SUMMARY
Edwardsiella ictaluri is an emerging bacterial pathogen that
affects farmed tilapia (Oreochromis spp.). This study reports the
arrival, establishment, and widespread findings of E. ictaluri in
farmed tilapia in Vietnam. Among 26 disease outbreaks from 9 provinces
in Northern Vietnam during 2019–2021, 19 outbreaks originated from
imported seeds, while outbreaks in seven farms were from domestic
sources. Clinically sick fish showed the appearance of numerous white
spots in visceral organs, and accumulative mortality reached 30%–65%.
Twenty-six representative bacterial isolates recovered from 26 disease
outbreaks were identified as E. ictaluri based on a combination
of phenotypic tests, genus- and species-specific polymerase chain
reaction assays, 16S rRNA and gyrB sequencing, and
phylogenetic analysis. All isolates harbored the same virulence gene
profiles esrC +, evpC +,ureA-C +, eseI- ,escD- , and virD4- .
Antimicrobial susceptibility tests revealed that 80.8%–100% of
isolates were multidrug resistant, with resistance to 4–8
antimicrobials in the groups of penicillin, macrolides, sulfonamides,
amphenicols, and glycopeptides. The experimental challenge successfully
induced disease that mimicked natural infection. The median lethal doses
(LD50) of the tested isolates (n = 4) were 42–61 colony
forming units/fish, indicating their extremely high virulence. This
emerging pathogen is established and has spread to various geographical
locations, causing serious impacts on farmed tilapia in northern
Vietnam. It is likely that this pathogen will continue to spread through
contaminated stocks (both imported and domestic sources) and persist.
Thus, increased awareness, combined with biosecurity measures and
emergent vaccination programs is essential to mitigate the negative
impact of this emerging disease on the tilapia farming industry.
Keywords : Edwardsiella ictaluri , outbreaks, tilapia,
virulence genes, antimicrobial resistance