Incidence of beta-haemolytic streptococcal infection associated with mass mortalities of cultured tilapia, Oreochromis mossambicus in brackish water ponds in India.

摘要

Invasive infections resulting in mass mortality of tilapia (Oreochromis mossambicus) have been frequently observed at fish farms in various districts of Kerala, India during summer. This study was conducted in a few polyculture ponds in Vypeen Island of Ernakulam district of Kerala during March to June 1999. The infected fish showed an exophthalmic condition along with corneal opacity, skin lesions and septicaemic conditions of different organs. The causative agents were gram-positive non-motile cocci occurring in chains of varying lengths. The morphological and biochemical characteristics confirmed that the causative organism was a biotype of Streptococcus iniae. This bacterium grew well in the range of 10-40℃ but optimum temperature was 25-35℃and optimum requirement of pH and salinity were 7.5-8.5 and 0-4% (w/v) respectively. All the isolates showed beta-haemolytic characters on blood base agar media containing 5% defibrinated fresh goat blood. Pathogenicity was studied after injecting the pathogen at 105-8 cells/ml intraperitoneally into healthy tilapia. There was 100% mortality observed within 7 days of infection. The moribund or dead fishes showed typical lesions and pathology as found in field samples. The physiological characteristicsand antibiotic sensitivity patterns of the bacteria are also described.