Epizootiology, pathogenicity and haemato-immunology associated with Streptococcus agalactiae serotype Ib infection in climbing perch (Anabas testudineus)

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.