The most virulent parasite determines virulence in coinfection: a meta-analysis

Coinfections of hosts by multiple parasite species and strains are widespread in nature. Theory suggests that these infections have a key influence on the virulence, or harm caused, to hosts. However, it is still unclear whether multiple parasites, which may compete for resources and space, are indeed worse for hosts across the tree of life. To test this hypothesis, we conducted separate meta-analyses based on different expectations derived from virulence in single infections. We included 68 effect sizes from 19 experiments on non-human animal host species and 38 parasite species combinations. We found that coinfections are overall more virulent than the mean degree of harm caused by both parasites in retrospective single infections. That said, the coinfection virulence level is similar to that of the most virulent parasite, and less than the additive virulence of both single infections. These results suggest that the most virulent parasite is the primary driver of virulence in coinfection. This finding has implications for parasite spread in nature and suggests we focus on controlling the more harmful parasites in the first instance, when trying to limit the damage caused by coinfection.