A previously unknown disease of farmed atlantic salmon: pathology and establishment of bacterial aetiology

Abstract

A previously unknown disease of Atlantic salmon Salmo salar occurred on a seawater farm in Ireland, during 1992 and 1993. The gross pathology and the histopathology of the disease are described. A distinctive histological feature of the disease was the presence of intracellular Gram-negative bacteria in several tissues. In particular, the endothelial cells of the kidney glomeruli were enlarged and contained densely packed bacteria within cytoplasmic vacuoles. During later stages of the disease, most tissues showed numerous areas of necrosis surrounded by lymphoid cells. A bacterium was isolated on axenic media, and was demonstrated to be the causative agent by laboratory infectivity trials. The bacterium was approximately 0.4 x 0.6 mum in size in vivo, but also showed filamentous forms (<5 mum) in vitro. In culture it was a slow-growing facultative or aerotolerant anaerobe, requiring serum or blood for growth. It was not possible to characterize the organism by standard biochemical reactions, but additional characterization tests are in progress.