Cestode infection facilitates co-infection by other parasites in a metapopulation of threespine stickleback

Parasitic infections are a global occurrence and impact the health of many species. Co-infections, where two or more species of parasite are present in a host, are a common phenomenon across species. Co-infecting parasites can interact directly or indirectly via their manipulation of (and susceptibility to) the immune system of their shared host. Helminths, such as the cestode Schistocephalus solidus, are well known to suppress immunity of their host (threespine stickleback), 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-zero S. solidus prevalence, we show there is an overall tendency towards facilitation: individuals with S. solidus infections have 28% higher diversity of other parasites, compared to S. solidus-uninfected individuals from the corresponding lakes. This facilitation effect, however, is stronger in lakes where S. solidus is particularly successful but tends towards inhibition in lakes with sparse and smaller cestodes (indicative of stronger host immune response). These results illustrate how even a single parasite species can vary geographically in their capacity for facilitation or inhibition of co-infections.