Short communication Labile resistance of Atlantic salmon, Salmo salar L., to infections with Gyrodactylus derjavini Mikailov, 1975: implications for host specificity S H Olafsdottir, H P Ø Lassen and K Buchmann Department of Veterinary Microbiology, Section of Fish Diseases, Royal Veterinary and Agricultural University, Frederiksberg C, Denmark Keywords: corticosteroid, dexamethasone, Gyrod- actylus derjavini, host resistance, host specificity, Oncorhynchus mykiss, rainbow trout, salmon, Salmo salar, susceptibility. A number of investigations have shown a clear host specificity of gyrodactylids parasitizing salmonid hosts. Gyrodactylus derjavini Mikailov, 1975 suc- cesfully infect brown trout, Salmo trutta L., but cannot reproduce satisfactorily on Atlantic salmon, S. salar L. (Buchmann & Uldal 1997; Bakke, Soleng & Harris 1999). In contrast, the congener G. salaris Malmberg, 1957 preferentially infects the latter host, whereas brown trout does not support significant population growth of this parasite (Bakke, Jansen & Hansen 1990; Bakke et al. 1999). The basic mechanisms behind this strict host specificity or differential resistance have remained enigmatic. However, some studies have shown that treatments of hosts with corticosteroids, such as hydrocortisone and dexamethasone, modu- late this association significantly. Thus, brown trout injected with the former steroid become susceptible to G. salaris (Harris, Soleng & Bakke 2000) and salmon from the Scottish (Conon) strain of Atlantic salmon treated with the latter hormone support population increases of G. derjavini (Nielsen & Buchmann, in press). It has been shown that the resistance of salmon to G. salaris differs according to their origin. Thus, Neva salmon from the Baltic are less susceptible to this particular parasite compared with Norwegian salmon (Bakke et al. 1990) and Baltic Iijoki salmon are relatively resistant to G. derjavini (Buchmann & Uldal 1997). Whether the resistance of Baltic salmon towards the latter gyrodactylid is sensitive to steroid treatment of the host is an important question. Thus, the present work elucidates the influence of injection with the corticosteroid dexamethasone on the host susceptibility of both Atlantic and Baltic strains of salmon to infection with G. derjavini. Two strains of Atlantic salmon were used for the experiments. Eyed eggs from the River Lule in Sweden and from the River Conon, Scotland (delivered by the Danish Wild Salmon Centre, Randers, Denmark) were brought to the hatching facility in spring 2001. The Conon salmon stock was previously found susceptible to infections with G. salaris (Bakke & MacKenzie 1993) and relatively resistant to G. derjavini (Buchmann & Uldal 1997). The status of the Baltic Lule salmon in this regard has not been investigated previously. Both egg batches were hatched and reared at the salmon hatchery of Bornholm (Nexø, Denmark) in patho- gen-free conditions. Approximately 8 months post- hatching the fish were brought to the laboratory for at least 4 weeks acclimatization before experimen- tation. The salmon used had a mean body weight of 2.3 g (SD 0.6) and a mean body length of 7.0 cm (SD 0.6) (n ¼ 70). As a control for parasite Journal of Fish Diseases 2003, 26, 51–54 Correspondence K Buchmann, Department of Veterinary Microbiology, Section of Fish Diseases, Royal Veterinary and Agricultural University, Stigbøjlen 4, DK-1870 Frederiksberg C, Denmark (e-mail: kub@kvl.dk) 51 Ó 2003 Blackwell Publishing Ltd