Investigation of mass mortality caused by multi-drug resistant Edwardsiella tarda in Oscar fish, Astronotus ocellatus from an ornamental fish farm, Kerala

Abstract

Oscar fish (Astronotus ocellatus) is one of the most domesticated and exported ornamental fishes from Kerala. The emergence of diseases caused by multi-drug-resistant bacteria is a serious obstacle faced in the ornamental fish industry. In the present study, six isolates were resolved from the diseased Oscar fish showing hemorrhages, necrosis and loss of pigmentation. After phenotypic and genotypic characterization, the bacteria were identified as Edwardsiella tarda, Klebsiella pneumoniae, Enterococcus fecalis, Escherichia coli, Brevibacillus borstelensis, and Staphylococcus hominis. Experimental challenge studies in healthy Oscar fish showed that E. tarda caused 100% mortality within 240 hours with 6.99×106 CFU/fish as LD50 and histopathology revealed the typical signs of infection. The pathogen could be successfully re-recovered from the moribund fish thereby confirmed Koch’s postulates. Edwardsiella species was confirmed as E. tarda through positive amplification of tarda specific gene and virulence genes viz., etfD and escB were also detected in the pathogen. The isolate showed multiple antimicrobial resistance (AMR) against aminoglycosides, tetracyclines, and quinolones with the MAR index of 0.32, which implicated the antibiotic pressure in the farm. Plasmid curing studies showed a paradigm shift in the resistance pattern with MAR index of 0.04, highlighting the resistance genes are of plasmid-borne. Moreover, tetracycline resistance gene (tetA) was the only one found to be chromosome-borne. This study is the first of its kind in detecting mass mortality caused by E. tarda in Oscar fish. Proper surveillance and appropriate action plans are crucial for the precise detection of pathogens responsible for diseases and AMR in aquaculture.