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 monogenean Gyrodactylus derjavini. Parasite populations increased on all naive hosts and peaked 4–5 weeks p.i. after which infection levels decreased. Introduction of naive fish into responding host populations resulted in heavy infections of the naive fish, whereas parasite expulsion continued in the responding host groups showing an acquired, non-sterile immunity. This non-sterile immunity lasted at least a month as these hosts were refractory to reinfection despite being exposed to a high infection pressure. Mucous cell hyperplasia was seen in some groups during the intermediary phase of infection, but at the termination of the study a significant depletion was evident. 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 passive immunity has been described.