Investigation of Disease Outbreaks in Farmed Nile Tilapia in Coastal Tanzania (Pwani region) Reveals Pseudomonas aeruginosa as the Most Frequently Isolated Pathogen

Since 2020, recurrent mortalities have been reported in grow-out farms in the Ruvu River basin in Tanzania’s Pwani region. This study investigated bacterial pathogens associated with these outbreaks. A cross-sectional survey was conducted between March 2022 and November 2023 in four affected tilapia farms. A total of 60 moribund fish were sampled. Clinical examination revealed skin ulcers, hemorrhages, fin erosion, abdominal distension, and darkened skin. Postmortem changes showed pale livers, enlarged spleens, and swollen gallbladders. Bacteria from skin lesions, kidneys, liver, and intestines were identified using selective media, morphology on agar, biochemical tests, and 16S rRNA gene sequencing, revealing six genera: Pseudomonas, Aeromonas, Bacillus, Staphylococcus, Enterobacter, and Micrococcus. Among these, Pseudomonas aeruginosa was the most frequently isolated from all farms and was strongly associated with the observed disease outbreaks. Phylogenetic analysis of P. aeruginosa further demonstrated close genetic relatedness among isolates obtained from different farms. The findings identify P. aeruginosa as the most frequently isolated bacterial pathogen associated with disease outbreaks in affected farmed tilapia within the Ruvu River basin and highlight the importance of improved biosecurity, stocking management, and aquatic animal health surveillance in Tanzania.