Coinfections Dampen the Effects of Temperature on Host-Parasite Interactions

Coinfections by multiple parasite species are widespread and often modify disease outcomes. To what degree other forms of ecological complexity, such as environmental change, influence the impacts of coinfecting parasites on disease remains unclear. We exposed nematodes (Caenorhabditis elegans) to naturally coinfecting parasites (Leucobacter celer and Leucobacter musarum) at temperatures across the host's thermal range. While we found that host mortality varied widely with temperature during single infections, coinfections buffered hosts against temperature-mediated variation via ongoing parasite competition. We extended our experiments with mathematical models indicating that changing virulence, rather than transmission, most plausibly underpins temperature-dependent host mortality. At moderate, but not extreme temperatures, we found coinfection continued to dampen mortality outcomes due to another source of host variation, genotypic susceptibility. Coinfection may provide a stabilizing effect on disease in a warming world-but this effect can falter when hosts are faced with multiple stressors.