The fecal odor of sick hedgehogs (Erinaceus europaeus) mediates olfactory attraction of the tick Ixodes hexagonus.

Parasite loads of animals vary among individuals, but the underlying mechanisms have not been fully identified. Here, we investigated whether health status of hedgehogs (Erinaceus europaeus) is correlated with tick burden, and whether chemical cues linked to the health status of the host mediate attraction of the tick Ixodes hexagonus. An ecological survey conducted over 10 years, involving 226 wild hedgehogs, revealed a strong association between health status and tick burden of hedgehogs, with healthy animals being less likely to carry ticks than unhealthy ones. Behavioral choice tests demonstrated that ticks display a preference for the fecal odor from sick hedgehogs compared with healthy ones. Chemical analysis of fecal odors using gas chromatography-mass spectrometry showed differences in the odor profile between sick and healthy hedgehogs. Sick animals tended to exhibit raised levels of the volatile aromatic heterocyclic compound indole in their feces. Ticks were attracted to indole when given the choice between indole and a solvent control. However, fecal matter from healthy hosts, with the addition of indole, was not attractive to ticks, suggesting that indole interacts with other, undetected compounds in mediating attraction. This study implies that it is the attraction to fecal odor that causes higher tick burdens in sick hedgehogs. Ticks might benefit from this preference by avoiding possible repulsion mechanisms of healthy hosts. We suggest that ticks potentially choose their host based on odor linked to the host's health status.