The idea that predators “help” their prey populations by reducing diseases is pervasive, and has recently received some theoretical and empirical support.  However, there are many populations that suffer both high predation and high disease, suggesting that predators do not always keep the herds healthy.  Here, we ask whether trait-mediated indirect effects can explain this “unhealthy herds” phenomenon.  We conducted transmission assays using an ecologically important Chaoborus predator-Daphnia host-yeast parasite system to look for trait-mediated indirect effects of the predator on Daphnia susceptibility.  We also measured an epidemiologically important trait, host body size, to assess whether changes in this trait might drive changes in susceptibility.  

Results/Conclusions

We found that chemical cues (kairomones) released by Chaoborus predators significantly increased susceptibility of Daphnia to a virulent yeast parasite, Metschnikowia.  This increase is due to trait-mediated indirect effects, since the prey/host was not directly exposed to the predator.  Daphnia that were exposed to Chaoborus kairomones were significantly larger, and this increased body size can explain the increased susceptibility in the presence of predator kairomones.  Finally, parasites produced more infectious stages (“spores”) from some host genotypes in the predator kairomone treatments; if these clones dominated a population, this would further increase disease burdens in the presence of Chaoborus.  Overall, our data suggest that predators can have strong indirect effects on host-parasite interactions that have the potential to reverse the “healthy herds” phenomenon.