Unravelling mass mortality in transported rohu (Labeo rohita) fingerlings: evidence of Flavobacterium columnare from Western India.

Columnaris disease caused by Flavobacterium columnare in freshwater farmed fishes is a severe problem, leading to considerable mortality. The present study investigated the mass mortality happened in Labeo rohita fingerlings within 48 h post-transportation from an organized fish farm in Maharashtra, India, to the laboratory. The moribund fish showed clinical signs analogous to columnaris and the bacterial isolation showed pale yellow rhizoid edged tightly adhered colonies which are found as slender rod-shaped bacteria by TEM. Molecular analysis of the isolate using 16S rRNA nucleotide homology and phylogenetic analysis showed ≥ 98% similarity with F. columnare BRTAGILLFC01 strain. RFLP analysis demonstrated that the isolate belongs to genomovar II. To test whether hypoxia during transportation influenced the pathogenicity of bacteria, the hypoxic stress (dissolved oxygen, DO: 2.50 ± 0.50 mg/L) was simulated in an experimental setting with F. columnare challenge to L. rohita, which was compared to the optimum setting of normoxic condition with DO: 6.00 ± 0.50 mg/L. Significant infection enhancement was observed in hypoxic condition as evident from 100% accumulated mortality within 48 h post-challenge compared to 80% in the normoxic group with varied LD50 values of 5.30 × 105 CFU/mL, and 1.24 × 106 CFU/mL, respectively. Severe to moderate histopathological alterations were recorded in the affected gill, kidney, and liver tissues such as the loss of secondary gill lamellae, tubular, epithelial and glomerular degeneration and pyknotic nucleus with increased sinusoidal space, in the respective tissues. The study evidenced that the hypoxic condition during transportation may induce F. columnare infection associated mass mortality in L. rohita fingerlings within the short span of post-transportation stress.