In vivo and in vitro cell‐mediated immune responses of rainbow trout, Oncorhynchus mykiss (Walbaum), against Cryptobia salmositica Katz, 1951 (Sarcomastigophora: Kinetoplastida)

. Delayed‐type hypersensitivity (DTH) reaction (an in vivo manifestation of cell‐mediated immunity) was detected in Oncorhynchus mykiss maintained on a pantothenic‐acid‐supplemented diet 2 weeks after infection with Cryptobia salmositica. The reaction was similar to that in mammals with mononuclear cell infiltration into the dermis and muscle layers and the presence of oedema. DTH reaction was also displayed by fish on a pantothcnic‐acid‐supplemented diet that had recovered from the infection and were protected against further infection. The reaction was less marked in infected or protected fish on a pantothenie‐acid‐deficient diet. Inhibition of macrophage migration (an in vitro expression of cell‐mediated immunity) was observed when head kidney cell suspensions from protected fish maintained on either pantothenic acid supplemented or deficient diets were incubated with Cryptobia antigen. No inhibition of migration was evident when head kidney cell suspensions from the above fish were incubated without antigen, nor was it evident when cells from uninfected fish were used. The occurrence of a typical DTH reaction in rainbow trout and the feasibility of assessing it by measuring the thickness of the induration provides a simple and practical method for assessing cell‐mediated immunity in large scale vaccination programmes against pathogens.