Opening a can of worms: a test of the coinfection facilitation hypothesis

Abstract

ABSTRACTParasitic infections are a global occurrence and impact the health of many species. Coinfections, where two or more species of parasite are present in a host, are a common phenomenon across species. Coinfecting parasites can interact directly, or indirectly via their manipulation of (and susceptibility to) the immune system of their shared host. Helminths, such as the cestodeSchistocephalus solidus, are well known to suppress immunity of their host (threespine stickleback,Gasterosteus aculeatus), potentially facilitating other parasite species. Yet, hosts can evolve a more robust immune response (as seen in some stickleback populations), potentially turning facilitation into inhibition. Using wild-caught stickleback from 21 populations with non-zeroS. solidusprevalence, we tested ana priorihypothesis thatS. solidusinfection facilitates infection by other parasites. Consistent with this hypothesis, individuals withS. solidusinfections have 18.6% higher richness of other parasites, compared toS. solidus-uninfected individuals from the same lakes. This facilitation-like trend is stronger in lakes whereS. solidusis particularly successful but is reversed in lakes with sparse and smaller cestodes (indicative of stronger host immunity). These results suggest that a geographic mosaic of host-parasite coevolution might lead to a mosaic of between-parasite facilitation/inhibition effects.