Parasites: Beat Them or Join Them?

When a parasite, virus, or other pathogen attacks, animals typically fight back with their immune system. But a new study of rodents infected with malaria shows that animals have another option: They can evolve to live with their invaders. The findings, reported in the 2 November issue of Science, may help scientists understand the evolution and spread of infectious diseases, as well as allow them to create hardier livestock.

Plants have two strategies for dealing with parasites: They can resist them--developing a hardy defense like tough leaves--or they can tolerate them--minimizing the damage the invaders cause by, say, increasing photosynthesis to boost energy stores. Most do a little of both. Lars Råberg, an ecologist at the University of Edinburgh in the U.K., wondered if animals make similar choices.

Råberg and his colleagues infected five strains of laboratory mice with a parasite that causes malaria. They then monitored changes in the animals' health, as measured by anemia and weight loss, with relation to the amount of parasites in the blood.

Like plants, the mice employed resistance and tolerance strategies. As parasites multiplied in the hosts, some mouse strains stayed healthier than others, indicating that they had developed a way to tolerate the parasites. Other strains were able to keep their parasite levels low, indicating that they were actively resisting the infection.

Resistance didn't correlate with tolerance. Mice with stronger resistance to the parasite were less able to tolerate them, for example, losing more weight and becoming more anemic than more tolerant mice. This indicates that the genetics behind tolerance might be distinct from those driving resistance, says Råberg. He also notes that the findings may have important implications for the evolution of pathogens. If pathogens are tolerated rather than destroyed, they are not pushed to evolve and become even more deadly, he notes. At the same time, tolerance may not be ideal because if the body doesn't destroy the pathogens, they are more likely to spread.

These factors could influence animal breeding strategies, says plant evolutionary biologist Mark Rausher of Duke University in Durham, North Carolina. Breeding for tolerance in chickens and pigs, for example, may provide a better defense than breeding for resistance.

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