CHEHALIS RIVER DISEASE - A UNIQUE GILL DISEASE OF SALMONIDS

Epidemic mortalities of chinook salmon alevins (Oncorhynchus tshawytsc ha) have occurred annually at a freshwater hatchery at the Chehalis Ri ver, British Columbia, Canada, since 1982. Previous work has identifie d branchial epithelial hyperplasia as the predominant pathological cha nge. Our initial work lead us to test the hypothesis that Chehalis Riv er disease (CRD) was actually a novel form of bacterial gill disease ( BGD). Histological and ultrastructural observations indicated that CRD -affected gills harboured a bacterial biofilm apparently complexed wit h particulate iron prior to and during clinical CRD. In one instance, extracellular viruslike particles were observed. Large numbers of a mi xed bacterial population were recovered from diseased gills. Flavobact erium sp. and Pseudomonas fluorescens were considered the predominant isolates from the 1990-91 outbreak. However, indirect immunofluorescen ce of gills with anti-Flavobacterium branchophila serum failed to dete ct antigenically similar bacteria on the gills of CRD-affected fish. C hemotherapeutic treatment of both naturally and experimentally infecte d fish was successful, thereby confirming the infectious nature of the disease. We conclude that CRD is a form of BGD, but the significance of the branchial viruslike particles and the presence of particulate i ron has not yet been established.