Increased susceptibility of salmonids to the monogenean Gyrodactylus salaris following administration of hydrocortisone acetate

Gyrodactylus salaris infects numerous salmonid species, ranging from the fu lly susceptible (Norwegian strains of Salmo salar), through species which, though initially susceptible, eventually eliminate their infections (Salvel inus alpinus and S. fontinalis) to entirely resistant (Salmo trutta) specie s. Here we describe experiments in which Salvelinus alpinus, S. fontinalis and Salmo trutta, implanted with hydrocortisone acetate to simulate stress- induced immunosuppression, were challenged with G. salaris. With previously uninfected Salvelinus fontinalis, G. salaris infections on fish treated wi th hydrocortisone acetate grew larger, and for longer, than on sham-treated controls. A similar result was obtained with S. trutta. Patterns of infect ion on Arctic charr, Salvelinus alpinus, were more complex, because individ ual fish varied from susceptible to highly resistant. Fish were therefore i nitially infected with G. salaris, and the most highly resistant group of i ndividuals identified and disinfected. After 6 months recovery from this pr imary infection, hydrocortisone acetate was administered to half the fish, and all were challenged with G. salaris. Parasite populations on the hydroc ortisone-treated individuals were consistently larger than those on the sha m-treated controls, exceeding 30 parasites per fish after 5 weeks, in compa rison with less than 10 parasites per fish on controls. These results indic ate that hydrocortisone administration can lead to enhanced gyrodactylid po pulations on a range of salmonids. This suggests that the response to G. sa laris is mediated by the immune system, and that the spectrum of responses observed in different species are, at least in part, due to the same mechan ism. At a practical level, stress-induced immunosuppression during handling and transport of cultured salmonids may prove an important factor in the d issemination of G. salaris between watersheds.