Chehalis River Disease: A Unique Gill Disease of Salmonids

Epidemic mortalities of chinook salmon alevins (Oncorhynchus tshawytscha) have occurred annually at a freshwater hatchery at the Chehalis River, British Columbia, Canada, since 1982. Previous work has identified branchial epithelial hyperplasia as the predominant pathological change. Our initial work lead us to test the hypothesis that Chehalis River 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 with particulate iron prior to and during clinical CRD. In one instance, extracellular viruslike particles were observed. Large numbers of a mixed bacterial population were recovered from diseased gills. Flavobacterium sp. and Pseudomonas fluorescens were considered the predominant isolates from the 1990–91 outbreak. However, indirect immunofluorescence of gills with anti-Flavobacterium branchophila serum failed to detect antigenically similar bacteria on the...