Manipulation of fish host by eye flukes in relation to cataract formation and parasite infectivity

Trophically transmitted parasites may predispose infected hosts to predation by altering host behaviour, which can be either an adaptation of the parasites to enhance transmission to the next hosts in the life cycle or a nonadaptive side-effect of infection. In an experimental study, we investigated host manipulation by Diplostomum spathaceum (Trematoda), an eye fluke of fish, to evaluate its adaptive value as a parasite strategy to increase transmission efficiency to bird hosts. The parasite induces cataract formation in the lenses of fish eyes, and predisposes fish to predation by reducing their escape response. We examined the effect of developmental stage and the number of parasites on parasite-induced cataract formation and the susceptibility of rainbow trout, Oncorhynchus mykiss, to simulated predation (capture by dip-net). We found that the catchability of fish increased with the coverage of parasite-induced cataract. Furthermore, cataract formation was most intensive after eye flukes had completed their development, and host manipulation took place only when parasites were infective to birds and thus capable of being transmitted. Intensity of infection, however, did not affect vulnerability of fish to capture by dip-net. These findings suggest that the ability of the parasite to manipulate fish behaviour by impairing its vision may have resulted from selection preferring parasite genotypes with higher transmission efficiency.