proteins that are not found in the mature virus particle but that are not found in the mature virus particle but which are used during replication in the infected which are used during replication in the infected cell). cell).

These include the These include the replicase and proteins that replicase and proteins that alter host cell metabolism. alter host cell metabolism.

VPg, the protein that is found at the end of each VPg, the protein that is found at the end of each of the positive sense genomic RNA molecules is of the positive sense genomic RNA molecules is formed from part of P3.formed from part of P3.

Release:Release: Release (in most cases) on the virus Release (in most cases) on the virus

from the cytoplasm occurs when the from the cytoplasm occurs when the cell lyses - probably a cell lyses - probably a 'preprogrammed' event which occurs 'preprogrammed' event which occurs a set time after the cessation of a set time after the cessation of 'housekeeping' macromolecular 'housekeeping' macromolecular synthesis at shutoff. synthesis at shutoff.

Important features of viruses that Important features of viruses that commonly infect the intestinal tract are commonly infect the intestinal tract are summarized in Table below.summarized in Table below.

Enteroviruses replicate optimally at 37°C, Enteroviruses replicate optimally at 37°C, whereas rhinoviruses grow better at 33°C, whereas rhinoviruses grow better at 33°C, in accordance with the lower temperature in accordance with the lower temperature of the nose. of the nose.

Enteroviruses are stable under acid Enteroviruses are stable under acid conditions (pH 3-5), which enables them to conditions (pH 3-5), which enables them to survive exposure to gastric acid, survive exposure to gastric acid,

whereas rhinoviruses are acid-labile. This whereas rhinoviruses are acid-labile. This explains why rhinovirus infections are explains why rhinovirus infections are restricted to the nose and throat.restricted to the nose and throat.

. . Features of viruses commonly Infecting the intestinal tract. Features of viruses commonly Infecting the intestinal tract. diarrheadiarrhea

VirusVirus Nucleic AcidNucleic Acid DiseaseDisease Number of Number of SerotypesSerotypes

Lifelong VaccineLifelong Vaccine AvailableAvailableImmunityImmunityTo diseaseTo disease

Antiviral TherapyAntiviral Therapy

PoliovirusPoliovirus RNARNA PoliomyelitisPoliomyelitis 33 Yes (type-Yes (type-specific) specific)

+ +

- -

EchovirusesEchoviruses RNARNA Meningitis, etcMeningitis, etc ManyMany No -No - --

CoxsackievirusesCoxsackieviruses RNARNA Meningitis, Meningitis, carditis, etccarditis, etc

ManyMany No -No - --

Hepatitis A virus Hepatitis A virus (enterovirus 72)(enterovirus 72)

RNARNA HepatitisHepatitis 11 Yes +Yes +

--

RotavirusRotavirus RNARNA DiarrheaDiarrhea SeveralSeveral11 No -2 No -2

--

Norwalk virus Norwalk virus (Norovirus)(Norovirus)

RNARNA DiarrheaDiarrhea UnknownUnknown No -No - --

AdenovirusAdenovirus DNADNA DiarrheaDiarrhea 41; of 41; of which2causewhich2causediarrheadiarrhea

Unknown -Unknown - --

In the previous table: 1 the exact In the previous table: 1 the exact number is uncertain for the number number is uncertain for the number of hepatitis A serotypes.of hepatitis A serotypes.

2 Rotavirus vaccine was released but 2 Rotavirus vaccine was released but was withdrawn because of side was withdrawn because of side effects .effects .

ENTEROVIRUSESENTEROVIRUSESPATHOLOGYPATHOLOGY

Enteroviruses are spread via the fecal-oral Enteroviruses are spread via the fecal-oral route. The ingested viruses infect cells of the route. The ingested viruses infect cells of the oro-pharyngeal mucosa and lymphoid tissue oro-pharyngeal mucosa and lymphoid tissue (tonsils) where they are replicated and shed into (tonsils) where they are replicated and shed into the alimentary tract..the alimentary tract..

From here they may pass further down the From here they may pass further down the gastrointestinal tract. Because of the acid gastrointestinal tract. Because of the acid stability of these viruses they can pass into the stability of these viruses they can pass into the intestine and set up further infections in the intestine and set up further infections in the intestinal mucosa.intestinal mucosa.

The virus also infects the lymphoid tissue The virus also infects the lymphoid tissue (Peyer's patches) underlying the intestinal (Peyer's patches) underlying the intestinal mucosa. mucosa.

At these sites, the virus replicates and At these sites, the virus replicates and are shed into the feces often for are shed into the feces often for months after the primary infection.months after the primary infection.

In the primary viremic phase, the In the primary viremic phase, the virus also enters the bloodstream at virus also enters the bloodstream at low levels. The tissues that are then low levels. The tissues that are then infected depend on the expression of infected depend on the expression of the correct receptors.the correct receptors.

For example, For example, CD155CD155, the polio virus , the polio virus receptor, is expressed in spinal cord receptor, is expressed in spinal cord anterior horn cells, dorsal root anterior horn cells, dorsal root ganglia, skeletal muscle, motor ganglia, skeletal muscle, motor neurons and some cells of the neurons and some cells of the lymphoid system. lymphoid system.

Expression of Expression of CD155CD155 within within embryonic structures giving rise to embryonic structures giving rise to spinal cord anterior horn motor spinal cord anterior horn motor neurons may explain the neurons may explain the restrictive restrictive host cell tropism of polio virus for this host cell tropism of polio virus for this cellular compartment of the central cellular compartment of the central nervous systemnervous system. .

The Coxsackie virus receptor (which The Coxsackie virus receptor (which also binds adenovirus) is a surface also binds adenovirus) is a surface protein with two immunoglobulin-like protein with two immunoglobulin-like domains is more widely expressed. domains is more widely expressed.

At this stage symptoms may occur and the At this stage symptoms may occur and the patient may experience fever and malaise. patient may experience fever and malaise.

A secondary viremia may occur at this time.A secondary viremia may occur at this time. The spread of the virus from the gastro-The spread of the virus from the gastro-

intestinal tract and the secondary viremia intestinal tract and the secondary viremia that occurs about 10 days after the initial that occurs about 10 days after the initial infection leads to a infection leads to a humoral and cell-humoral and cell-mediated immune response (the latter mediated immune response (the latter being of less importancebeing of less importance). ).

This rapidly limits the further replication of This rapidly limits the further replication of the virus in all tissues except the GI tract the virus in all tissues except the GI tract because the virus must pass through because the virus must pass through extracellular space to infect another cell. extracellular space to infect another cell.

In the GI tract In the GI tract replicationreplication may be sustained may be sustained for for several weeksseveral weeks even though a high titer even though a high titer of neutralizing antibody is achieved. of neutralizing antibody is achieved.

The cells in which this replication The cells in which this replication occurs are not known and it is occurs are not known and it is unclear why replication occurs in the unclear why replication occurs in the presence of the neutralizing presence of the neutralizing antibody.antibody.

Although each group of Although each group of enteroviruses share a receptor, the enteroviruses share a receptor, the various serotypesvarious serotypes of a group are of a group are usually usually not blocked by group-specific not blocked by group-specific antibodiesantibodies even though it would be even though it would be expected that they would have a expected that they would have a common receptor binding sitecommon receptor binding site. .

Human diseases caused by enterovirusesHuman diseases caused by enteroviruses PoliovirusPoliovirus

Coxsackie Coxsackie A virusA virus

Coxsackie Coxsackie B virusB virus

EchovirusEchovirus Enterovirus Enterovirus (other(other

AsymptomaAsymptomatic tic infectioninfection

yesyes yesyes yesyes yesyes yesyes

MeningitisMeningitis yesyes yesyes yesyes yesyes yesyes

ParalysisParalysis yesyes yesyes yesyes yesyes nono

Febrile Febrile exanthemsexanthems

nono yesyes yesyes yesyes yesyes

Acute Acute respiratory respiratory diseasedisease

nono yesyes yesyes yesyes yesyes

MyocarditisMyocarditis nono yesyes yesyes yesyes nono

OrchitisOrchitis nono nono yesyes yesyes nono

Figure 5 - Enterovirus pathogenesisFigure 5 - Enterovirus pathogenesis

Pathogenesis of enteroviruses. Cox = Pathogenesis of enteroviruses. Cox = Coxsackie virus A or B, Hep A = Coxsackie virus A or B, Hep A = hepatitis A virus, Echo = echovirus, hepatitis A virus, Echo = echovirus, Polio = poliovirus Polio = poliovirus

DISEASES CAUSED BY ENTEROVIRUSESDISEASES CAUSED BY ENTEROVIRUSES Most patients infected with an Most patients infected with an

enterovirus remain enterovirus remain asymptomaticasymptomatic but in small children benign fevers but in small children benign fevers caused by unidentified enteroviruses caused by unidentified enteroviruses are relatively common (non-specific are relatively common (non-specific febrile illness). febrile illness).

Many outbreaks of febrile illness Many outbreaks of febrile illness accompanied by rashes are also accompanied by rashes are also caused by enteroviruses.caused by enteroviruses.

POLIOVIRUSPOLIOVIRUS Poliovirus caused about 21, 000 cases of Poliovirus caused about 21, 000 cases of

paralytic poliomyelitis in the United States paralytic poliomyelitis in the United States each year in the 1940's - 50's prior to the each year in the 1940's - 50's prior to the introduction of the Salk (inactivated) and introduction of the Salk (inactivated) and Sabin (attenuated) vaccines. Sabin (attenuated) vaccines.

The height of the epidemic occurred in The height of the epidemic occurred in 1950 when there were 34,000 cases. Today, 1950 when there were 34,000 cases. Today, the number of cases of paralytic polio in the the number of cases of paralytic polio in the US is fewer than 10 and these are the result US is fewer than 10 and these are the result of the attenuated (Sabin) vaccine reverting of the attenuated (Sabin) vaccine reverting to virulence (see Vaccine section). to virulence (see Vaccine section).

Our first record of polio comes from an Our first record of polio comes from an Egyptian stele from the 18th dynasty (1580-Egyptian stele from the 18th dynasty (1580-1350 BCE) showing a victim of the disease 1350 BCE) showing a victim of the disease with a withered leg (figure 6).with a withered leg (figure 6).

Egyptian stele from the 18th dynasty Egyptian stele from the 18th dynasty showing a victim of polio with a withered showing a victim of polio with a withered leg figure .leg figure .

Fig.6Fig.6

PoliovirusPoliovirus

Important Properties:Important Properties:

- Infection is limited to the primates (receptor?)- Infection is limited to the primates (receptor?)

- Three serologic types are present- Three serologic types are present

- - CD155, the polio virus receptor, is expressed CD155, the polio virus receptor, is expressed (in (in

spinal cord anterior horn cells, dorsal root ganglia, skeletal spinal cord anterior horn cells, dorsal root ganglia, skeletal

muscle, motor neurons and some cells of the lymphoid muscle, motor neurons and some cells of the lymphoid

system)system)

- - For unknown reasons, polio virus does not For unknown reasons, polio virus does not spread to the cells of the central nervous system spread to the cells of the central nervous system in in all patientsall patients

This virus causes poliomyelitis.This virus causes poliomyelitis. The The host rangehost range is limited to is limited to primatesprimates, i.e., , i.e.,

humans and nonhuman primates such as apes humans and nonhuman primates such as apes and monkeys.and monkeys.

This limitation is due to the This limitation is due to the binding of the viral binding of the viral capsid protein to a receptor found only on capsid protein to a receptor found only on primate cell membranes. primate cell membranes.

However, note that purified viral RNA (without However, note that purified viral RNA (without the capsid protein) can enter and replicate in the capsid protein) can enter and replicate in many nonprimate cells—the RNA can bypass the many nonprimate cells—the RNA can bypass the cell membrane receptor; i.e., it is "cell membrane receptor; i.e., it is "infectious infectious RNARNA."."

There are three serologic (antigenic) types based There are three serologic (antigenic) types based on different antigenic determinants on the outer on different antigenic determinants on the outer capsid proteins. capsid proteins.

Because there is Because there is little cross-reactionlittle cross-reaction, protection , protection from disease requires the presence of from disease requires the presence of antibody antibody against each of the three types.against each of the three types.

Primary site of infection is lymphoid tissueassociated with the oropharynx and gut (GALT). Virus production at this site leads to a transient viraemia,following which the virus may infect the CNS. This is of interest because of this apparent 'dual tropism‘ of the virus for two distinct cell types - lymphoid/ epithelial cell in the gut and neurons in the CNS - different receptors, etc. Replication of the virus in the CNS occurs in the 'grey matter', particularly motor neurons in the anterior horns of the spinal cord and brain stem. Distinctive 'plaques' produced in the grey matter are due to lytic replication of the virus & probably inflammation caused by an over-enthusiastic immune response.

Replication CycleReplication Cycle Interaction of the virus with its receptorInteraction of the virus with its receptor

Enters the cell, uncoatingEnters the cell, uncoating

The genome RNA functions as mRNAThe genome RNA functions as mRNA

Translated into one very large polypeptideTranslated into one very large polypeptide

viral RNA polymerase synthesized the viral RNA polymerase synthesized the

progeny RNA genomesprogeny RNA genomes Replication Replication (+ve ssRNA::-ve ssRNA:: +ve ssRNA)(+ve ssRNA::-ve ssRNA:: +ve ssRNA)

Assembly occurs at the cytoplasmAssembly occurs at the cytoplasm

Release occurs by lysis of the cellsRelease occurs by lysis of the cells

Life cycleLife cycle Poliovirus infects human cells by binding to an Poliovirus infects human cells by binding to an

immunoglobulin-likeimmunoglobulin-like receptor, receptor, CD155CD155, (also known , (also known as the as the poliovirus receptorpoliovirus receptor (PVR)) on the cell surface. (PVR)) on the cell surface.

Interaction of poliovirus and CD155 facilitates an Interaction of poliovirus and CD155 facilitates an irreversible conformational change of the viral irreversible conformational change of the viral particle necessary for viral entry. particle necessary for viral entry.

The precise mechanism poliovirus uses to enter the The precise mechanism poliovirus uses to enter the hosthost cell has not been firmly established. Attached cell has not been firmly established. Attached to the host to the host cell membranecell membrane, entry of the viral nucleic , entry of the viral nucleic acid was thought to occur by one of two ways:acid was thought to occur by one of two ways:

via the formation of a via the formation of a porepore in the plasma in the plasma membrane through which the RNA is then membrane through which the RNA is then “injected” into the host cell “injected” into the host cell cytoplasmcytoplasm,,

or that the virus is taken up by or that the virus is taken up by receptor-mediated receptor-mediated endocytosisendocytosis. .

Recent experimental evidence supports the Recent experimental evidence supports the latter hypothesis and suggests that poliovirus latter hypothesis and suggests that poliovirus binds to CD155 and is taken up via endocytosis. binds to CD155 and is taken up via endocytosis.

Immediately after internalization of the particle, Immediately after internalization of the particle, the viral RNA is released. the viral RNA is released.

Translation of the viral RNA occurs by an IRES-Translation of the viral RNA occurs by an IRES-mediated mechanism.mediated mechanism.

Poliovirus is a positive stranded Poliovirus is a positive stranded RNA virusRNA virus. Thus . Thus the genome enclosed within the viral particle the genome enclosed within the viral particle can be used as can be used as messenger RNAmessenger RNA and and immediately immediately translatedtranslated by the host cell. by the host cell.

On entry the virus hijacks the cell's translation On entry the virus hijacks the cell's translation machinery; causing inhibition of cellular protein machinery; causing inhibition of cellular protein synthesis in favor of virus–specific protein synthesis in favor of virus–specific protein production. production.

Unlike the host cell's mRNAs the 5' end of Unlike the host cell's mRNAs the 5' end of poliovirus RNA is extremely long—over poliovirus RNA is extremely long—over 700 nucleotides—and is highly structured.700 nucleotides—and is highly structured.

The polio virus RNA comprises 7741 bases The polio virus RNA comprises 7741 bases with a large 5' leader sequence of 743 with a large 5' leader sequence of 743 bases that does not code for viral protein bases that does not code for viral protein (untranslated region). (untranslated region).

This region of the viral genome is called This region of the viral genome is called internal ribosome entry site (IRES) and it internal ribosome entry site (IRES) and it directs translation of the viral RNA. directs translation of the viral RNA.

Genetic mutations in this region prevent Genetic mutations in this region prevent viral protein production.viral protein production.

Poliovirus mRNA is translated as one long Poliovirus mRNA is translated as one long polypeptidepolypeptide called noncapsid viral protein called noncapsid viral protein 0000. .

This polypeptide is then cleaved This polypeptide is then cleaved by a virus-encoded by a virus-encoded proteaseprotease to form both the to form both the capsid proteinscapsid proteins of the of the progeny virions and several progeny virions and several noncapsid proteinsnoncapsid proteins, , including the including the RNA polymeraseRNA polymerase that synthesizes the that synthesizes the progeny RNA genomes,progeny RNA genomes,approximately approximately 1010 individual individual viral proteins, including:viral proteins, including:

3Dpol3Dpol, an , an RNA dependent RNA polymeraseRNA dependent RNA polymerase whose whose function is to copy and multiply the viral RNA function is to copy and multiply the viral RNA genome. genome.

2Apro2Apro and and 3Cpro/3CDpro3Cpro/3CDpro, , proteasesproteases which cleave which cleave the viral polypeptide. the viral polypeptide.

VPgVPg (3B), a small protein that binds viral RNA and is (3B), a small protein that binds viral RNA and is necessary for synthesis of viral positive and negative necessary for synthesis of viral positive and negative strand RNA. strand RNA.

2BC, 2B, 2C, 3AB, 3A, 3B2BC, 2B, 2C, 3AB, 3A, 3B proteins which comprise proteins which comprise the the protein complexprotein complex needed for virus replication. needed for virus replication.

VP0, VP1, VP2, VP3, VP4VP0, VP1, VP2, VP3, VP4 proteins of the viral capsid. proteins of the viral capsid.

Replication of the genome occurs by Replication of the genome occurs by synthesis of a complementary negative synthesis of a complementary negative strand, which then serves as the template strand, which then serves as the template for the positive strands.for the positive strands.

Some of these positive strands function as Some of these positive strands function as mRNA to make more viral proteins, and mRNA to make more viral proteins, and the remainder become progeny virion the remainder become progeny virion gene RNA. gene RNA.

The assembly of new virus particles, (i.e. The assembly of new virus particles, (i.e. the packaging of progeny genome into a the packaging of progeny genome into a capsid which can survive outside the host capsid which can survive outside the host cell) is poorly understood. cell) is poorly understood. Assembly of the Assembly of the progeny virions occurs by coating of the progeny virions occurs by coating of the genome RNA with capsid proteins. genome RNA with capsid proteins.

Fully assembled poliovirus leaves the Fully assembled poliovirus leaves the confines of its host cell 4 to 6 hours confines of its host cell 4 to 6 hours following initiation of infection in following initiation of infection in cultured mammalian cells. cultured mammalian cells.

Virions accumulate in the cell cytoplasm Virions accumulate in the cell cytoplasm and are released upon death of the cell. and are released upon death of the cell. They do not bud from the cell They do not bud from the cell membrane.membrane.

The mechanism of viral release from The mechanism of viral release from the cell is unclear, but each dying cell the cell is unclear, but each dying cell can release up to 10,000 polio virions.can release up to 10,000 polio virions.

Origin and serotypesOrigin and serotypes Poliovirus is structurally similar to other human Poliovirus is structurally similar to other human

enteroviruses (enteroviruses (coxsackievirusescoxsackieviruses and and echovirusesechoviruses), ), as well as to human as well as to human rhinovirusesrhinoviruses, which also use , which also use immunoglobulin-like molecules to recognize and immunoglobulin-like molecules to recognize and enter host cells. enter host cells.

PhylogeneticPhylogenetic analysis of the RNA and protein analysis of the RNA and protein sequences of poliovirus suggests that PV may have sequences of poliovirus suggests that PV may have evolved from a C-cluster evolved from a C-cluster coxsackiecoxsackie A virus A virus ancestorancestor, , that arose through a mutation within the capsid.that arose through a mutation within the capsid.

The distinct The distinct speciationspeciation of poliovirus probably of poliovirus probably occurred as a result of change in cellular receptor occurred as a result of change in cellular receptor specificity from specificity from intercellular adhesion molecule-1intercellular adhesion molecule-1 (ICAM-1), used by C-cluster coxsackie A viruses, to (ICAM-1), used by C-cluster coxsackie A viruses, to CD155CD155; leading to a change in pathogenicity, and ; leading to a change in pathogenicity, and allowing the virus to infect nervous tissue. There allowing the virus to infect nervous tissue. There are three are three serotypesserotypes of poliovirus, of poliovirus, PV1PV1, , PV2PV2 , and , and PV3PV3; each with a slightly different ; each with a slightly different capsidcapsid protein. protein.

Capsid proteins define cellular receptor specificity Capsid proteins define cellular receptor specificity and virus antigenicityand virus antigenicity..

Transmission & Transmission & EpidemiologyEpidemiology

Poliovirus is spread via the fecal-oral route Poliovirus is spread via the fecal-oral route

Most disease results from type 1 polio virus Most disease results from type 1 polio virus

Poliovirus caused about 21, 000 cases of Poliovirus caused about 21, 000 cases of paralytic poliomyelitis in the United States paralytic poliomyelitis in the United States each year in the 1940's - 50's each year in the 1940's - 50's

Today, the number of cases of paralytic Today, the number of cases of paralytic polio in the US is fewer than 10 and these are polio in the US is fewer than 10 and these are the result of the attenuated (Sabin) vaccine the result of the attenuated (Sabin) vaccine reverting to virulence reverting to virulence

PV1PV1 is the most common form encountered is the most common form encountered in nature, however all three forms are in nature, however all three forms are extremely extremely infectiousinfectious. Wild polioviruses can be . Wild polioviruses can be found in approximately 10 countries. found in approximately 10 countries.

PV1 is highly localized to regions in India, PV1 is highly localized to regions in India, Pakistan, Afghanistan, and Egypt, but Pakistan, Afghanistan, and Egypt, but following outbreaks of poliomeyletis in 2003–following outbreaks of poliomeyletis in 2003–2004 it remains widespread in West and 2004 it remains widespread in West and Central Africa. Central Africa.

Wild poliovirus Wild poliovirus type 2type 2 has probably been has probably been eradicated; it was last detected in October eradicated; it was last detected in October 1999 in 1999 in Uttar PradeshUttar Pradesh, India. , India.

Wild Wild PV3 PV3 is found in parts of only five is found in parts of only five countries (Nigeria, Niger, Pakistan, India, and countries (Nigeria, Niger, Pakistan, India, and Sudan).Sudan).

Specific strains of each serotype are used Specific strains of each serotype are used to prepare vaccines against polio. to prepare vaccines against polio.

Inactive polioInactive polio vaccine (IPV) is prepared by vaccine (IPV) is prepared by formalin inactivation of three wild, virulent formalin inactivation of three wild, virulent reference strains, Mahoney or Brunenders reference strains, Mahoney or Brunenders (PV1), MEF-1/Lansing (PV2), and (PV1), MEF-1/Lansing (PV2), and Saukett/Leon (PV3). Saukett/Leon (PV3). Oral polioOral polio vaccine vaccine (OPV) contains live attenuated (weakened) (OPV) contains live attenuated (weakened) strains of the three serotypes of poliovirus. strains of the three serotypes of poliovirus.

Passaging the virus strains in monkey Passaging the virus strains in monkey kidney epithelial cells introduces mutations kidney epithelial cells introduces mutations in the viral in the viral IRESIRES, and hinders (or , and hinders (or attenuates) the ability of the virus to infect attenuates) the ability of the virus to infect nervous tissue.nervous tissue.

Pathogenesis & ImmunityPathogenesis & Immunity Replicate 1st in oropharynx & GIReplicate 1st in oropharynx & GI

Then, spread via blood to the CNSThen, spread via blood to the CNS

The virus replicates in the The virus replicates in the motor neuronsmotor neurons located located

in the in the anterior hornanterior horn of the spinal cord of the spinal cord

Paralysis occurs due to the death of these cellsParalysis occurs due to the death of these cells

Immunity is lifelong Immunity is lifelong type-specifictype-specific

In infected individuals, the immune response In infected individuals, the immune response

consists of consists of both intestinal IgA and humoral IgG to both intestinal IgA and humoral IgG to

the specific serotype. the specific serotype.

Immune system avoidanceImmune system avoidance Poliovirus uses two key mechanisms to evade Poliovirus uses two key mechanisms to evade

the the immune systemimmune system. . First, it is capable of surviving the highly First, it is capable of surviving the highly

acidicacidic conditions of the conditions of the gastrointestinal tractgastrointestinal tract, , allowing the virus to infect the host and allowing the virus to infect the host and spread throughout the body via the spread throughout the body via the lymphatic systemlymphatic system. .

Second, because it can replicate very quickly, Second, because it can replicate very quickly, the virus overwhelms the host organs before the virus overwhelms the host organs before an immune response can be mounted.an immune response can be mounted.

Individuals who are exposed to poliovirus, Individuals who are exposed to poliovirus, either through infection or by either through infection or by immunizationimmunization with with polio vaccinepolio vaccine, develop , develop immunityimmunity. .

In immune individuals, antibodies against In immune individuals, antibodies against poliovirus are present in the tonsils and poliovirus are present in the tonsils and gastrointestinal tract (specifically IgA gastrointestinal tract (specifically IgA antibodies) and are able to antibodies) and are able to blockblock poliovirus poliovirus replicationreplication; ;

IgG and IgM antibodies against poliovirus IgG and IgM antibodies against poliovirus can can prevent prevent the spread of the virus to the spread of the virus to motor motor neurons of the central nervous systemneurons of the central nervous system..

Infection with one serotype of poliovirus Infection with one serotype of poliovirus does not provide immunity against the does not provide immunity against the other serotypes, however second attacks other serotypes, however second attacks within the same individual are extremely within the same individual are extremely rare.rare.

Clinical FindingsClinical Findings Asymptomatic polio infection:Asymptomatic polio infection: - - This occurs when the replication of the virus is This occurs when the replication of the virus is

restricted to the GI tractrestricted to the GI tract - Asymptomatic infection is quite common. Roughly - Asymptomatic infection is quite common. Roughly

1% of infections are clinically apparent1% of infections are clinically apparent - IP is 10-14 days- IP is 10-14 days Abortive poliomyelitis (minor illness):Abortive poliomyelitis (minor illness):

- i- is febrile disease and occurs in the first week of s febrile disease and occurs in the first week of

infection infection

- - occurs in about 5% of infected individualsoccurs in about 5% of infected individuals

- - general malaise which may be accompanied by general malaise which may be accompanied by

vomiting, a headache and sore throat.vomiting, a headache and sore throat.

- Most patients - Most patients recover spontaneouslyrecover spontaneously. .

Clinical Findings Cont.Clinical Findings Cont. Non-paralytic poliomyelitis:Non-paralytic poliomyelitis: - - TThis is similar to his is similar to aseptic meningitisaseptic meningitis with fever, with fever,

headache & a stiff neck .This also usually headache & a stiff neck .This also usually resolves resolves spontaneouslyspontaneously..

Paralytic polio:Paralytic polio: - - flaccid paralysisflaccid paralysis is the predominant finding is the predominant finding - Respiratory paralysis can occurs due to brain stem - Respiratory paralysis can occurs due to brain stem

involvmentinvolvment

Post-polio syndrome:Post-polio syndrome: - Occurs many years after the acute illness- Occurs many years after the acute illness - - involves further loss of function in affected muscles involves further loss of function in affected muscles

perhaps as a result of further neuron loss.perhaps as a result of further neuron loss.

Human poliovirusHuman poliovirus Viral particles seen by Viral particles seen by transmission electron transmission electron microscopy (TEM) at microscopy (TEM) at

a a magnification of magnification of

350,000x). 350,000x). This image is from This image is from

Dennis Dennis Kunkel's excellent Kunkel's excellent

Microscopy Science and Microscopy Science and Photography Th Photography Th

rough a rough a Microscope Microscope

web site. web site.

Laboratory DiagnosisLaboratory Diagnosis

Isolation of the virusIsolation of the virus

Detection a Detection a riserise in antibody titer in antibody titer

Clinical spicemens include: throat swabs, stool or Clinical spicemens include: throat swabs, stool or

spinal fluid spinal fluid by inoculation of cell cultures. by inoculation of cell cultures.

The virus causes a cytopathic effect (CPE) and can The virus causes a cytopathic effect (CPE) and can

be identified by neutralization of the CPE with be identified by neutralization of the CPE with

specific antisera.specific antisera.

Treatment & PreventionTreatment & Prevention

No antiviral therapyNo antiviral therapy

Prevented by vaccination using:Prevented by vaccination using:

1- killed vaccine (1- killed vaccine (Salk vaccineSalk vaccine, , inactivated vaccine, inactivated vaccine, IPVIPV))

2- live attenuated vaccine (2- live attenuated vaccine (Sabin Sabin vaccinevaccine, oral vaccine, , oral vaccine, OPVOPV))

(current version of IPV is called (current version of IPV is called enhanced polio vaccine enhanced polio vaccine „eIPV“„eIPV“))

CoxsackievirusesCoxsackieviruses TThere are many infections caused by here are many infections caused by

Coxsackie viruses, most of which are Coxsackie viruses, most of which are nevernever diagnosed precisely diagnosed precisely

Algonquin indian name of village in N.Y. where Algonquin indian name of village in N.Y. where first isolated (Daldorf and Sickles/suckling first isolated (Daldorf and Sickles/suckling mice/1948). Two groups, based on pathology mice/1948). Two groups, based on pathology in suckling mice: in suckling mice:

Coxsackie type ACoxsackie type A usually is associated usually is associated with surface rashes (exanthems) with surface rashes (exanthems) 24 24 serotypes serotypes

Type BType B typically causes internal symptoms typically causes internal symptoms (pleurodynia, myocarditis) (pleurodynia, myocarditis) 6 serotypes 6 serotypes

Both can also cause paralytic disease or Both can also cause paralytic disease or mild respiratory tract infection. mild respiratory tract infection.

CoxsackievirusesCoxsackieviruses Meningitis:Meningitis: - - Enteroviruses are the major cause of viral Enteroviruses are the major cause of viral

meningitis meningitis

- B- Both Coxsackie virus A and B can cause oth Coxsackie virus A and B can cause

aseptic meningitis aseptic meningitis

- - Viral meningitis typically involves a headache, Viral meningitis typically involves a headache,

stiff neck, fever and general malaise stiff neck, fever and general malaise

- - Most patients recover from the disease unless Most patients recover from the disease unless

encephalitis occurs encephalitis occurs

- - The disease is most prevalent in the summer and fallThe disease is most prevalent in the summer and fall

CoxsackievirusesCoxsackieviruses

Herpangina:Herpangina:

- - Coxsackie virus ACoxsackie virus A can cause a fever with can cause a fever with painful ulcers on the palate and tongue painful ulcers on the palate and tongue leading to problems swallowing and vomitingleading to problems swallowing and vomiting

- - Treatment of the symptoms is all that is Treatment of the symptoms is all that is required as the disease required as the disease subsides in a few subsides in a few daysdays. .

CoxsackievirusesCoxsackieviruses Hand, foot and mouth diseaseHand, foot and mouth disease: :

(HFMD)(HFMD) - - This is an exanthem (rash) caused by This is an exanthem (rash) caused by Coxsackie Coxsackie

type A16 type A16 - - Symptoms include fever and blisters on the Symptoms include fever and blisters on the

hands, palate and feet. hands, palate and feet. - - it subsides in a few days it subsides in a few days

Myocarditis:Myocarditis: - - Type A and BType A and B can cause myocarditis in can cause myocarditis in

neonates and young childrenneonates and young children - Fever, chest pains, arrhythmia and even - Fever, chest pains, arrhythmia and even

cardiac failure can result cardiac failure can result - - Mortality rates are highMortality rates are high. .

Laboratory DiagnosisLaboratory Diagnosis The diagnosis is made either by isolating The diagnosis is made either by isolating

the virus in the virus in cell culturecell culture or suckling mice or or suckling mice or by observing a by observing a rise in titer of neutralizing rise in titer of neutralizing antibodies.antibodies.

A rapid (2.5 hour) A rapid (2.5 hour) PCR-based testPCR-based test for for enteroviral RNA in the spinal fluid is enteroviral RNA in the spinal fluid is usefuluseful for making a prompt diagnosis of viral for making a prompt diagnosis of viral meningitis because culture techniques meningitis because culture techniques typically take days to obtain a result.typically take days to obtain a result.

Treatment & PreventionTreatment & Prevention There is neither antiviral drug therapy nor There is neither antiviral drug therapy nor

a vaccine available against these viruses. a vaccine available against these viruses. No passive immunization is recommended.No passive immunization is recommended.

EchovirusesEchoviruses

ECHO = ECHO = eenteric nteric ccytopathic ytopathic hhuman uman oorphanrphan

Cause aseptic meningitis, URT, febrile Cause aseptic meningitis, URT, febrile

illness, infantile diarrhea & illness, infantile diarrhea & hemorrhagic hemorrhagic

conjunctivitisconjunctivitis

Transmitted by fecal-oral routeTransmitted by fecal-oral route

Diagnosed by isolation in cell cultureDiagnosed by isolation in cell culture

No vaccine or antiviral availableNo vaccine or antiviral available

The structure of echoviruses is similar to The structure of echoviruses is similar to

that of other enteroviruses. that of other enteroviruses. More than 30 serotypes have been More than 30 serotypes have been

isolated.isolated. In contrast to coxsackieviruses, they are In contrast to coxsackieviruses, they are

not pathogenic for mice. not pathogenic for mice. Unlike polioviruses, they do not cause Unlike polioviruses, they do not cause

disease in monkeys. disease in monkeys. They are transmitted by the fecal-oral They are transmitted by the fecal-oral

route and occur worldwide. route and occur worldwide. Pathogenesis is similar to that of the other Pathogenesis is similar to that of the other

enteroviruses.enteroviruses.

Along with coxsackieviruses, Along with coxsackieviruses, echoviruses are one of the leading echoviruses are one of the leading causes of aseptic (viral) meningitis.causes of aseptic (viral) meningitis.

The diagnosis is made by isolation of The diagnosis is made by isolation of the virus in cell culture.the virus in cell culture.

Serologic tests are of little value, Serologic tests are of little value, because there are a large number of because there are a large number of serotypes and no common antigenserotypes and no common antigen..

There is no antiviral therapy or There is no antiviral therapy or vaccine available.vaccine available.

Other enterovirus Other enterovirus diseasesdiseases

Acute hemorrhagic conjunctivitisAcute hemorrhagic conjunctivitis is is caused by caused by Coxsackie A24Coxsackie A24 and and Enterovirus 70.Enterovirus 70. The disease resolves The disease resolves in a week or twoin a week or two

Enterovirus 71Enterovirus 71 is one of the leading is one of the leading causes of causes of viral CNS diseaseviral CNS disease, including , including meningitis, encephalitis, & paralysismeningitis, encephalitis, & paralysis

Enterovirus 72 is hepatitis A virusEnterovirus 72 is hepatitis A virus

‘‘New' Enteroviruses:New' Enteroviruses:Since 1969, 'new' Enteroviruses have been Since 1969, 'new' Enteroviruses have been assigned numbers, not names: assigned numbers, not names:

Type: Type: Illness: Illness:

6868 Pneumonia Pneumonia

6969 None (?) None (?)

7070 Acute haemorrhagic Acute haemorrhagic conjunctivitis (1969-1974 conjunctivitis (1969-1974 pandemic) pandemic)

7171 Meningitis Meningitis

7272 Hepatitis A virus (now a separate Hepatitis A virus (now a separate genus: Hepatovirus) genus: Hepatovirus)

RHINOVIRUSES RHINOVIRUSES Rhinoviruses are the main cause of the Rhinoviruses are the main cause of the

common cold common cold (but not the only one!). (but not the only one!).

There are more than 100 serotypes There are more than 100 serotypes

They are They are sensitive to low pHsensitive to low pH and and replicate replicate well at 33 well at 33 °°CC

Infect the URT & spread directly via Infect the URT & spread directly via aerosolsaerosols or by or by fomitesfomites such as hands and such as hands and other forms of direct contactother forms of direct contact

RHINOVIRUS DISEASERHINOVIRUS DISEASE There are nearly There are nearly 62 million cases62 million cases of the of the

common cold annually in the US common cold annually in the US

52.2 million52.2 million of these cases affect Americans of these cases affect Americans under under age 17age 17   

There are nearly There are nearly 22 million22 million school-loss days school-loss days annually due to the common cold annually due to the common cold 

There are approximately There are approximately 45 million bed45 million bed

days annually associated with the common days annually associated with the common cold  cold 

Clinical FindingsClinical Findings

IP is IP is 2-42-4 days days

Symptoms include Symptoms include discharging or discharging or

blocked nasal passages often blocked nasal passages often

accompanied by sneezes,  and perhaps a accompanied by sneezes,  and perhaps a

sore throatsore throat

The illness lasts about 1 weekThe illness lasts about 1 week other viruses cause symptoms that are other viruses cause symptoms that are

similar to those of rhinoviruses similar to those of rhinoviruses (parainfluenzaviruses, coronaviruses and enteroviruses)(parainfluenzaviruses, coronaviruses and enteroviruses)

They replicate better at They replicate better at 33°C33°C than at than at 37°C, which explains why they affect 37°C, which explains why they affect primarily the nose and conjunctiva primarily the nose and conjunctiva rather than the lower respiratory tract.rather than the lower respiratory tract.

Because they are Because they are acid-labileacid-labile, they are , they are killed by gastric acid when swallowed. killed by gastric acid when swallowed.

This explains why they do not infect This explains why they do not infect the gastrointestinal tract, unlike the the gastrointestinal tract, unlike the enteroviruses.enteroviruses.

The host range is limited to humans The host range is limited to humans and chimpanzeesand chimpanzees

Summary of Replicative CycleSummary of Replicative Cycle Replication is similar to that of Replication is similar to that of

poliovirus. The cell surface receptor poliovirus. The cell surface receptor for rhinoviruses is ICAM-1, an for rhinoviruses is ICAM-1, an adhesion protein located on the adhesion protein located on the surface of many types of cellssurface of many types of cells

Pathogenesis & ImmunityPathogenesis & Immunity The portal of entry is the upper The portal of entry is the upper

respiratory tract, and the infection is respiratory tract, and the infection is limited to that region. limited to that region.

Rhinoviruses rarely cause lower Rhinoviruses rarely cause lower respiratory tract disease, probably respiratory tract disease, probably because they grow poorly at 37°C.because they grow poorly at 37°C.

Immunity is serotype-specific and is Immunity is serotype-specific and is a function of nasal secretory a function of nasal secretory IgAIgA rather than humoral antibodyrather than humoral antibody..

Laboratory DiagnosisLaboratory Diagnosis

Diagnosis can be made by isolation of the virus Diagnosis can be made by isolation of the virus from nasal secretions in cell culture, but this is from nasal secretions in cell culture, but this is rarely attempted. Serologic tests are not done.rarely attempted. Serologic tests are not done.

Treatment & PreventionTreatment & Prevention

No specific antiviral therapy is available.No specific antiviral therapy is available. Vaccines appear impractical because of the large Vaccines appear impractical because of the large

number of serotypes. number of serotypes. Paper tissues impregnated with a combination of Paper tissues impregnated with a combination of

citric acidcitric acid (which inactivates rhinoviruses) and (which inactivates rhinoviruses) and sodium lauryl sulfatesodium lauryl sulfate (a detergent that inactivates (a detergent that inactivates enveloped viruses such as influenza virus and enveloped viruses such as influenza virus and respiratory syncytial virus) limit transmission respiratory syncytial virus) limit transmission when used to remove viruses from fingers when used to remove viruses from fingers contaminated with respiratory secretions.contaminated with respiratory secretions.

High doses of vitamin C have little ability to High doses of vitamin C have little ability to prevent rhinovirus-induced colds.prevent rhinovirus-induced colds.

Lozenges containing zinc gluconate are Lozenges containing zinc gluconate are available for the treatment of the common available for the treatment of the common cold, but their efficacy remains unproved.cold, but their efficacy remains unproved.

But But rehydration and keeping the airways rehydration and keeping the airways unblocked is advisableunblocked is advisable

Prescription of aspirin to relive fever Prescription of aspirin to relive fever ???? Interferon nasal sprays have little effect Interferon nasal sprays have little effect The best way to avoid spreading the virus The best way to avoid spreading the virus

is interrupt the infection chain by hand is interrupt the infection chain by hand washing washing

Human rhinovirus 1AHuman rhinovirus 1A Rhinovirus 14Rhinovirus 14, as solved by cryo-electron , as solved by cryo-electron

microscopy and image reconstructionmicroscopy and image reconstruction

Molecular surface of Molecular surface of Rhinovirus 14Rhinovirus 14, radially , radially depth cued, as solved by X-ray crystallography depth cued, as solved by X-ray crystallography

Rhinovirus 14Rhinovirus 14, radially depth cued with antigenic sites , radially depth cued with antigenic sites highlighted, as solved by X-ray crystallographyhighlighted, as solved by X-ray crystallography

This image is based of the real This image is based of the real atomic co-ordinates of atomic co-ordinates of

rhinovirus 14. The rhinovirus 14. The antigenic antigenic

sites on the surface of sites on the surface of this this

particle are highlighted particle are highlighted in purple. Note how in purple. Note how the distribution of the distribution of the antigenic sites the antigenic sites on the capsid on the capsid emphasises its emphasises its icosahedral symmetry.icosahedral symmetry.

Rhinovirus 14Rhinovirus 14 complexed with neutralizing complexed with neutralizing antibodies, as solved by cryo-electron microscopy antibodies, as solved by cryo-electron microscopy

and image reconstructionand image reconstruction

Rhinovirus 14Rhinovirus 14 complexed with the ICAM-1 receptor, complexed with the ICAM-1 receptor, as solved by cryo-electron microscopy and image as solved by cryo-electron microscopy and image reconstruction reconstruction

No effective prophylaxis or No effective prophylaxis or treatment.treatment.

There is little or no cross-protection There is little or no cross-protection between serotypes.between serotypes.

Protection relies on levels of Protection relies on levels of secreted Ab in URT - may be secreted Ab in URT - may be relatively short-lived (e.g. a few relatively short-lived (e.g. a few years rather than life-long).years rather than life-long). Medscape Article:Medscape Article: What's New What's New With Common Colds? Complications With Common Colds? Complications and Management. and Management.

2- CALICIVIRUSES2- CALICIVIRUSES They are almost like Picornaviruses They are almost like Picornaviruses

But differ in having:But differ in having: 1- larger genome1- larger genome 2- having distinctive spikes on 2- having distinctive spikes on

the surfacethe surface

Two human pathogens in this family:Two human pathogens in this family: 1- Norwalk virus (norovirus)1- Norwalk virus (norovirus) 2- Hepatitis E virus2- Hepatitis E virus

Norwalk virus (Norovirus)Norwalk virus (Norovirus)

One of the most common causes One of the most common causes of of viral gastroenteritis in adultsviral gastroenteritis in adults

It is named after an It is named after an outbreakoutbreak

(Ohio, 1969)(Ohio, 1969) It has It has +ve ssRNA+ve ssRNA, ,

nonenveloped, icosahedralnonenveloped, icosahedral

Uncertain serotypesUncertain serotypes Replicative cycle is unclear but Replicative cycle is unclear but

similar to Picornavirusessimilar to Picornaviruses

Transmission & Transmission & EpidemiologyEpidemiology

Fecal-oral (Fecal-oral (seafoodseafood & water) & water)

Person-to-person (group settings)Person-to-person (group settings)

Outbreaks in Outbreaks in cruise shipscruise ships, schools, , schools, campscamps

Low infectious dose, resistance to Low infectious dose, resistance to chlorination & drying, are factors chlorination & drying, are factors enhancing the infectionenhancing the infection

There are many animal caliciviruses, but There are many animal caliciviruses, but there is no evidence that they cause human there is no evidence that they cause human infection.infection.

Infection is enhanced by several features of Infection is enhanced by several features of the virus: the virus:

low infectious dose, low infectious dose, excretion of virus in the stool for several excretion of virus in the stool for several

weeks after recovery, weeks after recovery, and resistance to inactivation by and resistance to inactivation by

chlorination and to drying in the chlorination and to drying in the environment.environment.

It is thought to remain infectious for several It is thought to remain infectious for several days on environmental surfaces such as days on environmental surfaces such as door handles.door handles.

Pathogenesis & Pathogenesis & ImmunityImmunity

Infection is limited to the mucosal cells Infection is limited to the mucosal cells

of GIof GI

Watery diarrhea Watery diarrhea W/O bloodW/O blood

Mostly asymptomaticMostly asymptomatic

Short immunity (reinfection)Short immunity (reinfection)

Clinical FindingsClinical Findings

Sudden onset of Sudden onset of vomiting & diarrheavomiting & diarrhea

With With low-grade feverlow-grade fever & & abdominal abdominal

crampscramps

But But W/O bloodW/O blood

It lasts for several daysIt lasts for several days

Treatment & PreventionTreatment & Prevention

Diagnosed clinically and rarely by Diagnosed clinically and rarely by

PCRPCR

NoNo antiviral drugs or vaccine antiviral drugs or vaccine

But IV fluid may requiredBut IV fluid may required

Personal hygiene Personal hygiene

ApthovirusesApthoviruses

This is the group of viruses This is the group of viruses responsible for foot-and-mouth responsible for foot-and-mouth disease (FMD) – disease (FMD) –

a major economic pest worldwide, a major economic pest worldwide, especially in S.America and especially in S.America and Australasia. Australasia.

Controlled Controlled largely by vaccinationlargely by vaccination (inactivated vaccine - occasional (inactivated vaccine - occasional vaccine-linked outbreaks) vaccine-linked outbreaks)

or slaughter of infected animals. or slaughter of infected animals.

They are physically quite distinct They are physically quite distinct from other Picornaviruses: from other Picornaviruses:

Acid-labileAcid-labile - below pH 7.0. - below pH 7.0. AntigenicallyAntigenically - 7 serotypes, (A; C; - 7 serotypes, (A; C;

O; SAT1,2,3; Asia-1), O; SAT1,2,3; Asia-1), location of antigenic sites on capsid location of antigenic sites on capsid

quite different from Enteroviruses. quite different from Enteroviruses. GenomeGenome - Larger than other - Larger than other

Picornaviruses, ~ 8.5kb; Picornaviruses, ~ 8.5kb; 5' non-translated5' non-translated region contains region contains

poly-Cpoly-C tract of ~100-170nt - function tract of ~100-170nt - function not known (encapsidation?). not known (encapsidation?).

Foot-and-mouth disease virus OFoot-and-mouth disease virus O

Molecular surface of Foot and Mouth Disease Molecular surface of Foot and Mouth Disease Virus, radially depth cued, as solved by X-ray Virus, radially depth cued, as solved by X-ray

crystallographycrystallography

The virus is found in breath, saliva, faeces, The virus is found in breath, saliva, faeces, urine, milk and semen of infected animals urine, milk and semen of infected animals as well as meat and by-products in which as well as meat and by-products in which have remained above pH 6.0. have remained above pH 6.0.

Infection causes:Infection causes: Vesicles or blisters on the tongue, gums, Vesicles or blisters on the tongue, gums,

cheek, palate, lips, nostrils, muzzle, teats, cheek, palate, lips, nostrils, muzzle, teats, udder, snout and hooves. udder, snout and hooves.

Pyrexia, anorexia, shivering, reduction in Pyrexia, anorexia, shivering, reduction in milk production for 2-3 days, grinding of milk production for 2-3 days, grinding of the teeth, drooling, lameness, stamping or the teeth, drooling, lameness, stamping or kicking of the feet. kicking of the feet.

Death of young stock. Death of young stock.

Clinically indistinguishable from:Clinically indistinguishable from: Vesicular stomatitis (Rhabdovirus) Vesicular stomatitis (Rhabdovirus) Swine vesicular disease (Enterovirus - Swine vesicular disease (Enterovirus -

very similar to human coxsackievirus B5) very similar to human coxsackievirus B5) Vesicular exanthema of swine Vesicular exanthema of swine

(Calicivirus) (Calicivirus) Very similar to:Very similar to: Rinderpest (Paramyxovirus) Rinderpest (Paramyxovirus) Infectious bovine rhinotracheitis Infectious bovine rhinotracheitis

(Herpesvirus) (Herpesvirus) Bluetongue (Reovirus) Bluetongue (Reovirus) Bovine viral diarrhoea (Flavivirus) Bovine viral diarrhoea (Flavivirus)

Affects:Affects: Bovidae (cattle, zebus, domestic buffaloes, yaks), Bovidae (cattle, zebus, domestic buffaloes, yaks),

sheep, goats, swine, all wild ruminants and pigs. sheep, goats, swine, all wild ruminants and pigs. Camelidae (camels, dromedaries, llamas, vicunas) Camelidae (camels, dromedaries, llamas, vicunas)

have have lowlow susceptibility. susceptibility. Horses may be carriers but do not become sick. Horses may be carriers but do not become sick. Virus is Virus is highly infectioushighly infectious and can persist in and can persist in

contaminated fodder and the environment for up to contaminated fodder and the environment for up to 1 month1 month, depending on the temperature and pH., depending on the temperature and pH.

FMD is one of the most contagious animal diseases. FMD is one of the most contagious animal diseases. Transmission occurs by:Transmission occurs by:

Direct or indirect contact (droplets) Direct or indirect contact (droplets) Animate vectors (humans, etc) Animate vectors (humans, etc) Inanimate vectors (vehicles, implements) Inanimate vectors (vehicles, implements) Airborne, especially temperate zones (up to 60 km Airborne, especially temperate zones (up to 60 km

overland and 300 km by sea - e.g. 1967 UK outbreak overland and 300 km by sea - e.g. 1967 UK outbreak spread from France to Isle of Wight) spread from France to Isle of Wight)

Can humans catch foot and mouth?Can humans catch foot and mouth?Yes - just:Yes - just:Foot and mouth disease is a zoonosis, a Foot and mouth disease is a zoonosis, a disease transmissible to humans, but it disease transmissible to humans, but it crosses the species barrier with difficulty crosses the species barrier with difficulty and with little effect. and with little effect.

Given the high incidence of the disease in Given the high incidence of the disease in animals, both in the past and in more animals, both in the past and in more recent outbreaks worldwide, its occurrence recent outbreaks worldwide, its occurrence in man is in man is rarerare so experience of the human so experience of the human infection is limited. infection is limited.

The last human case reported in Britain The last human case reported in Britain occurred in 1966, during the last epidemic occurred in 1966, during the last epidemic of foot and mouth disease. of foot and mouth disease.

The circumstances in which it does occur in The circumstances in which it does occur in humans are not well defined, though all humans are not well defined, though all reported cases have had reported cases have had close contact with close contact with infected animals. infected animals.

There is one report from 1834 of three There is one report from 1834 of three veterinarians acquiring the disease from veterinarians acquiring the disease from deliberately drinking raw milk from deliberately drinking raw milk from infected cows.infected cows.

There is no report of infection from There is no report of infection from pasteurised milk, and the Food Standards pasteurised milk, and the Food Standards Agency considers that foot and mouth Agency considers that foot and mouth disease has no implications for the human disease has no implications for the human food chain. food chain.

The type of virus most often isolated in The type of virus most often isolated in humans is type O followed by type C and humans is type O followed by type C and rarely A. rarely A.

The incubation period in humans is 2-6 The incubation period in humans is 2-6 days. days.

Symptoms have mostly been mild and self Symptoms have mostly been mild and self limiting, mainly uncomfortable tingling blisters limiting, mainly uncomfortable tingling blisters on the hands but also fever, sore throat, and on the hands but also fever, sore throat, and blisters on the feet and in the mouth, including blisters on the feet and in the mouth, including the tongue.the tongue.

Patients have usually recovered a week after Patients have usually recovered a week after the last blister formation. the last blister formation.

Foot and mouth disease should Foot and mouth disease should not be confusednot be confused with the human disease hand, foot, and mouth with the human disease hand, foot, and mouth disease. This is an unrelated and usually mild disease. This is an unrelated and usually mild viral infection, principally of children, caused by viral infection, principally of children, caused by different viruses, principally coxsackie A virus. different viruses, principally coxsackie A virus. BMJ 322: 565-566, 2001.The UK was finally BMJ 322: 565-566, 2001.The UK was finally declared oficially free from FMDV again in declared oficially free from FMDV again in January 2002, 11 months after the beginning of January 2002, 11 months after the beginning of the outbreak.the outbreak.

CardiovirusesCardioviruses

One serotype. One serotype. Includes encephalomyocarditis virus (EMCV) Includes encephalomyocarditis virus (EMCV)

(model infection of mice), mengovirus, (model infection of mice), mengovirus, Maus-Elberfield virus, Maus-Elberfield virus, Columbia virus Columbia virus all considered to be strains of EMCV (really all considered to be strains of EMCV (really

a mouse virus, but can infect man, a mouse virus, but can infect man, elephants, squirrels...).elephants, squirrels...).

Genome size ~7.8kb; 5' non-translated Genome size ~7.8kb; 5' non-translated region contains poly-C tract of ~100-170nt region contains poly-C tract of ~100-170nt (like Apthoviruses). (like Apthoviruses).

Encephalomyocarditis virusEncephalomyocarditis virus Molecular surface of Mengovirus, radially depth Molecular surface of Mengovirus, radially depth

cued, as solved by X-ray crystallography cued, as solved by X-ray crystallography