Unhealthy herds: Indirect effects of predators enhance two drivers of disease spread

1.Predators could reduce disease prevalence in prey populations by culling infected hosts and reducing host density. However, recently observed positive correlations between predator density and disease burdens in prey/hosts suggest that predators do not always 'keep the herds healthy'. Several possible mechanisms could explain this 'unhealthy herds' effect, including a predator-induced change in prey/host traits which enhances susceptibility or alters other epidemiologically important traits. 2.Here, we use an invertebrate predator, zooplankton host, yeast parasite system to demonstrate such trait-mediated indirect effects. We exposed ten genotypes of the prey/host Daphnia dentifera to infochemicals ('kairomones') produced by the invertebrate predator Chaoborus and to a yeast parasite. 3.We found that kairomone exposure induced larger and more susceptible D. dentifera. Clones that showed substantial increases in body length also yielded more spores upon death. However, exposure to kairomones did not alter reproduction from uninfected hosts. All of these results were captured with a dynamic energy budget model of parasitism. 4.Overall, our empirical and theoretical results show that predators can have strong indirect effects on host-parasite interactions that could produce positive correlations between predation intensity and disease burden.