Clinical and pathological features of common gill diseases of cultured salmonids in Ontario.

We reviewed the clinical presentations and histopathology of 118 diagnostic submissions of trout with infectious gill diseases from commercial trout farms within Ontario. Bacterial gill disease (BGD) (56%) and nodular gill disease (NGD) (26.2%) together accounted for 82.2% of these submissions. Submissions of fish with BGD occurred in every month of the year, but were proportionally more frequent in late winter and spring than in late summer and fall. Chemotherapy of groups of fish with gill diseases was common prior to submission, but the success rate of such treatment was low (29% for BGD; 21% for NGD). Specific therapeutic protocols implemented following etiological diagnosis of BGD were successful in 80% of the previously unresponsive cases and in 88.8% of previously untreated cases. The gills of trout collected within 48-96 h of treatment and apparent clinical recovery lacked bacteria and necrotic epithelial cells, but features such as lamellar fusion and epithelial hyperplasia were similar between recovered and affected fish.