Characterization, virulence and pathology of Flavobacterium sp. KG3 associated with gill rot in carp, Catla catla (Ham.)

Abstract

“Gill rot” caused by the Gram negative, long, rod shaped bacterium Flavobacterium sp. in a sewage fed farm growing Indian major carps in West Bengal, India, is described. The phenotypic and molecular characteristics, virulence and pathology of Flavobacterium sp. strain KG3 are also documented. The infected catla Catla catla had cutaneous haemorrhages, white patches on body and gill, gill rot and excess mucus secretion. On selective cytophaga agar, the isolate produced non-rhizoid yellow pigmented colonies. Phylogenetically, the strain KG3 was distinctly different from the closely related type strains of Flavobacterium species, though it branched with type strain F. hydatis DSM 2063 (NCBI accession number AM230487) with low node value. Histologically, the naturally infected catla gills showed cartilaginous tissue inflammation, mucus secretion, necrosis of gill filament with associated reduction in the number of lamellae per filament, obliteration of interlamellar water channels, and fusion of lamellae. Catla catla fingerlings experienced about 57% moralities within 5days of abrasion and immersion in a bath containing 4.7×106Flavobacterium sp. cells/mL. The abraded and unchallenged fingerlings recorded 20% mortality in 3days. These results demonstrated the virulence of Flavobacterium sp. in catla especially during the winter season, which warrants its management measures. Statement of relevanceFlavobacterium spp. are emerging as major pathogens of Indian freshwater fish. They can cause chronic mortalities in fish at low water temperature. Correct diagnosis and understanding the pathology would help manage flavobacterial infection in fish.