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

<p><span><span><span><span><span><span><span><span><span><span><span>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 <i>Schistocephalus solidus</i>, 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 <i>S. solidus</i> prevalence, we show there is an overall tendency towards facilitation: individuals with <i>S. solidus</i> infections have 28% higher diversity of other parasites, compared to <i>S. solidus</i>-uninfected individuals from the corresponding lakes. This facilitation effect, however, is stronger in lakes where <i>S. solidus</i> 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.</span></span></span></span></span></span></span></span></span></span></span></p>