Muayad A Mahmud, Janette E Bradley, Andrew D C MacColl
Infectious parasites vary greatly in their severity (virulence), e.g. from the common cold to the Ebola virus, but the reasons why are poorly understood. To be successful, parasites have to spread to new victims (hosts), and to do so they use host resources to multiply. This is what makes hosts sick. If parasites multiply too quickly they may kill their host before they can spread. Theory suggests that this trade-off completely determines the virulence of different parasites, which thus lie along a continuum from deadly quick spreaders to slow spreaders that hardly affect their host’s health. This idea completely ignores the possibility that the environment in which infections take place could have important effects on parasites or their hosts that might affect virulence. We examined variation in the virulence of a tiny parasitic worm which infects the skin of a small fish, the three-spined stickleback. Stickleback live in lakes, and environmental conditions, such as the acidity of the water, vary greatly among lakes. The worm, feeding on the skin of the fish, is constantly exposed to the surrounding environment. We found that, when isolated from fish, worms survive better in the water from their own than from other lakes. We also found that worms from different lakes vary greatly in their virulence, even when infecting the same fish in the same water. This suggests that the virulence of these parasites is partly genetically determined. We further found that virulence is closely, negatively, related to the acidity of the lakes from which the parasites originated, consistent with the idea that disease severity can evolve in response to the environment in which infection normally takes place. Thus we should pay attention to the environment in which farmed or aquacultured animals are raised, to minimise the virulence of infectious parasites.
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