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