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

- Pathology of Flavobacterium sp. KG3 isolated from the Catla catla gill rot was described.
- It elicited pathogenesis on catla gills with necrosis and other cellular and tissue level alterations.
- Flavobacterium sp. caused 57% morality in abraded catla fingerlings within 5 days of challenge at 4.7 × 106 cells/mL level.
- Flavobacterial infection is emerging as major problem of cultured freshwater fish during the winter.

“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 5 days of abrasion and immersion in a bath containing 4.7 × 106Flavobacterium sp. cells/mL. The abraded and unchallenged fingerlings recorded 20% mortality in 3 days. 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.