Natural Concurrent Infection of Vibrio harveyi and V. alginolyticus in Cultured Hybrid Groupers in Malaysia.

In September 2016, a marine fish farm operator in Selangor, Malaysia reported a disease outbreak affecting juvenile Hybrid Groupers (Camouflage Grouper Epinephelus polyphekadion x Tiger Grouper E. fuscoguttatus). The average daily mortality was 120 fish, resulting in a cumulative mortality rate of 29% within 10 days. The affected Hybrid Groupers displayed lethargy, excessive mucus production, rotten fins, congestion of livers and kidneys, and enlargement of spleens. Microscopically, there were generalize congestion of the brains and internal organs. Vibrio harveyi and V. alginolyticus were successfully isolated from the diseased fish. The isolated pathogens were found sensitive to oxytetracycline and tetracycline, but resistance towards ampicillin and vancomycin. Experimental infections using the isolated V. harveyi (108 CFU/ml), V. alginolyticus (108 CFU/ml), and concurrent infection by V. harveyi (108 CFU/ml) and V. alginolyticus (108 CFU/ml) in juvenile Asian Seabass Lates calcarifer resulted in 60%, 100% and 100% mortality within 240 h post infection, respectively. The experimentally infected Asian Seabass demonstrated similar clinical signs and histopathological changes as naturally infected Hybrid Groupers. However, concurrently infected fish demonstrated severe clinical signs and histopathological changes compared to single infections. These results suggest that both isolates of Vibrio are pathogenic to fish and responsible for the disease outbreak. However, concurrent infection involving V. alginolyticus and V. harveyi leads to a more devastating impact to the cultured fish. This is the first report of concurrent Vibrio infection in cultured marine fish in Malaysia. This article is protected by copyright. All rights reserved.