Acquired resistance in rainbow trout against Gyrodactylus derjavini

Investigations were conducted on the host response in rainbow trout and the associated changes in mucous cell density during infection with the skin m onogenean Gyrodactylus derjavini. Parasite populations increased on all nai ve hosts and peaked 4-5 weeks p.i. after which infection levels decreased. Introduction of naive fish into responding host populations resulted in hea vy infections of the naive fish, whereas parasite expulsion continued in th e responding host groups showing an acquired, non-sterile immunity. This no n-sterile immunity lasted at least a month as these hosts were refractory t o reinfection despite being exposed to a high infection pressure. Mucous ce ll hyperplasia was seen in some groups during the intermediary phase of inf ection, but at the termination of the study a significant depletion was evi dent. Passive immunization of naive host (with sera from immune hosts) did not confer protection. This indicates differences between host responses to G. derjavini compared to responses against other pathogens where such a pa ssive immunity has been described.