First insights into Weissella tructae infections in rainbow trout (Oncorhynchus mykiss) in Vietnam: Bacterial characterization, pathogenicity and antimicrobial susceptibility

Weissellosis has emerged as a significant disease impacting farmed rainbow trout (Oncorhynchus mykiss), resulting in high global mortality rates. In the summer of 2023, an outbreak in Northern Vietnam's rainbow trout farms was linked to Weissellosis, with mortality rates ranging from 20 % to 60 %, prompting an investigation into the causative agent. A total of 106 diseased fish exhibiting panophthalmitis and hemorrhagic eyes were collected from 25 affected farms. Twenty-five representative isolates, each sourced from a different farm, were identified as Weissella tructae through phenotypic and biochemical characterisation, PCR, and 16S rRNA sequencing. Challenge experiments indicated a relatively high virulence of W. tructae in rainbow trout weighing 35–40 g, with LD₅₀ values estimated at 1.2 × 10⁴ CFU/fish. Histopathological examination revealed significant tissue damage, including hemorrhages in the brain, spleen, and kidney. Antimicrobial susceptibility testing of eight antibiotics authorized for use in Vietnam showed the lowest minimum inhibitory concentration (MIC) values for Amoxicillin, Amoxicillin-Clavulanic Acid, and Erythromycin (0.06–1 mg/L). In contrast, most isolates exhibited MIC values of ≥ 512 mg/L for Sulfamethoxazole/Trimethoprim. This study represents the first report of Weissellosis in Vietnamese rainbow trout, highlighting the geographic expansion of the disease and underscoring the urgent need for effective strategies to prevent and control this pathogen.