Virology

I. Viral characteristics

Viruses are extremely small (20 - 400 nm)
- Cannot be easily cultured or seen by microscope
- cannot be easily removed by filtration
Viruses are non-living
- virus particles (virions) may survive in environment for long time
Viruses are obligate intracellular parasites
- infection of cells often causes disease
- Virus infections are hard to treat - few truly anti-viral drugs exist
- cell-mediated immunity is often important
Viruses can infect most any type of cell
- host range of virus: which animal can be infected
- tissue tropism: which cell in the body can be infected
Viruses can cause cancer
- oncoviruses
Viruses can become dormant (provirus)
- Latent viruses may cause chronic infections s.a. AIDS, hepatitis
- requires integration into host cell DNA (integrase) or stable viral DNA

II. Viral Structure

1. Genetic material

ss RNA Viruses: HIV, Influenza virus

ds RNA viruses: Orbivirus

ss DNA Viruses: Parvovirus

ds DNA viruses: Herpes viruses, T-4 Bacteriophage

Can contain as few as 2 (Circovirus) and as many as > 1000 (Mimivirus) genes

2. Capsid

Protein coat: Capsomeres/protomers

icosahedral: Adenovirus

helical: Influenza Virus

complex: Poxvirus

3. Envelope (Herpes Simplex Virus, Influenza Virus, HIV)

Lipid/carbohydrate membrane → host-derived

budding off; host cell often survives

Protein spikes → viral products → Enzyme function, attachment

Shape: Pleomorphic unless attached (Rabies virus)

Ether-inactivation

sensitive to acid, heat, drying, detergents

4. Other proteins: Reverse transcriptase, RNA polymerase

III. "Life" cycle of a virus

Adsorption → Receptor proteins determines host range & tissue tropism

Rabies: AchR
HIV: CD4, CCR5
COVID-19: ACE2

Penetration/Entry

Capsid rearrangement/nucleic acid injection
Envelope/membrane fusion
Receptor-mediated endocytosis

Viral NA replication (early genes?)
- RNA viruses use RNA polymerase or reverse transcriptase
Viral protein synthesis (late genes?)
- self-assembly
Virion release (may involve cell death)

Naked viruses
Enveloped viruses bud

IV. Viral infection and pathogenesis

Transmission to the host
1. most commonly respiratory
2. skin (cuts, sores)
3. arthropod-borne (arboviruses)
Infection of target cell and first site replication
1. may produce early non-specific symptoms (prodrome)
2. may result in bloodstream spread (viremia)
Amplification in target tissues
1. amplification may occur in macrophages, vascular endothelium or liver
  - leads to secondary viremia
2. ultimately, disease symptoms due to affected target tissue
  - CNS: meningitis, encephalitis, rabies, polio
  - Liver: Hepatitis
  - Skin: measles, rubella, chickenpox etc.
3. Outcome of cellular infection (many viral infections are sub-clinical!)
  - abortive (e.g. non-permissive cell)
  - lytic
  - chronic
  - latent
  - transforming
Host damage results from
1. viral cytopathic effect (inclusions, syncytia, lysis)
2. immunopathogenesis
  - cytokine release: interferon
  - inflammation (immune complexes, complement)
  - cell-mediated defenses
3. transformation (cancer)

Virology

I. Viral characteristics

1. Genetic material

2. Capsid

3. Envelope (Herpes Simplex Virus, Influenza Virus, HIV)

4. Other proteins: Reverse transcriptase, RNA polymerase

IV. Viral infection and pathogenesis