Sequential pathology of the gills of Coho salmon with a combined diatom and microsporidian gill infection.

A large group of 40 gram Coho salmon smolts experienced 60% mortality within the first week after introduction to cages in sea-water. Histological and ultrastructural examination of sequential samples of gill tissue revealed a dramatic suppurative branchitis accompanied by extensive fusion of gill lamellae. This dramatic host response and subsequent high mortality appeared to be in response to a bloom of Corethron-like diatoms. A further complication was an extensive infection of endothelial cells of the gill vasculature by a microsporidian protozoan.