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

Many animal viruses are enveloped

How can the virus enter the cell? Injection Endocytosis Fuse with membrane http://darwin.bio.uci.edu/~faculty/wagner/hsvbindin g.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