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 +, eseI-, 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.

The objective of this study was to determine the causative agent of an outbreak with clinical signs similar to those of piscine streptococcosis in farmed snakeskin gourami ( Trichopodus pectoralis). Initial microscopic examination revealed the predominance of a Gram-positive, cocci bacteria in the brain and kidney of the diseased fish. This bacterium was successfully isolated and identified as Streptococcus suis based on nucleotide homology of 16S rDNA and species-specific PCR. This isolate tested negative for serotype 2, one of the major zoonotic serotypes. Experimental infection was then performed to investigate the pathogenicity of the bacterium and its histopathological manifestation. Naïve juvenile and adult snakeskin gourami were injected intraperitoneally with a low dose (1.2×10 5 CFU/fish) and a high dose (1.2×10 7 CFU/fish) of S. suis. Cumulative mortality appeared to be dose- and size-dependent. Experimentally diseased fish exhibited clinical signs consistent with naturally diseased fish. Severe histopathological changes in multiple organs were observed in both juvenile and adult fish, including meningitis, severe congestion in the brain and eyes, thickened stromal layers of the retina, severe hepatic lipidosis and tissue degeneration. Notably, numerous granulomas containing massive bacterial cells in the necrotic core were observed in the infected fish. Relatively pure colonies of S. suis were recovered from tissues of experimentally diseased fish. Taken together, this study fulfilled Koch’s postulates, indicating that S. suis is a new piscine pathogen. Although this is a case report, public awareness and biosecurity measures should be considered to prevent the spread of the disease. Further surveillance of the pathogen’s distribution and research into the underlying causes of fish-host adaptation will provide insights into the genuine impact and appropriate disease control strategies.