Gyrodactylus derjavini infections in four salmonids: Comparative host susceptibility and site selection of parasites

The susceptibility of 4 salmonids [rainbow trout Oncorhynchus mykiss, brown trout Salmo trutta, a Baltic (Iijoki) and an Atlantic (Conon) strain of Salmo salar] to infection with a Danish isolate of Gyrodactylus derjavini was tested in the laboratory (12 to 13 degrees C). Initial parasite attachment numbers did not differ between species, but parasite populations increased significantly faster and to a higher level in rainbow trout compared to the other salmonids, with S. trutta exhibiting an intermediate position. The 2 strains of S. salar became only slightly infected during the following weeks despite a heavy infection pressure. The parasites' selection of microhabitat on the hosts also differed between species and changed over time on the same host species. The tail fin was the preferred site in all salmonids although the pectoral, pelvic and anal fins were important sites in the initial period of the 6 wk investigation. It is notable that the corneal surface, especially in the Baltic salmon, became increasingly more heavily infected in the later stages of the infection period. Explanations for the differing degrees of susceptibility are suggested and some of these were investigated. Lysozyme activity in the fin epithelia and the density of mucous cells in the fins from the hosts were compared. Marked lysozyme activity was detected in tail fins from the 4 salmonids, but the activity was not associated with host resistance to infection. In contrast, the density of superficial mucous cells, which showed regional variation on the tail fins, was negatively correlated with susceptibility to infection. Thus, fish with high density of epithelial mucous cells showed the lowest susceptibility to infection, and this finding is discussed in relation to host resistance.