Dexamethasone treatment increases susceptibility of rainbow trout, Oncorhynchus mykiss (Walbaum), to infections with Gyrodactylus derjavini Mikailov

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Abstract
The parasite population dynamics of a Danish isolate of Gyrodactylus derjavini Mikailov 1975 on immunosuppressed fry of rainbow trout, Oncorhynchus mykiss (Walbaum), were investigated and compared with immunocompetent fry. Immunosuppression was induced 14 days prior to infection by intraperitoneal injections of dexamethasone (fluoro‐methyl‐prednisolone). Treated and untreated fish were infected by cohabitation with Gyrodactylus‐infected donorfish, carrying 4‐week‐old infections. Initial infections did not differ between the two groups. However, parasite populations increased significantly faster and reached significantly higher levels in the dexamethasone‐treated group from day 13 post‐exposure and during the following 3 weeks when compared with the untreated group. Microhabitat distribution of parasites on the two host groups differed from day 20 post‐exposure, which could indicate a different host response to infection. Changes in the density of superficial mucous cells in the caudal fins were investigated during the course of infection. No differences in mucous cell density between untreated and dexamethasone‐treated groups were found.

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