Edwardsiella ictaluri, the causative agent of bacillary necrosis of Pangasius (BNP), has been a significant challenge to the farmed catfish industry in Vietnam since it was first reported over two decades ago. However, little is known about the current heterogeneity of E. ictaluri strains circulating striped catfish (Pangasianodon hypophthalmus) farms in Vietnam and how these populations have changed over time. Therefore, the aim of this study was to characterise E. ictaluri strains associated with BNP in farmed striped catfish in Vietnam over a twenty-year period.
A total of 203 strains were investigated in this study, which were recovered from clinical disease outbreaks across six Mekong Delta provinces in Vietnam between 2001 and 2021. Bacterial strains were identified using primary identification methods and their biochemical profile was characterised using the commercial API20E® system. In addition, their sensitivity to six antibiotic compounds used in global aquaculture was profiled using the Kirby-Bauer disk diffusion method. Then, a subset (n = 83) of bacterial strains were further typed by pulse-field gel electrophoresis methods and screened for the presence of eight virulence genes by standard PCR.
In general, E. ictaluri strains were found to be homogenous in their biochemical profiles (Table 1), irrespective of geographical location or year of recovery. Antibiotic resistance was found to increase over time, with high-level, multidrug resistance, associated with strains recovered between 2018 and 2021. Pulse-field gel electrophoresis differentiated selected strains into 16 pulsotypes. Furthermore, strains recovered between 2001 and 2011 were found to cluster distinctly from those recovered after 2017. Screening of virulence genes using PCR methods revealed six conserved virulence mechanisms in selected strains, however differences were noted in the presence of eseJ and traD, associated with the type IV and IV secretion systems, respectively. Continued surveillance of E. ictaluri in Vietnam will be important to inform biosecurity strategies and aid in the development of effective vaccines against BNP in striped catfish.