Abstract
During the period from late July through September 1974, an epizootic occurred among cultured yellowtails in Tosashimizu Bay in Kochi Prefecture. The typical symptoms of the disease were bilateral exophthalmos (Fig. 1), petechiae on the inside of the opercula and congestion of the pectoral and caudal fins. Internally there was congestion and haemorrhagia of the intestine, liver, spleen and kidney. Tne causative organism was Gram-positive, nonsporing ovoid cells occurring in short chains and about 1.4×0.7μ (Fig. 2). Growth occurred at 10-45°C and optimum temperature appeared to be 20-37°C. Optimum salt concentration for growth was 0%, and growth occurred at the concentration range of 0-6.5%. Optimum pH for growth was about 7.5. The organism was inert to production of ammonia, gelatinase, urease, indol and hydrosulfide, and to reductions by methylene blue, litmas milk and nitrate. Catalase, starch hydrolysis, cytochrome oxdase, casein digestion and PT test were negative. MR test, VP reaction and TTC reduction were positive. Because of the growth in 6.5% NaCl, at pH 9.6 and in 0.1% methylene blue milk, the organism was placed in the genus Streptococcus, group III. The organism was found to be not identical in its biochemical characteristic and pathogenicity to fish to any formerly established species of the genus Streptococcus.
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