BIO220H1 Lecture Notes - Lecture 8: Antigenic Drift, Flu Season, Antigenic Shift

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1 Feb 2019
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The Evolution of Virulence Part 2 and the Evolution of Influenza
More on virulence evolution
A graphical explanation
The impact of multiple infections
‘Accidental infections’ (or spillover events)
Evolution of influenza
Pandemic flu & antigenic shift
Seasonal flu & antigenic drift
Summary: Virulence evolution (so far)
Conventional wisdom
Parasites should not harm their hosts
Enlightened theory
Parasites evolve to intermediate virulence based on a
relationship (trade‐off) between virulence and transmission (optimal strategies)
Other complicating factors
Transmission strategy
More things well learn about today
Predicting optimal virulence
Virulence may be an unavoidable consequence of parasitetransmission
Parasites need to exploit their host in order to get
transmitted to new hosts
An increase in transmission (as a result of increasing
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replication) comes with the cost of an increase in virulence
(mortality associate with virulence)
Benefit – Cost is maximized at intermediate levels of virulence
At low replication rates, marginal gain > marginal cost
At high replication rates, marginal gain < marginal cost
At intermediate replication rates, marginal gain = marginal cost
The predictions we just made implicitly assume that hosts are infected with a single
parasite genotype.
But for some diseases, infection with multiple strains is common
e.g., malaria infections in humans
(individua affected by multiple strainsaccumulate many parasite genotypes)
Do multiple infections influence virulence evolution?
The Tragedy of the Commons’
Relative transmission is what matters
A host can be infected by several parasite strains (or species) because
of multiple infections or rapid within‐host evolution
If the strains compete for the same resource, there is a tragedy of the
commons
Strains that exploit the host more will gain a
bigger share of host resources, outcompeting other strains, and transmitting at a higher
rate
Even though overall transmission from the
host is lower, the more virulent strain has higher relative transmission
Multiple infections & the tragedy of the commons
Theory predicts that multiple infections select for increasing virulence. Any evidence
of this?
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Multiple infections select for higher virulence
More virulent
(Host is more sick, more anemic)
Less virulent
(Host is less sick, less anemic)
The more virulent strains do better in competition
(multiple infections)
Summary: Virulence evolution
Conventional wisdom
Parasites should not harm their hosts
Enlightened theory
Parasites evolve to intermediate virulence based on a
relationship (tradeoff) between virulence and transmission
Other complicating factors
Transmission strategy
Within‐host competition
Accidental infections
Three stages in the evolution of virulence
Stage 1: Accidental Infection (or spillover events)
Many pathogens can cause infections in novel hosts.
some may fail to cause secondary infections (rabies, West Nile)
some may cause a short chain of infections and then quickly die out (Ebola)
Accidental infections can be particularly virulent, but are not
necessarily so. Low virulence infections are probably underestimated
No evolutionary prediction about their virulence is possible.
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