Priming of host resistance to protect cultured rainbow trout Oncorhynchus mykiss against eye flukes and parasite-induced cataracts.

In the present study, immunologically naive rainbow trout Oncorhynchus mykiss were experimentally exposed to a low-level Diplostomum spathaceum (Trematoda) infection to stimulate acquired resistance and, along with unexposed controls, were subsequently exposed to natural infection for 8 weeks. The priming of the host resistance, designed to simulate a procedure applicable in aquaculture, decreased the number of establishing parasites compared to untreated controls by the end of the experiment. This effect was slow and did not protect the fish against the parasite-induced cataracts. The results suggest that this type of priming of host resistance is probably inefficient in preventing the deleterious effects of D. spathaceum infection in aquaculture conditions.