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The present study report on epizootiology, pathogenicity and haemato-immunological aspects of Streptococcus agalactiae (Group B Streptococcus, GBS) serotype Ib (GBS-Ib) affecting climbing perch (Anabas testudineus) in southern Thailand. Study on growth of GBS-Ib under various conditions indicated that this bacterium grew from 25 to 35C, pH 5 to 9 and 0.5 to 4.0% NaCl. Climbing perch inoculated by intraperitoneally injected GBS-Ib at concentrations of 101 to 104 CFU/mL exhibited 60.00-96.67% mortality, with 60 h-LD50 and 72 h-LD50 at 4.5x102 CFU/mL and 6.3x101 CFU/mL inoculums, respectively. Study of the haemato-immunological changes in the fish infected with GBS-Ib revealed declines of haematocrit, haemoglobin, serum protein, red blood cells, and respiratory burst activity. On the other hand, white blood cells, lysozyme activity, phagocytic index, and average beads per cell, were significantly increased in the infected fish. Study on the effect of environmental factors on the susceptibility of fish to GBS-Ib infection indicated that high water temperature and low dissolved oxygen levels proved to increase mortality of climbing perch. These findings indicate that GBS-Ib is a pathogenic bacterium responsible for fish mortality in Thailand.

Streptococcus agalactiae (Group B Streptococcus; GBS) causes serious damage to fish aquaculture worldwide. The present study reports S. agalactiae infection in climbing perch (Anabas testudineus) and Günther's walking catfish (Clarias macrocephalus) polycultured in southern Thailand between 2011 and 2015. During the disease outbreak, a mortality rate of 10-40% was observed, affecting climbing perch and Günther's walking catfish weighing from 60-150 g and 30-90 g, respectively. The infected fish exhibited a variety of symptoms typical of streptococcosis, including lethargy, exophthalmia, corneal opacity, ascites, haemorrhage and erratic swimming. One hundred and twenty six isolates from the infected fish were identified as S. agalactiae serotype Ib by biochemical, serological, as well as molecular analyses. The S. agalactiae isolates from the present study were completely sensitive to chloramphenicol, erythromycin, lincomycin and oxytetracycline, but resistant to oxolinic acid and sulfamethoxazole/trimethoprim. Investigation into virulence-associated genes (bca, bac, scpB, lmb and GBSi1) indicated that the S. agalactiae isolates from climbing perch and Günther's walking catfish contained only bca, which differed from S. agalactiae previously isolated from infected tilapia. The S. agalactiae isolates from the present study were found to be strongly virulent for climbing perch with 80-100% mortality within 7 days following intraperitoneal (i.p.) injection with S. agalactiae at a concentration of 107 CFU/ml. Major histopathological changes of naturally infected climbing perch revealed diffuse haemorrhage in several organs. To our knowledge, this is the first isolation of S. agalactiae serotype Ib from climbing perch and Günther's walking catfish polycultured in southern Thailand. [ABSTRACT FROM AUTHOR]