Atypical piscine mycobacteriosis in Japanese medaka (Oryzias latipes)

Japanese medaka, (Oryzias latipes), small, freshwater, tropical cyprinodonts, are principally used for toxicologic and carcinogenicity assays, but are finding more applications in developmental genetic and biological research. An increase in mortality began in brood stock of adult medaka that had been shipped and housed separately by sex. Initially, mortality averaged one fish daily and began in females two weeks after they were received. Cohabitation began eight weeks after arrival. After four to six weeks of cohabitation in different spawning aquaria, mortality was observed in males. Clinical signs of disease included loss of scale luster and color, with subsequent blanching of dorsal flank musculature, small raised nodules on various external surfaces, emaciation, fraying of fin tips, and equilibrium disturbances. Histologic examination of affected adults revealed multi-organ granulomatous inflammation with intracellular acid-fast bacilli. Specimens from 46 juvenile medaka that were spawned from affected adults, were submitted for culture and histologic evaluation. Of 18 fish, two had lesions similar to those of adults. The organism isolated from the remaining fish was identified as Mycobacterium fortuitum. Due to atypical rapid progression of disease, spread of M. fortuitum to progeny, and poor prognosis, the entire colony was euthanized.