Cytophaga sp. (Cytophagales) infection in seawater pen-reared Atlantic salmon Salmo salar

Abstract

A Cytophaga sp. was associated with skin and muscle lesions of moribund Atlantic salmon Salmo salar smolts shortly after they were introduced to seawater in the late winter and spring of 1985, 1986, and 1987. Concurrent with high mortalities, the lesions were most prevalent 1 wk after introduction. Fish introduced to seawater in summer were less severely affected. The lesions initially appeared as white patches on the flanks in the posterior region, and, as the disease progressed, enlarged and extended deep into the underlying muscle. Fish with the lesions exhibited elevated plasma sodium levels, and a probable cause of mortality was osmotic stress. Wet mounts of the lesions from all epizootics revealed numerous filamentous gliding bacteria. Gliding bacteria were isolated on Marine Agar and on seawater Cytophaga Medium from samples obtained during the 1987 epizootics, and were identified as a Cytophaga sp. based on biochemical, morphological, and G + C ratio data. The organism is a marine bacterium requiring at least 10 YO seawater for growth in Cytophaga Medium. In addition to differences in growth characteristics, this isolate was serologically distinct from C. psychrophila, Flexibacter columnaris, and F. maritimus. We have experimentally induced the lesions in Atlantic salmon smolts with a pure culture of the organism, thus supporting the hypothesis that the Cytophaga sp. described here is the etiological agent of the disease.