BIO220H1 Lecture Notes - Lecture 23: Marginal Utility, Marginal Cost, Influenza A Virus

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27 Mar 2020
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March 24th 2020
BIO220 LEC0101
BIO220 Lecture 23: The Evolution of Virulence Part 2 and the Evolution of
Influenza
Conventional wisdom parasite should not kill their host
Enlightened theory due to trade-offs, parasite should evolve into an intermediate virulence
Other factors transmission strategy, and todays content
Predicting Optimal Virulence
- To get transmitted, parasite needs to get replicated within the host.
- More replication = more transmission
- Benefit of replication increase then plateaus
- The cost of virulence also increases as replication rate increase (mortality rate also
increases if it’s too virulent)
- Benefit cost is maximized for intermediate level virulent parasite.
- 1) at low replication rates, marginal gain > marginal cost thus will select for
increase virulence
- 2) at high replication rates, marginal gain < marginal cost thus will select for lower
virulence
- 3) at intermediate (optimal) replication rates, marginal gain = marginal cost select
intermediate virulence
*prediction assumes that the host is infected with only one parasite genotype. (could be
many parasites inside the host, but here we assume the host contain only one type of parasite
and is replicating asexually all genetically identical)
Some diseases can contain many strains
e.g. malaria infections in humans, three different strains
- could get multiple bites where mosquitoes are the vector
- Some people can get multiple bites from different strains of parasite accumulate different
parasite genotype
- results from Malawi suggest that inflections are not from a single parasite genotype what
does that do to virulence?
Do Multiple Infections Influence Virulence Evolution?
- For different strains of parasite, each would want to compete with different strains
inside the host
- The orange parasite would want to replicate faster within the host and have higher
transmission benefits, and the cost of virulence and the host dies sooner is spread
across to all the different parasite strains inside the host the marginal benefit
is >>> than the marginal cost
- Cost is spread among different strains, but the benefit is weighted more to the strain
that replicate the fastest
*’The tragedy of the Common benefit is specific to individual while the cost is
shared.
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