A peek into mass mortality caused by antimicrobial resistant Edwardsiella tarda in goldfish, Carassius auratus in Kerala

Edwardsiella tarda is one of the serious threats affecting the worldwide aquaculture. In the present study, four isolates were recovered from diseased goldfish, showing hemorrhages, reported with 60% mass mortality in an ornamental fish farm, Ernakulam, Kerala. Based on the phenotypic and genotypic analysis, the bacteria were identified as Edwardsiella tarda, Citrobacter freundii, Acinetobacter junii and Comammonas testosteronii. Experimental challenge studies using healthy goldfish revealed that among the four isolates, E. tarda alone leads to 100% mortality of experimental fish within 175 degree days and the pathogen could be successfully re-isolated from the moribund fish. The LD50 value of E. tarda was calculated as 9.9 × 10(5) CFU/fish. The histopathology of the infected tissues of goldfish had shown the typical features of E .tarda infection. The pathogen was found positive for the virulence genes viz., hly, etfA, etfD and eseD as detected using PCR. Thus E. tarda was confirmed as the real causative agent of the disease outbreak. Multiple antimicrobial resistance (AMR) exhibited by the pathogen towards 19 tested antibiotics with the MAR index of 0.46 highlighted the exposure of antibiotics to the fish in the farm. The existence of antibiotic resistant genes within the plasmid as revealed through plasmid curing studies pointed out the possibility of rapid dissemination of AMR in aquaculture. Hence proper surveillance and appropriate diagnostic methods need to be implemented at regular intervals to mitigate the menace.