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Lecture 8

BIO220 Lecture 8.docx

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Department
Biology
Course Code
BIO220H1
Professor
Doug Thomson

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BIO220 Lecture 8 Deadly Viruses & The Evolution of Virulence
1918 Influenza Epidemic
- The 1918 flu was a leading cause of death among all disease and infectious disease
Ebola virus
- First saw ebola in 1976 outbreak and then suddenly disappeared
Why is the common cold so benign when the flu and ebola are so deadly?
- Conventional wisdom
o Host has infection and contains replicating pathogens that replicate so they can be
transmitted to another host
o The highly virulent pathogens may be new to the species of host over time a
mutualistic association develops
o Ex: ebola originates in bat and is a highly virulent pathogen because it keeps
transferring, for example from bats to monkeys to humans
SARS
- Some of the sars virus genes suggest that it is a coronavirus that crossed the species and has not
coevolved
Myxoma virus
- Originated when Europeans brought rabbits to Australia and their population exploded
o In order to control rabbit population, they created a virus that targets rabbits.
o Evolution occurred in population to decrease to level of virulence
o Virulence also declines over time because virus does not want to kill the host
o HOWEVER, a mutual association did not occur between rabbits and virus with evolution,
as is expected. What happened?
- Parasite adaptation hypothesis
o High replication rates high virulence maximize transmissionhigh fitness
o Model : T(v) x L(v), where:
T(v) number of new infections per day
L(x) number of days infection lasts
o Virulence is doing 2 things
Increase transmission
Decrease longevity of transmission
Trade off
o Optimizing virulence
Most virulent strains increase transmission rate but kills host quickly
Least virulent strain tend to be cleared by the host quickly
- Ewald’s Theory (verbal optimization)
o Trait to be optimized
o Fitness
o Constraint
- Model of disease transmission
o Direct host to host transmission
o Host must be mobile and functioning relatively well in order to transmit disease
- Prediction
o Low replication rate and therefore low virulence
- There are very few high virulent diseases that are directly transferred
- What can we do to cause the evolution of lower virulence
o Reduce transmission by eliminating vectors

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Description
BIO220 Lecture 8 – Deadly Viruses & The Evolution of Virulence 1918 Influenza Epidemic - The 1918 flu was a leading cause of death among all disease and infectious disease Ebola virus - First saw ebola in 1976 – outbreak and then suddenly disappeared Why is the common cold so benign when the flu and ebola are so deadly? - Conventional wisdom o Host has infection and contains replicating pathogens that replicate so they can be transmitted to another host o The highly virulent pathogens may be new to the species of host –over time a mutualistic association develops o Ex: ebola originates in bat and is a highly virulent pathogen because it keeps transferring, for example from bats to monkeys to humans SARS – - Some of the sars virus genes suggest that it is a coronavirus that crossed the species and has not coevolved Myxoma virus - Originated when Europeans brought rabbits to Australia and their population exploded o In order to control rabbit population, they created a virus that targets rabbits. o Evolution occurred in population to decrease to level of virulence o Virulence also declines over time because virus does not want to kill the host o HOWEVER, a mutual association did not occur between rabbits and virus w
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