Necrotic myositis in cage cultured rainbow trout, Oncorhynchus mykiss (Walbaum), caused by Flexibacter psychrophilus

A severe necrotic myositis characterized by large intramuscular bullae was identified in 100–500 g rainbow trout, Oncorhynchus mykiss (Walbaum), maintained in freshwater netpens. Mortality increased as the temperature decreased, with mortality rates peaking at 10% per month. Flexibacter psychrophilus was recovered from the muscle lesions and kidneys of affected fish. The gross and histological description of the disease, and the phenotypic characteristics of the causative agent are described. The necrotic myositis isolate was utilized to experimentally reproduce the disease.