Natural occurrence of edwardsiellosis caused by Edwardsiella ictaluri in farmed hybrid red tilapia (Oreochromis sp.) in Southeast Asia

Infections caused by Edwardsiella ictaluri bacteria have mainly affected the catfish aquaculture industry in Asia and America, while few reports reveal E. ictaluri-associated mortality in non-catfish species. Here, we report a natural occurrence of E. ictaluri in hybrid red tilapia juveniles farmed in a floating cage system in Northern Vietnam Southeast Asia. Cumulative mortality reached 40-50% within the first month after stocking. Diseased fish exhibited the presence of numerous white spots in the swollen spleen and head kidney. Pure cultures of pinpoint bacterial colonies were recovered from internal organs of all naturally diseased fish (n = 10) and subsequently four representative isolates were chosen for further identification and analyses. The bacteria were later confirmed as E. ictaluri based on biochemical characteristics, specific PCR for both genus and species levels, partial 16S rRNA and gyrB sequencing. Artificial infections using Nile tilapia juveniles produced Edwardsiellosis with typical signs of visceral white spots as observed in naturally diseased fish. Discovery of E. ictaluri infection in farmed red tilapia adds to the growing list of emerging pathogens in tilapia aquaculture in Vietnam and the region of which better awareness needs to be made.