Cryptobia salmositica: an in vitro and in vivo study on the mechanism of anaemia in infected rainbow trout, Salmo gairdneri Richardson

Abstract. There are two basic antigenic components in Cryptobia that are responsible for the anaemia in infected rainbow trout. A 'lytic component', which is dosage-dependent, causes lysis of red blood cells independent of antibody or complement. The second, an 'immune-complex-forming component', attaches to red blood cells, forms immune complexes with specific antibody and activates complement resulting in haemolysis. These two antigenic components, from both live and lysed Cryptobia , were present in the serum of infected fish. When sonicated antigen or heat-inactivated antiserum (from infected fish) was incubated with red cells from uninfected fish, a portion of the red cells was lysed and a positive Coombs' reaction was observed with the remaining intact red cells. The positive Coombs' reaction was due to immune complexes adsorbed onto the red cells and these lysed when incubated with complement. Antibody by itself did not adsorb onto the red cells. From the fourth week post-infection, a positive Coombs' reaction was observed in all infected fish and haemolysis occurred with complement. The authors suggest that, in infected fish, one or more components of the complement cascade is depleted continually during infection and that the anaemia is due to the lytic action of the antigen and immune complex formation on red cells. These lead to intra-vascular haemolysis as well as erythrophagocytosis. In general, the mechanism of anaemia in cryptobiosis appears similar to that in African trypanosomiasis.