Atypical bacterial gill disease: a new form of bacterial gill disease affecting intensively reared salmonids

An unusual form of bacterial gill disease (BGD) was identified which affect ed five species of cultured salmonids from Canada (i.e, rainbow trout, chi- nook salmon and Atlantic salmon), Norway (i.e. brown trout) and Chile (i.e. coho salmon). All outbreaks occurred at low water temperatures (< 10 degre es C) and with clinical presentations distinct from classical BGD, which is caused by Flavobacterium branchiophilum. In contrast to classical BGD, fis h did not show marked respiratory distress with flaring of the opercula, th e animals did not orientate at the surface of the water column near inflow water or at the margins of the tanks, and the feed response of the fish was varied. While mortality was increased, it was not precipitous as in classi cal BGD. Eight outbreaks were examined in greater detail using histopatholo gy, scanning electron microscopy, bacteriology and immunohistochemistry. La rge numbers of small bacterial rods were seen adhering to the lamellar epit helium of affected gills from all outbreaks. Histologically, the lamellar e pithelium appeared swollen, often with evidence of single cell degeneration and exfoliation. In more severe instances, the formation of lamellar synec hiae was seen, usually associated with sequestration of bacteria between fu sed lamellae. By contrast with typical BGD, overt epithelial hyperplasia, l amellar fusion and filamental clubbing were not common sequelae to infectio n; instead, the end result was shortened and somewhat: stubby lamellae cove red with swollen epithelial cells. The predominant bacterium recovered from affected gills was a small, Gram-negative, motile, fluorescent pigment-pro ducing rod that shared phenotypic characteristics with Pseudomonas fluoresc ens. Polyclonal antisera prepared against three representative isolates ind icated a weak antigenic similarity among them. Immunohistochemistry corrobo rated this finding, in that the antisera reacted strongly with gill section s containing the homologous bacteria, but not against morphologically simil ar bacteria in heterologous sections. A Gram-negative, yellow pigmented bac terium (YPB), identified as Flavobacterium psychrophilum, was also recovere d, but only from the gills in the Ontario outbreaks. Antiserum prepared aga inst this YPB indicated an antigenic similarity among isolates recovered fr om the Ontario outbreaks, but immunohistochemistry failed to recognize anti genically related bacteria on the gills of fish from the other outbreaks. B ased on the unusual clinical presentation and the histopathological appeara nce of the gills, in conjunction with the absence of filamentous bacteria a ssociated with and recovered from affected gills, the present authors have called this condition 'atypical bacterial gill disease' or ABGD.