Evolution of Virulence: Malaria, a Case Study

Published on October 1, 2007   34 min

Other Talks in the Series: Evolution and Medicine

The problem of the evolution of virulence is essentially the question, why do infectious agents harm us? If their survival depends on our survival, why have they not evolved to be harmless? What I'm going to do is illustrate one way evolutionary biologists attempt to answer this question, and I'm going to do so using malaria as an example. I hope to persuade you that it is both intellectually interesting and important to know why malaria parasites-- and by analogy other parasites-- harm us. I have prepared these slides, and the opinions expressed are mine. However, my presentation draws heavily on work of members of my research group and other collaborators. In particular, my view of malaria evolution has been heavily shaped by long and very fruitful collaboration with Margaret Mackinnon.
Evolutionary biologists often try to understand how natural selection acts. Here I will ask how natural selection acts on malaria virulence. But before discussing the natural selection, I need to define virulence. The word is used to mean a variety of different things by different people. I use it to mean the harm done to us following infection. In other words, the things physicians worry about, morbidity and mortality. Other things, like a parasite's ability to infect or replicate or transmit, are related and very important, but they are not part of my definition of virulence. I'm going to ask how natural selection shapes the virulence of malaria parasites in two steps. First, I'm going to ask, why are they so virulent? What evolutionary advantages are there for parasites which harm their hosts? Why is malaria nasty? It turns out that that is relatively easy to answer. In fact, it's so easy to answer that it begs the next question, which is, why aren't malaria parasites more virulent? Why aren't they nastier? And I'm going to spend most of the lecture on those two questions. I then want to spend a few minutes discussing the implications of evolutionary analysis for medicine. I want to persuade you that we cannot ignore parasite evolution in public health planning. This is obvious in the context of drug resistance. I hope to persuade you that it is very likely to be important for virulence as well, and we are currently overlooking this, possibly to our peril. And then I will end by very briefly discussing two other diseases to illustrate a precautionary plea.