Be afraid, be very afraid: how host cues determine attractiveness to parasite infectious stages and the resulting infection loads

The likelihood of infection is influenced by both innate and environmental factors, including host defences and contacts with infectious stages. Although theory predicts that motile parasites ought to select susceptible host species, few studies have considered parasite preference among individuals of a single host species. By experimentally manipulating the presence, activity, and susceptibility of tadpoles, we tested the importance of host cues (chemical and mechanical) and host resistance on intraspecific host choice by free-swimming trematode (flatworm) cercariae. Cercariae could 'choose' among four chambers with these combinations in a first set of trials but could not contact (and infect) hosts. In a second set of trials with the same tadpoles, cercariae were allowed to select and infect hosts, allowing us to analyze the relationship between initial parasite choice and subsequent infection establishment. Cercariae showed a trend for greater attraction to anesthetized tadpoles over negative controls (empty chambers), suggesting the use of chemical cues to locate hosts, but were most attracted to active (non-anesthetized) tadpoles, indicating an important role for host movement. Cercariae showed no preference for tadpoles subjected to an immunosuppressive treatment, despite their greater susceptibility to infection. Importantly, the initial number of cercariae that chose each tadpole in the first round positively predicted parasite load in the second round of exposures. Highly active hosts, which initially attracted the most cercariae, ultimately supported the highest infections, either because parasites made 'good' host choices, or, alternatively, prior host exposure (without actual infection) increased susceptibility.