Population increase of Gyrodactylus derjavini on rainbow trout induced by testosterone treatment of the host

Field observations indicating a possible effect of testosterone on the susceptibility of teleosts to gyrodactylid infections were tested experimentally. Female rainbow trout Oncorhynchus mykiss (O+) were injected with testosterone (5 or 10 pg) or vehicle alone (placebo) and subjected to infection with Gyrodactylus derjavini Mikailov, 1975 through cohabitation with infected fish. The number of parasites on testosterone-treated fish increased significantly compared to parasites on untreated fish. The site selection of parasites differed shghtly between experimental groups. Infected trout used for cohabitation expenenced a marked decrease in infection, indicating acquired resistance. This was further supported by the different site selection of parasites on these fishes. The superficial mucous cell density on the tail fins of testosterone-treated fish was not significantly lower than in the placebo trout, whereas previously Infected fish with a low parasite burden showed a significantly higher mucous cell density. A poss~ble immunosuppressive effect of testosterone is discussed, but although testosterone is likely to act on the trout immune system, the possibhty that the male sex hormone affects the parasites' reproduction directly cannot be excluded.