The arrival, establishment and spread of a highly virulent Edwardsiella
ictaluri strain in farmed tilapia, Oreochromis spp.
Edwardsiella ictaluri is an emerging bacterial pathogen affecting
farmed tilapia ( Oreochromis spp.). This study reports the
arrival, establishment and widespread 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
stocked seeds while outbreaks in 7 farms were from domestic sources.
Clinically sick fish showed appearance of numerous white spots in
visceral organs, and accumulative mortalities reached 30-65%.
Twenty-six representative bacterial isolates recovered from 26 disease
outbreaks were then identified as E. ictaluri based on a
combination of phenotypic tests, genus- and species-specific PCR assays,
16S rRNA and gyrB sequencing and phylogenetic analysis.
All isolates harbored the same virulence gene profile esrC
+, evpC +, ureA-C
escD- and virD4-.
Antimicrobial susceptibility tests revealed 80.8-100% isolates were
multidrug-resistant, with resistance to 4-8 antimicrobials in the groups
of penicillin, macrolide, sulfonamide, amphenicols, and glycopeptide.
Experimental challenge successfully induced disease that mimicked
natural infection. The median lethal dose (LD 50) of the
tested isolates (n = 4) were 42 to 61 CFU/fish, indicating their
extremely high virulence. This emerging pathogen has already been
established and spread to various geographical locations and causing
serious impact to farmed tilapia in Northern Vietnam. It is likely that
this pathogen will continue to occur through possibly contaminated
stocks (both imported and domestic sources) and persist spreading. Thus,
increased awareness combined with biosecurity measures and emergent
vaccination programs are essential to mitigate the negative impact of
this emerging disease on the tilapia farming industry.