Streptococcus agalactiae infection in tilapia (Oreochromis niloticus)

Abstract

Streptococcus agalactiae was isolated from cultured tilapia in Surat Thani province. Isolates were Gram-positive cocci, catalase negative, alpha-haemolytic and serogroup B. Streptococcal-infected fish showed various swimming abnormalities such as swimming on their side, erratic surface or bottom swimming including serpentine movement, exophthalmia and opacity. Internally, splenomegaly, ascites as well as pale liver discoloration were observed. Fish experimentally infected by peritoneal injection using 101 -108 CFU/ fish showed 20-90% mortality within 10 days and the LD50 was 3.60x101 - 1.72x107 CFU. Haematocrit, haemoglobin, plasma protein and blood cell values of infected and moribund fish were significantly decreased. Histopathological findings included the occurrence of inflammation, cells necrosis, infiltration of lymphocytes and the formation of granulomas in the infected organs.

Keywords

• Streptococcus agalactiae
• tilapia
• blood parameters
• histopathology