Scientist and Science Journalist

I have a broad interest in evolutionary biology, but the question that keeps me busiest is why parasites make their hosts sick. Intuitively parasites should not disease or indeed kill their hosts, because they are dependent on them for their survival. Thus, there must be some evolutionary reason for parasites to be harmful.

Evolutionary biologists have come up with quite a few of such reasons, but the most popular view parasite virulence as a consequence of parasites trying to maximise their fitness. Thus, in order to spread, parasites need to grow in their current hosts to produce the life stages with which they can infect new hosts. At the same time, however, such within-host growth also causes damage to the hosts, as parasites destroy host tissues and use up resources. Moreover, parasite strains often have to share their hosts with other strains, which causes competition between them. As it is generally assumed that more aggressive parasites outcompete less aggressive parasites, such competition should select for parasites that cause more disease, so that aggressive parasites should have an evolutionary advantage.

Interestingly, although these ideas have been around for decades, there is hardly any experimental evidence to support these claims, so that a proper understanding of why parasites cause disease is still not with us. During my PhD with Andrew Read at the University of Edinburgh I studied within-host competition between malaria parasites, showing that aggressive parasites indeed outcompete less harmful parasites. I also showed that the extent to which competition occurs, depends on host genotype, the presence or absence of drugs, and the order in which competing strains infect their host.

Currently, I am working with Sonia Altizer to study the protozoan parasite Ophryocystis elektroscirrha in monarch butterflies. These butterflies occur in both migratory (eastern and western USA) and non-migratory populations (for example in South Florida and Hawaii), and Sonia’s work has shown that parasite prevalence differs between these populations. Using a combination of field work, laboratory experiments and molecular genetics we ultimately hope to explain how different ecological conditions and migration strategies have shaped the evolution of this parasite.