Pathological and microbiological characterisation of a streptococcosis outbreak in Genetically Improved Farmed Tilapia, reared in a brackish water cage farm, India

Sumithra, T G and Anusree, V N and Sharma, S R Krupesha and Reshma, K J and Suja, G and Amala, P V and Sayooj, P (2025) Pathological and microbiological characterisation of a streptococcosis outbreak in Genetically Improved Farmed Tilapia, reared in a brackish water cage farm, India. Journal of Marine Biological Association of India, 67 (1). pp. 22-30. ISSN 2321-7898

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Abstract

Genetically Improved Farmed Tilapia (GIFT) is a scientifically validated species for brackish-water cage aquaculture. However, only limited information exists regarding the disease occurrences under cage farming conditions in India. The present study reports a disease outbreak incident in GIFT from a cage farm, six months post-stocking, resulting in a cumulative mortality of 45% over two weeks. Necropsy and microbiological analyses consistently isolated a common bacterium from different moribund fish. Experimental infection trials confirmed Koch’s postulates, with an estimated LD50 of 6.5 × 106.65 CFU/ fish via intramuscular injection. The pathogen was identified as Streptococcus agalactiae, and its Multiple Antibiotic Resistance (MAR) index was 0.08, indicating a low level of antibiotic resistance risk. Serum bactericidal assays revealed the pathogen’s ability to proliferate in healthy tilapia serum, with significantly enhanced serum survival at elevated temperatures (p ≤ 0.05). Histologically, infected fish tissues revealed mild congestion and haemorrhage in the brain, multifocal hemosiderin deposits, sub-capsular haemorrhage, capsular thickening and macrophage infiltration in spleen, primary lamellar congestion, secondary lamellar hyperplasia and fusion in gills, and necrotic changes of hepatic parenchyma. This report provides the first description of S. agalactiae infection in brackish-water cage-cultured GIFT in India, highlighting a potential health threat to tilapia aquaculture in such environments.

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