Abdominal body fat was generally adequate but not excessive. We observed mucus clumps microscopically in gill clip specimens. Infectious agents such as ectoparasites were not evident in any of the wet mount preparations. Kidney swab specimens cultured aerobically yielded no growth. Affected rainbow trout with operculum removed.
Gills are pale and thickened, with gray clumps of mucus.
Frequently, the surfaces of the thickened gill areas were lined by a single layer of angular, dome-shaped cells that appeared somewhat distinct from the underlying epithelium. Small mats of Gram-negative filamentous bacteria were commonly observed in the nonhyperplastic gill regions, and these were infrequently associated with necrosis of individual gill epithelial cells. Sections of gill demonstrating the segmental nature of the epithelial hyperplasia. Arrows denote relatively unaffected gill areas. Arrowhead indicates site of filament fusion.