Isolation and Molecular Characterization of Streptococcal Species recovered from Clinical Infections in Farmed Nile Tilapia (Oreochromis niloticus) in the Philippines

Citation

Legario F, Choresca Jr CH, Turnbull JF & Crumlish M (2020) Isolation and Molecular Characterization of Streptococcal Species recovered from Clinical Infections in Farmed Nile Tilapia (Oreochromis niloticus) in the Philippines. Journal of Fish Diseases, 43 (11), pp. 1431-1442. https://doi.org/10.1111/jfd.13247

Abstract
Streptococcosis cause severe losses for global tilapia farming especially in developing countries. The aim of this study was to identify and characterize streptococci recovered from Nile tilapia farmed in the Philippines. Moribund and apparently healthy fish were sampled from grow-out cages, ponds and hatcheries. Clinical signs observed included exophthalmia, eye opacity, ascites, lethargy, erratic swimming, and haemorrhages. Results showed that both Streptococcus iniae and Streptococcus agalactiae were associated with disease in these sites. Consistent with global reports, including those from Southeast Asia, S. agalactiae was more widespread than S. iniae. Molecular serotyping of the S. agalactiae isolates identified the serotype Ia and serotype Ib. Histopathological findings were meningitis, meningoencephalitis and septicaemia. Identical virulence profiles were found for all strains of S. iniae, while S. agalactiae strains were separated into virulence profile I and profile II. All strains were susceptible to the tested antibiotics and resistant to oxolinic acid. Only S. agalactiae serotype Ib showed resistance to sulphamethoxazole-trimethoprim. This is the first study from the Philippines to characterize the streptococci involved in disease outbreaks in tilapia aquaculture. Outputs from this study will promote development of efficacious disease control strategies in tilapia farming for the Philippines and in Southeast Asia.

Keywords
molecular typing; Streptococcus agalactiae; Streptococcus iniae; serotype Ia; serotype Ib; virulence genes

Journal
Journal of Fish Diseases: Volume 43, Issue 11

• Published version