Parasite resistance and avoidance behaviour in preventing eye fluke infections in fish.

This paper examines the efficiency of acquired resistance in protecting the fish host, rainbow trout (Oncorhynchus mykiss), against the trematode parasite Diplostomum spathaceum, and the hypothesis that fish recognize areas where infective stages are aggregated and show avoidance behaviour. We found that when fish with a low level of infection were held in restricted cages in natural conditions they became infected and developed cataracts as a result of this infection. This suggests that acquired resistance is insufficient in protecting fish against the parasite or the deleterious effects of infection in conditions where fish could not avoid the parasite. Behavioural experiments in the laboratory showed that fish reacted to the parasite cercariae by avoiding the infection source, which decreased the rate of parasite establishment. We conclude that by using a combination of behavioural avoidance and physiological resistance, fish could defend against the parasite more effectively.