Vagococcus salmoninarum, a bacterium of pathological significance in rainbow trout Oncorhynchus mykiss farming

A chronic condition of juvenile and adult rainbow trout was recorded in a trout farm over several years and resulted in significant mortality. The disease occurred mainly at low temperature. Necropsy findings were associated with hyperaemia and haemorrhage, in gills and viscera, with extensive cardiovascular lesions including myocarditis and epicarditis. Most of the losses were attributed to heart lesions and mortality peaked under stress conditions. A Gram-positive, chain-forming, coccoid to ovoid-shaped coccus identified as Vagococcus salmoninarum on the basis of phenotypic and genetic characteristics was regularly isolated from diseased fish. It was demonstrated to be the causative agent of the condition by infection experiments and histopathology, which also confirmed the affinity of the bacterium for the circulatory system. V. salmoninarum was resistant to inost of the antimicrobial drugs tested in vitro, and the treatments conducted in the field were ineffective. An attempt at vaccination did not provide encouraging results. V salmoninarum joins a list of Gram-positive bacteria responsible for chronic infections which do not respond to control procedures and which pose a serious threat to broodstock and large-sized fish.