Antihelmintic treatment alters the parasite community in a wild mouse host

Individuals are often co-infected with several parasite species, yet the consequences of drug treatment on the dynamics of parasite communities in wild populations have rarely been measured. Here we experimentally reduce nematode infection in a wild mouse population and measure the effects on other non-target parasites. A single oral dose of the antihelmintic, Ivermectin, significantly reduces nematode infection. However, we find a reciprocal increase in other gastrointestinal parasites, specifically coccidial protozoans. Recapture rates were also lower in Ivermectin-treated individuals, suggesting the unexpected increase in these other parasites can be harmful to the host. These results highlight that drug therapy may have unintended consequences for non-target parasites and that host - parasite dynamics cannot always be fully understood in the framework of single host – parasite interactions.