Species Interactions in a Parasite Community Drive Infection Risk in a Wildlife Population

Web of Parasite Interactions We live under constant assault from a variety of pathogens. Pathogen exposure will be more or less harmful depending on host factors, including immune status, and, as Telfer et al. (p. 243; see the Perspective by Lafferty) point out, the presence of co-infecting pathogens. In a time-series study of wild voles and four pathogens, co-infection had a larger effect on disease than any other factor. For example, infection with cowpox virus increased susceptibility and prolonged bacterial co-infections. Conversely, an ongoing infection with the bacterium Anaplasma reduced the rodents' susceptibility to the protozoan Babesia. In turn, chronic infection with Babesia limited susceptibility to the bacterium Bartonella. Coinfecting pathogens have specific interacting effects on host resources, immune responses, and other pathogens. Most hosts, including humans, are simultaneously or sequentially infected with several parasites. A key question is whether patterns of coinfection arise because infection by one parasite species affects susceptibility to others or because of inherent differences between hosts. We used time-series data from individual hosts in natural populations to analyze patterns of infection risk for a microparasite community, detecting large positive and negative effects of other infections. Patterns remain once variations in host susceptibility and exposure are accounted for. Indeed, effects are typically of greater magnitude, and explain more variation in infection risk, than the effects associated with host and environmental factors more commonly considered in disease studies. We highlight the danger of mistaken inference when considering parasite species in isolation rather than parasite communities.