virology. basics of virology how are viruses transmitted from host to host? how does a virus...

Virology

Basics of Virology•How are viruses transmitted from host to

host?

•How does a virus▫Enter the body?▫Enter a host cell?▫Replicate?▫Exit the cell?

•How does the immune system work against a viral infection?

Viral entry

Viruses are very specific

•Species – few related organisms▫Rabies – only affects mammals▫Polio – primates

•Tissue▫Flu – lining of respiratory tract▫HIV – T-cells

What determines the host range?•Entry

•Replication

•Exit

How are viruses transmitted from host to host?

•Direct contact•Indirect contact•Air-born – droplets•Food-born•Water-born•Fecal-oral•Vector

Reservoirs

•Organisms where virus is abundant

•Probably don’t get sick from the virus

•Ex. Wild birds

Viral life cycle is important to understand•Virologists study:

▫Mode of entry▫Integration, replication & synthesis▫Method of exit

•Why??

•Interrupt one of these steps▫Prevention▫Treatment

How does a virus enter the body?

• Respiratory tract▫ Flu▫ Rhinovirus▫ Hantavirus

• GI Tract▫ Polio▫ Rotavirus

• Skin▫ Rabies▫ papillomavirus

• Genitals▫ Papillomavirus▫ HIV▫ Herpes

• Blood▫ Hepatitis▫ HIV

Many animal viruses are enveloped

How can the virus enter the cell?• Injection

• Endocytosis

• Fuse with membrane▫http://darwin.bio.uci.edu/~faculty/wagner/

hsvbinding.html

• Low pH dissolves envelope▫Uncoat genome▫RNA/DNA released

How do viruses replicate?

•Genome copies▫Method varies depending on type of viruses

•Translation makes viral proteins

•Produce new capsids

•Self-assembly

Retroviruses

•Single-stranded RNA viruses

•Contain reverse transcriptase▫Enzyme that synthesizes DNA from RNA

•http://www.whfreeman.com/kuby/content/anm/kb03an01.htm

•Ex. HIV

Viruses Exit Cell

•Budding ▫Doesn’t kill host cell

•Bursting ▫Kills cell

Stability of Viruses

Which types of viruses are most stable?•DNA DNA

▫Proofreading mechanisms▫Few errors▫1/1 billion bp

•DNA RNA▫Errors 1 million x greater

•RNA DNA▫Errors

Influenza A

•Has 8 segments of RNA

•Changes frequently

•New version of flu vaccine made yearly

Why does Influenza A change frequently?•Antigenic drift

▫Gradual change▫RNA has high mutation rate▫Minor changes in RNA = minor changes in

surface protein

•Antigenic Shift▫Major changes based on reassortment of

RNA segments

Reassortment

•Pigs may serve as a mixing vessel

•Co-infection of host by 2 viruses▫May mix some human segments & some

bird segments▫Ex. 1957 Flu had 3 avian segments, 5

human segments

Since mid-1700s, major shifts in Influenza A have occurred about every 30 years

Spanish Flu - 1918