Streptococcus agalactiae serotype Ib, an emerging pathogen affecting climbing perch (Anabas testudineus) and Günther’s walking catfish (Clarias macrocephalus) polycultured in southern Thailand

Abstract

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.