Viruses

Egyptian stele(1580-1350 B.C)Crippled priest

Poxviruses(Poxviridae)

• The Pharaoh Ramses V died of smallpox (variola virus) in 1157 B.C.

• In 1520: 22 - 3.5 million Aztecs died

• Last case in Somalia on 26th October 1977

• Sept.11—variola virus as a bioterrorism weapon?

• Don’t confuse with chickenpox (varicella)

VIRUSES• Virus = Latin for poison• Isolation of the first Virus

– Dmitri Iwanowski in 1892–Tobacco mosaic virus which

affects >150 plants world wide stunted growth

–“filterable agent”

                        

      

VIRUSES, LIVING ORGANISMS?

• Life=A complex set of processes resulting from the actions of proteins specified by nucleic acids

• Viruses:–Inert outside host cells–Obligate intracellular parasites–Do not divide

VIRION• A complete, fully developed viral

particle• Virions are the transmissible state of a

virus. Metabolically inert• Virions must be able to adhere and

allow entry into some host cell(s)• Also to survive outside of host cell

environment. • Some virions more hardy than others

(hepatitis virus A can withstand short periods of boiling; most virions are destroyed by this)

VIRUS SIZE• Most, 20 to 300 nm

–(0.02 to 0.3 m)• Filoviruses up to 14,000 nm

–Exceptional

STRUCTURE OF VIRUSES• Only one type of nucleic acid

(NA)–DNA or RNA

• NA enclosed by a protein coat

Circular Linear

Segmented

Types of viral nucleic acids

VIRAL CAPSOMERS• Protein

subunits, building blocks of the viral capsid

STRUCTURE OF VIRUSES

• Capsid composed of repeating subunits - capsomers

• helical, icosahedral, complex • protection, attachment,

ENVELOPED VIRUSES• Envelope present

–Part of host’s cell membrane

Structure of Viruses

•Envelope derived from host membrane lipids and virus proteins •nuclear, plasma membrane by budding •necessary for attachment

Capsid

Envelope

Nucleicacid

NONENVELOPED OR NAKED VIRUSES

• Envelope absent• More resistant than enveloped viruses

• "Naked" viruses require host death so viruses can be released

• Enveloped viruses: are shed virus particles ; they shed by budding out, continued release from cell membrane

• Cell does not die (immediately), continues to serve as factory for virus assembly and release. Virus typically acquires a coating of host cell membrane, and will include virus-specific proteins. This is the "envelope"

Examples of enveloped viruses include:

•Retrovirus, I.e. HIV

•Paramyxovirus, I.e. influenza

•Rhabdovirus, I.e. rabies

•Enveloped Herpes virus

Nucleic acid

Capsid

Naked virus (no envelope)

TYPES OF VIRUSES• Animal viruses• Plant viruses• Bacteriophages (Virus that

infects Bacteria)–Ex: Coliphage infect E.coli cells

Types of Viral Infections

Oncogenic viruses

Herpes viruses

VIRAL METABOLISM• Multiply using the machinery

of the host cell• Have few or no enzymes

HOST RANGE• Species specific

–Small pox virus, humans• Broad host range

–Rabies virus, mammals

VIRAL SPIKES• Carbohydrate/protein complexes

embedded in the viral envelope• Used as means of identification

–Influenza virus, avian flue (H5N1)/Adenovirus

Adenovirus spikesRespiratory Illness, Common Cold,

“Pink Eye”, Gastrointestinal Illness

VIRAL MORPHOLOGY• Helical

–Cylindrical capsid with a helical structure

• Polyhedral–Icosahedral (20 triangular faces and 12 corners)

Helical nucleocapsid

RHABDOVIRUS

POLYHEDRAL

VIRAL MORPHOLOGY (cont.)

• Complex viruses–Structures attached to capsid

•Tail, tail fibers–Bacteriophages

Bacteriophage

Tail

POXVIRUS

Infections due to the poxviruses (members of the Poxviridae family) occur in humans and animals.

The orthopoxviruses include smallpox (variola), monkeypox, vaccinia, and cowpox viruses.

CLASSIFICATION OF ANIMAL VIRUSES

• Type of nucleic acid–DNA or RNA–Single stranded (ss) or double stranded (ds)

• Presence of envelope

CLASSIFICATION OF ANIMAL VIRUSES

• Strategy for Replication–Where do they replicate?

• Morphology–Structures–Described by common Names–Where is their niche?

(Enteroviruses)

Non-enveloped EnvelopeddsDNA Adenovirus

PapovavirusHerpesvirusPoxvirusHepadenavirus

ssDNA Parvovirus -dsRNA Reovirus RetrovirusssRNA Picornavirus Togavirus

FlavivirusCoronavirusRhabdovirus

Classification of viruses

CLASSIFICATION OF ANIMAL VIRUSES (cont.)

• ssDNA, nonenveloped–Parvovirus (Parvovirus B19 Fetal

Death, GI)• dsDNA, nonenveloped

–Adenovirus (Respiratory, GI)

–Papovavirus (Warts, Tumors)

CLASSIFICATION OF ANIMAL VIRUSES (cont.)

• dsDNA, enveloped–Poxvirus (Smallpox, Cowpox)

–Herpesvirus (Fever blisters, Chicken pox, Shingles, Mononucleosis)

–Hepadnavirus (Hepatitis B, Tumors)

HERPESVIRUS

CLASSIFICATION OF ANIMAL VIRUSES (cont.)

• ssRNA, nonenveloped–Picornavirus (Polio, Common Cold,

GI)• dsRNA nonenveloped

–Reovirus (GI, Respiratory)• dsRNA enveloped

–Retrovirus (Tumors, AIDS)

POLIOVIRUS

Polivirus structure

REOVIRUS

reo (for respiratory, enteric, and orphan, the latter meaning not associated with human disease)

Reoviruses are infectious agents of the virus family Reoviridae, transmitted by respiratory and fecal-oral routes. They are not major human pathogens

RETROVIRUSThey are enveloped viruses, with

an RNA genome. The name is derived from the fact that the

virus particle contains an RNA-dependent DNA Polymerase

(Reverse transcriptase)

This enzyme converts the RNA genome into DNA, which then

integrates into the host chromosomal DNA. The reverse

transcriptase is highly error prone and rapid genetic variation

is a feature of this group

CLASSIFICATION OF ANIMAL VIRUSES (cont.)

• ssRNA, enveloped–Togavirus (Encephalitis) –Flavivirus (Dengue Fever,

Yellow Fever, West Nile Virus)

–Coronavirus (Common Cold)

–Rhabdovirus (Rabies)

CORONAVIRUS

CLASSIFICATION OF ANIMAL VIRUSES (cont.)

• ssRNA, enveloped–Filovirus (Ebola, Marburg) –Arenavirus (Hemorrhagic Fever)

–Paramyxovirus (Mumps)

–Orthomyxovirus (Influenza)

–Bunyavirus (Hantavirus)

FILOVIRUS

Filovirus. Although very "hot" in the news, these viruses are very poorly characterized because of their extreme pathogenicity. They are class IV pathogens, meaning they can only be cultured in total containment facilities, of which there are only two in the U. S. They are thought to be enveloped viruses with - RNA genomes.

Marburg and Ebola virus

Appear in many different shapes (pleomorphic)

First ID in Germany/Yugoslavia from tissues of green monkeys who developed hemorrhagic fever

Filo = threadlike

PART II

GROWTH OF VIRUSES• Bacteriophages

–Bacterial cells• Animal viruses

–Animal cells–Animals and embryos

Growing Viruses•Animal Viruses

•A. Living Animals

•mice, rabbits, guinea pigs

•B. Chicken Embryos (Eggs)

•used to be most common method to grow viruses

•Still used to produce many vaccines (Flu Vaccine)

•C. Cell Cultures

•Most common method to grow viruses today

PLAQUE ASSAY

LIVING ANIMALS• Diagnostic• Vaccine research• Expensive• Regulated

NEWBORN MOUSE

EMBRYONATED EGGS• Diagnostic• Vaccine production• Inexpensive• Unregulated

CHIKEN EMBRYO

CHIKEN EMBRYO

CHICKEN EMBRYO

ANIMAL CELL CULTURES• Derived from animals, or

humans• Culture in special medium• Infected cells show

cytopathic effect (CPE)

Non-infectedmonkey cells

Infectedmonkey cells

Multinucleated (giant) cellsinfected with reovirus

INCLUSION BODIES POX VIRUS

ANIMAL CELL CULTURES• Primary cell lines

–Tissues• Diploid cell lines

–Human embryos• Continuous cell lines

– Cancerous cells

“...This lecture is dedicated to the memory of Mrs. Henrietta Lacks,

whose very cells,after her death,

have helped scientist all around the world to find

ways to alleviate suffering and prevent disease

among all of us, who are still here...”

VIRAL IDENTIFICATION METHODS

• Electron microscopy• Serological

–Specific antibodies • Molecular biology

–Nucleic acid sequences

VIRAL MULTIPLICATION• Viral genes

–Viral proteins–A few enzymes (if any) usually involved with viral nucleic acid replication

VIRAL MULTIPLICATION (cont.)

• Infected cell provides:–Enzymes and machinery for viral nucleic acid and protein synthesis

–Energy

BACTERIOPHAGE MULTIPLICATION

• Lytic cycle (lytic phages)–Lysis and death of host cell

• Lysogenic cycle (Lysogenic or temperate phages)–Host cell carries phage NA and divides normally

LYSOGENIC (TEMPERATE) PHAGES

• Upon infection phage inserts its DNA into the host chromosome (prophage)

• Prophage genes may alter host cell characteristics by transduction

TRANSDUCTION• Generalized

–Phage picks up, randomly, fragments of host DNA instead of phage’s DNA

–May transfer any gene

TRANSDUCTION (cont.)• Specialized

–Phage DNA that has been integrated to host DNA is exised along with a few adjacent host genes

LYSOGENIC PHAGES (cont.)• Toxins produced by:

–Corynebacterium diphteriae–Streptococcus pyogenes–Clostridium botulinum

VIRAL MULTIPLICATION• Attachment• Penetration• Biosynthesis• Maturation• Release

ONCOGENIC VIRUSES• Viral nucleic acid gets

integrated into host genome• Alteration of cellular growth

genes

LATENT VIRAL INFECTIONS

• Virus/host equilibrium–Herpes viruses

•Cold sores•Shingles (chicken pox)

SLOW VIRAL INFECTIONS• Virus builds up gradually• Usually fatal

–Subacute sclerosing panencephalitis

PRIONS• Protein• Undetectable nucleic acid?, a

bacterium?–Neurological diseases–Creutzfeldt-Jacob disease–Mad cow disease

HOST CELL DEFENSES

INTERFERON (IFN)• Antiviral proteins (AVP)

produced by virus-infected cells

• Interfere with viral multiplication

INTERFERON (cont.)• Host-cell specific• Not virus specific• Short lived• No effect on infected cells

INTERFERON TYPES• Alpha (IFN• Beta (IFN• Gamma (IFN)

INTERFERON ACTIVITY• IFNs diffuse and gain

entrance to non-infected neighboring cells

• Induce synthesis of antiviral proteins

INTERFERON ACTIVITY • Antiviral proteins:

–Destruction of viral mRNA–Inhibit translation of viral mRNA

–Inhibit polypeptide elongation

RABBITS AND MYXOMATOSIS

AUSTRALIA

• Introduced Rabbits First arrived in 1778 from Europe

• Spread 125 Km/year• Soon became a pest

MYXOMATOSIS• Caused by a poxvirus• Mild infection in Australian

native rabbits• Fatal infection in the

introduced European rabbits

TRANSMISSION OF MYXOMATOSIS

• Mosquito• Flea• Direct contact

MYXOMA VIRUS IN AUSTRALIA

• Introduced in the Summer 1950-1951

• By 1956 the European rabbit population dropped 90%

MYXOMA VIRUS IN AUSTRALIA (cont.)

• As time went on:• Rabbit population began to

recover• Appearance of less virulent

viruses• Appearance of more resistant

rabbits