Two disease outbreaks were encountered in cage cultured cobia fingerlings with the mortality of 36.80% and 83.70% during summer month at Palk Bay and Gulf of Mannar, respectively. The mortality started with the appearance of clinical signs like off feed, erratic swimming behavior with circling movement, surfacing, and acute death. The gross lesions observed were corneal opacity, subsequent congestion, exophthalmosis, abdominal distention with ascites, severe congestion at the caudal lobe of the liver, and moderate congestion of the brain. Histopathologically, moderate vacuolation and spongiosis were observed in brain. The viral nervous necrosis was confirmed by RT-PCR from the brain samples collected from the affected fish. Addition to this viral disease, Vibrio alginolyticus (KX989540) was isolated from Palk Bay cage cultured fingerlings and Pseudomonas aeuruginosa (KX860116) from Gulf of Mannar. The sea surface water temperature during the disease outbreak was significantly (P <0.05) higher. In conclusion, two outbreaks were due to co-infection by VNN and bacteria. The higher temperature during the months of April and May (33.2 ±0.25 ◦C 34.1 ±0.25 ◦C), increased stocking density, and occlusion of barnacles and clams at the cage inner net were attributed as main stress factors that might have induced the infection resulted in mass mortalities of cobia fingerlings.

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