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

Abstract

Gyrodactylus salaris infects numerous salmonid species, ranging from the fully susceptible (Norwegian strains of Salmo salar), through species which, though initially susceptible, eventually eliminate their infections (Salvelinus alpinus and S. fontinalis) to entirely resistant (Salmo trutta) species. 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 with hydrocortisone acetate grew larger, and for longer, than on sham-treated controls. A similar result was obtained with S. trutta. Patterns of infection on Arctic charr, Salvelinus alpinus, were more complex, because individual fish varied from susceptible to highly resistant. Fish were therefore initially infected with G. salaris, and the most highly resistant group of individuals identified and disinfected. After 6 months recovery from this primary infection, hydrocortisone acetate was administered to half the fish, and all were challenged with G. salaris. Parasite populations on the hydrocortisone-treated individuals were consistently larger than those on the sham-treated controls, exceeding 30 parasites per fish after 5 weeks, in comparison with less than 10 parasites per fish on controls. These results indicate that hydrocortisone administration can lead to enhanced gyrodactylid populations on a range of salmonids. This suggests that the response to G. salaris 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 mechanism. At a practical level, stress-induced immunosuppression during handling and transport of cultured salmonids may prove an important factor in the dissemination of G. salaris between watersheds.

Effect of Tetramicra brevifilum (Microspora) infection on respiratory-burst responses of turbot (Scophthalmus maximus L.) phagocytes

“…Fourth, serum cytokines such as soluble macrophage-activating factor (MAF), although alone enhances O 2 production by fish macrophages (Graham & Secombes, 1988), in the presence of LPS may inhibit O 2 production, as reported for gilthead seabream macrophages (Mulero & Meseguer, 1998); similarly, interleukin-1 has immunoregulatory e#ects (Secombes et al, 1999). Finally, the observed e#ects may be due to other soluble factors such as eicosanoids (prostaglandins, prostacyclin and thromboxanes) that downregulate macrophage respiratory burst activity (Novoa et al, 1996;Tafalla et al, 1999), or hormones such as melanin-concentrating hormone (MCH) (Harris et al, 1998) or cortisol, which is capable of inhibiting the inflammatory response (Pickering & Pottinger, 1985;Narnaware et al, 1994;Espelid et al, 1996;Harris et al, 2000).…”

Comparative susceptibility of two races of Salmo salar (Baltic Lule river and Atlantic Conon river strains) to infection with Gyrodactylus salaris

“…Furthermore, host macrophages colonize and kill gyrodactylids in vitro (Buchmann & Bresciani 1999) and dexamethasone significantly down-regulates macrophage activity (Pagniello et al 2002). In addition, other immunosuppressants, such as hydrocortisone (Harris et al 2000), have been shown to inhibit the host resistance towards infection with specific gyrodactylids, which further suggests the role of immune factors in host specificity. However, the effects of corticosteroids on mammals are quite complex (Wilckens & De Rijk 1997) and it cannot be excluded that other mechanisms in fish might be affected by the steroid treatment.…”

Methodological issues affecting the study of fish parasites. I. Duration of live fish storage prior to dissection

“…In addition to such factors as stress-induced immunosuppression, which affects the transmission and dissemination of monogeneans (Harris et al 2000), or high fish density in the holding tanks, water temperature is also likely to have played an important role in the rapid reproduction of gyrodactylids in the cold season and dactylogyrids in the warm season. Throughout the experiment, all fish were held in outdoor tanks with natural levels of light and temperature.…”