Multiple etiological factors led septicemic disease with mass mortality of fishes in a freshwater reservoir of Odisha, India

Abstract

A detailed investigation on mass mortality of fishes was conducted in a small tropical reservoir- Derjang (20o50’32.0”N, 85o01’14.8″E), Odisha, India. Mortality mostly occurred in Systomus sarana followed by Labeo rohita, Cirrhinus mrigala, Labeo catla, Ompok bimaculatus, Labeo calbasu and Mastacembelus armatus. During 20 days of disease occurrence in May–June 2019, a total of about 3000 kg of fish died. The clinical signs in Cyprinid group were haemorrhagic spots, ulcerative lesions, rotten and pale patches in gills due to septicemic disease whereas haemorrhagic spots were the only prominent symptoms observed in Silurid group. Bacteriological isolation and identification through conventional and molecular techniques revealed that Klebsiella pneumoniae was the most common pathogen recovered from S. sarana, C. mrigala and O. bimaculatus. Further Aeromonas hydrophila, Acinetobacter baumannii were isolated from L. rohita and L. catla respectively. The role of these pathogens for this disease outbreak in multiple fish species is discussed in perspective of environmental factors. Sudden environmental alternation by the supercyclone Fani (3rd May 2019) on the coastal part of Odisha might have played a key role to translate the aquatic bacteria into the virulent infective pathogens. In the experimental challenge study, isolated bacteria showed pathogenicity in respective hosts as that in the reservoir. Thus this further revealed both bacteria and fish specific virulency with a variation in LD50 values. All the gram negative bacterial isolates were found to resist ampicillin and amoxicillin-clavulanic acid and most of them were TEM gene positive. However, the bacteria were found to be susceptible to the rest of the nineteen antibiotics. These findings suggested that the sudden cyclone is an enormous threat to reservoir aquaculture, and should be taken into consideration before breeding, stocking and harvesting of fishes.