Associations between epidermal thionin-positive cells and skin parasitic infections in brown trout Salmo trutta

The dynamics of the densities of epidermal thionin-positive cells (putative mast cells) in the skin of brown trout fry were investigated during experi mental infections with the skin parasites Ichthyophthirius multifiliis (Cil iophora) and Gyrodactylus derjavini (Monogenea). It was shown that the meta chromatic thionin-stained cells were extremely sensitive to parasite exposu re, as the density of cells in the skin of trout decreased markedly after e xposure to the pathogens. As early as 7 d post infection the cell counts we re significantly reduced and almost totally depleted following 9 d infectio n, which suggests that degranulation of the cells occurs following parasite exposure. No recruitment of new cells was seen during the study period. So me reduction in uninfected control groups indicates that the putative mast cells are sensitive to stress as well. A notable variation in densities of thionin-stained cells between different fins was found and the corneal surf ace was devoid of these cells. The possible implications of these cells in host-parasite interactions are suggested and discussed.