Identification and Characterisation of <scp> <i>Vibrio vulnificus</i> </scp> Associated With an Outbreak in Brackish Water–Farmed Nile Tilapia ( <scp> <i>Oreochromis niloticus</i> </scp> ) in Thailand

Vibriosis caused by Vibrio vulnificus is a significant threat in brackish-water aquaculture, yet its involvement in Nile tilapia (Oreochromis niloticus) outbreaks in Thailand has not been previously confirmed. This study investigated seven disease events in brackish water-farmed tilapia to identify the causative agent and characterise its pathogenicity. Moribund fish exhibited enophthalmos, cutaneous haemorrhages, vent hyperemia, and gastrointestinal congestion. Bacteria were consistently identified as V. vulnificus by MALDI-TOF MS and confirmed by qPCR targeting the 16S rRNA and vvhA genes. All isolates carried key virulence genes (vvhA, vuuA, and pilFVV). Phylogenetic analysis of partial 16S rRNA sequences revealed that outbreak isolates formed a monophyletic clade within V. vulnificus and clustered closely with other V. vulnificus strains. Antibiotic susceptibility testing showed uniform susceptibility to oxytetracycline, enrofloxacin, sulfamethoxazole-trimethoprim, and chloramphenicol, while most isolates exhibited intermediate susceptibility to amoxicillin. Histopathology of naturally infected fish demonstrated systemic vascular lesions with congestion, haemorrhage, melanomacrophage proliferation, and mixed inflammatory infiltrates affecting the brain, heart, liver, spleen, kidney, and gastrointestinal tract. Experimental infection reproduced the disease, with tilapia challenged at 10⁷ CFU/fish showing 100% mortality within 6 h, whereas lower doses caused transient illness followed by complete recovery and no detectable infection. The findings indicate the pathogen's high virulence, broad organ involvement, and susceptibility to commonly used aquaculture antibiotics. Early diagnosis, appropriate antimicrobial selection, and improved health management are essential to reduce future outbreaks.