OUTBREAKS OF A DISEASE CAUSED BY RICKETTSIA-LIKE ORGANISM IN CULTURED TILAPIAS IN TAIWAN

An organism, measuring 0.86 +/- 0.32 X 0.63 +/- 0.24 mum, likely to be member of family Rickettsiaceae, was detected for the first time from an outbreak of a disease with mass mortality among pond-reared tilapia in Taiwan. The pathogen was demonstrated to be Gram-negative and stained positively with Liu's solution and to grow on To2 and EPC cell cultures but not on nine artificial media selected and the pathogenicity was confirmed by Koch's postulates. Haematocrit in affected fish was decreased to 16.1 +/- 5.2% versus 28.3 +/- 5.1% for normal value. Typical lesions in moribund fish included marked white nodules and microscopical granulomatous formation all over the organs. The spleen enlarged to 5-50 times of the normal organ. Horizontal transmission without parasite vectors occurred. The experimental infection showed higher mortality at 15-degrees-C than at 30-degrees-C. In vitro experiments revealed that oxytetracycline which is legally permitted would probably be an effective antibiotic to control this disease. A field test showed that oral administration of oxytetracycline at a dose of 30-50 mg per kg body weight for 10-14 days was effective to control the disease. Clinical signs, susceptibility of various fishes to the pathogen and transmission electron-microscopy findings are described.