Unusual eosinophilic granule cell proliferation in coho salmon (Oncorhynchus kisutch)

Whitish, opaque, coalescing masses were observed coating the visceral organs of numerous adult coho salmon when they were dissected for artificial spawning in the autumn of 1983 at the Puntledge River hatchery, British Columbia. One affected fish was observed in November 1991. Histological examination of 10 of these fish (nine in 1983 and one in 1991) revealed that the lesions consisted of unusual proliferations of eosinophilic granule cells (EGCs) with some characteristics of neoplasia. The proliferative lesions extended throughout the gastrointestinal tract and infiltrated through all layers of the gut. Pancreatic tissue and adipose tissues associated with the pyloric caeca were also infiltrated and replaced by the proliferating EGCs. A mild diffuse infiltration was also observed in the spleen, liver and kidney. Histochemical analysis and ultrastructural examination showed the proliferating cells to be essentially identical with the EGCs which are usually concentrated in the stratum granulosum of the intestine in normal salmonid fishes.