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Virions, Prions, and Viroids:

Infectious Agents of Animalsand PlantsTopics

StructureClassification

Methods of study

Viral infections

Viral tumors

Human tumors

Viral host range

Prions

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StructureAnalogous to bacteriophages

Capsid (protein coat) made up of capsomeres Naked vs. enveloped viruses Attachment proteins or spikes in enveloped viruses

Distinct from bacteriophages

Segmented viruses (contain more than one RNAmolecule)

Plant viruses do not enter via receptor attachment

Shapes:

Isometric

Helical

Pleomorphic

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Classification

Taxonomy in constant fluxMost common taxonomic criteria for animal viruses:

1. Genome structure (DNA or RNA, ss vs. ds,segmented or not)

2. Virus particle structure (isometric, helical,pleomorphic)

3. Presence or absence of viral envelope

Viruses infecting vertebrates are divided into 14 RNAfamilies and 7 DNA families (Ending: -viridae)

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Coronavirus

Classification:

Coronaviridae (Family)

Coronavirus (Genus)

Common cold virus (Species) (together

with rhinoviruses)

Structure: non-seg., lin., ssRNA, helical, env.

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Herpesvirus

Classification:

Herpesviridae (Family)

Herpesvirus (Genus)

Herpes simplex type 1 / type 2 (Species)

Structure:

non-seg., lin., dsDNA, helical, env.

http://darwin.bio.uci.edu/~faculty/wagner/movieindex.htm

http://darwin.bio.uci.edu/~faculty/wagner/movieindex.htmlhttp://darwin.bio.uci.edu/~faculty/wagner/movieindex.html

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Cells Infected with a Herpes Virus

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Groupings Based on Routes ofTransmission

Not a taxonomic grouping more than one family may bincluded in one transmission grouping

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Methods of Study

Much more expensive and difficultto study animal viruses thanbacteriophages

Cultivation in host cells Living animal Embryonated chicken eggs

Cell or tissue culture (= invitro)

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Methods of Study cont: Quantitation

Plaque assay (useful for infective and lytic viruses)

Virion counting with EM

Quantal assay

(ID50 or LD50)

Hemagglutination(e.g.influenza virus)

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Acute Viral Infections

Productive infection of relative short duration

Naked viruses usually cause cell lysis, whileenveloped viruses do not

Symptoms due to localized or widespread tissue

damage

Host defense mechanisms gradually eliminate virus

Examples of acute infections

Time course

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Mumps, Measles, Influenza, andPoliomyelitis

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Time Course of Acute Viral Infection

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HEV

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Reproductive Cycle

1. Attachment

2. Entry

3. Uncoating

4. Replication of NA andprotein

5. Maturation of viral particles

6. Cell lysis

7. Spreading and shedding

8. Transmission to next host

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Transcription Strategy

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Mechanisms of Release

1. Cytopathic effect: Unlike virulentphages most animal viruses do not

encode for cell lytic enzymes.Instead degenerative changesassociated with the virus lead to

cell death.

2. Budding: from plasma membrane(most common) or from Golgi

apparatus. May or may not kill cell.Enveloped viruses

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Persistent Viral Infections

Virus continually present in body. Released by

budding.

May or may not cause disease

Carrier able to spread disease

Four categories (if more than one applies = complexinfections):

1. Late complications from acute infections

2. Latent infections

3. Chronic infections

4. Slow infections

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Late Complications from AcuteInfections

1. Subactue sclerosing panencephalitis (SSPE) -years following measles in 1 in 300,000. Viruspersistent in brain tissue with decreased

transcription of viral genome. Lack of envelopeprotein production prevents immune response toeliminate infected cells. High Ab, low CMI. Exactmechanism unknown.

2. Progressive (pan)encephalitis following rubella

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Latent Infections

Acute infection symptomless periodreactivation of disease

Symptoms of reactivation may differ fromoriginal disease

No measurable viral particles during

symptomless period

Examples: HSV-1 and HSV-2; varicella

C

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Chickenpox- Varicella

Blister-like rash on surfaceof skin and mucousmembranes. Blistersusually appear first ontrunk and face, thenspread to almost

everywhere else.

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Shingles orHerpes Zoster

About 20 % of those peoplewho have had chicken poxwill get zoster at some timeduring their lives. Most

people will get zoster onlyonce.

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Shingles comes frolatin cingulum, whichmeans girdle or belt.

It occurs in an area ofthe skin that is suppliedby the sensory fibers

a single nerve-dermatome. Rashappears as well-defineband on one side of

body, or on one side offace, arms or legs.

Ch i I f i

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Chronic Infections

Virus can be demonstrated at all times

Disease may or may not be present forextended time periods or show up late(carriers!)

Sl I f i

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Slow Infections

Gradual increase of infectious agent over long

time period often no apparent symptoms for longtime (=preclinical phase)

Usually slowly progressive lethal diseases

Examples AIDS

Lentivirus

prions

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HIV: Example of Complex Infection

Retrovirus ssRNA, envelope

RNA DNA (with the help of reversetranscriptase) permanent integration into host

genome (=provirus)

Polyprotein is cleaved into individual proteins withviral protease assembly of virions budding

Vi l T (N l )

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Viral Tumors (Neoplasms)

Benign

Malignant cancer, metastasizes

Proto-oncogenes and oncogenes are regulatorygenes

Properties of normal and transformed cells Only about 15% of human tumors are due to viruses

Examples of human tumors:

Kaposis sarcoma (herpes virus)

Squamous cell carcinomas (HPV)

Hepatocellular carcinoma (HBV and HCV)

K i S

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Kaposis Sarcoma

Purplish lesions of a

skin cancer not usuallyseen in young men

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Viral Host Range

Mostly species and even cell type specific

Exception:Zoonotic viruses are transmissiblefrom animals (arthropods, vertebrates) to man

(zoonosis) Arboviruses (West Nile virus), rabies etc.

Modification of host range due to

Phenotypic mixing Genetic reassortment

G ti R t t

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Genetic Reassortent

In segmented viruses

Simultaneous infection of onecell with 2 different types ofviruses leads to exchange of

genetic information

Creation of major new influenza

strains resulting in pandemics

Antigenic shift vs. antigenic drift

Oth I f ti A t P i

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Other Infectious Agents: Prions

Small proteinaceous infectious particles (resistinactivation by procedures that modify nucleic acids)

Prion diseases are often called spongiformencephalopathies because of the post mortemappearance of the brain with large vacuoles in the

cortex and cerebellum

Human prion diseases CJD: Creutzfeld-Jacob Disease

BSE: Mad cow disease (BSE) GSS: Gerstmann-Straussler-Scheinker syndrome

Kuru