Evolution of Virulence Matthew H. Bonds The François-Xavier Bagnoud Center for Health and Human Rights Harvard School of Public Health Partners in Health.

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Presentation transcript:

Evolution of Virulence Matthew H. Bonds The François-Xavier Bagnoud Center for Health and Human Rights Harvard School of Public Health Partners in Health

Outline of Presentation 1.Background to Disease Evolution - Evolution of virulence - Antibiotic resistance - Disease emergence 2.Evolutionary Stable Strategies (ESS) 3.Evolution of Virulence - single infection - multiple infections

Background to Disease Evolution 1.Evolution of Virulence 2.Antibiotic Resistance 3.Disease Emergence

Background to Disease Evolution 1.Evolution of Virulence 2.Antibiotic Resistance 3.Disease Emergence

Background to Disease Evolution 1.Evolution: A change in genetic material in a population from generation to the next. - We say that organisms evolve to “maximize their fitness”

Background to Disease Evolution 1.Evolution: A change in genetic material in a population from generation to the next. - We say that organisms evolve to “maximize their fitness” 2. Fitness ≅ Reproductivity

Background to Disease Evolution 1.Evolution: A change in genetic material in a population from generation to the next. - We say that organisms evolve to “maximize their fitness” 2. Fitness ≅ Reproductivity = number of surviving offspring or number of reproductive offspring

Background to Disease Evolution 1.Evolution: A change in genetic material in a population from generation to the next. - We say that organisms evolve to “maximize their fitness” 2. Fitness ≅ Reproductivity = number of surviving offspring or number of reproductive offspring

Evolutionarily Stable Strategy Evolutionarily Stable Strategy (ESS): A strategy which, if adopted by a population, cannot be invaded by any alternative strategy that is initially rare. An ESS is a kind of Nash equilibrium.

What is the ESS for a Pathogen?

A strategy adopted by some kind of pathogen, for which an alternative (mutant) strategy, cannot invade.

What is the ESS for a Pathogen? A strategy adopted by some kind of pathogen, for which an alternative (mutant) strategy, cannot invade.

What is the ESS for a Pathogen? A strategy adopted by some kind of pathogen, for which an alternative (mutant) strategy, cannot invade.

What is the ESS for a Pathogen? A strategy adopted by some kind of pathogen, for which an alternative (mutant) strategy, cannot invade.

What is the ESS for a Pathogen? A strategy adopted by some kind of pathogen, for which an alternative (mutant) strategy, cannot invade.

What is the ESS for a Pathogen? A strategy adopted by some kind of pathogen, for which an alternative (mutant) strategy, cannot invade. A strategy that maximizes the Basic Reproductive Ratio is evolutionarily stable (Anderson and May, 1982)

ESS for a Pathogen? What are the evolutionary tradeoffs faced by pathogens?

ESS for a Pathogen? What are the evolutionary tradeoffs faced by pathogens? Pathogens should evolve to maximize the transmission rate and minimize the disease-induced death rate.

ESS for a Pathogen? What are the evolutionary tradeoffs faced by pathogens? Pathogens should evolve to maximize the transmission rate and minimize the disease-induced death rate. There must be a tradeoff between transmission and virulence! ?

ESS for a Pathogen?

v Tradeoff between transmission and killing the host

ESS for a Pathogen? v Maximize Ro with respect to v v*

ESS for Multiple Pathogens? v Maximize Ro with respect to v v*

ESS for Multiple Pathogens?

Maximize Ro with respect to v

ESS for Multiple Pathogens?

CoESS

Summary The evolutionarily stable strategy for a pathogen is the strategy that maximizes its basic reproductive ratio Typically, the phenotype that we consider to be evolving is the disease-induced mortality rate (virulence) There may be a tradeoff between virulence and transmission The ESS level of virulence depends on coinfection. The host represents a common property resource, and the Co-evolutionarily stable strategy is the outcome of a prisoner’s dilemma.