Molecular characterization and pathogenicity assessment of bacteria causing infectious diseases in cage-farmed fish in the Lake Victoria

Abstract

Cage aquaculture in Lake Victoria Tanzania has expanded rapidly increasing from 460 in 2021 to over 1000 in 2025, yet disease outbreaks remain a major constraint to productivity and farmer livelihoods. A cross-sectional study was conducted to investigate bacterial pathogens affecting Nile tilapia (Oreochromis niloticus) in cage farms across six districts in the Lake Victoria Basin (LVB), Tanzania. A total of 90 moribund fish were collected from thirty cage fish farm purposively and from each fish liver and kidney were sampled. Bacterial isolates were identified using biochemical profiling and 16S rRNA gene sequencing. Three pathogenic bacteria namely Citrobacter freundii, Pseudomonas aeruginosa, and Streptococcus agalactiae were isolated and identified. Pathogenicity trials demonstrated that P. aeruginosa exhibited the highest mortality rate (86.7%), followed by C. freundii (53.3%) and S. agalactiae (40%). Clinical and post-mortem findings from the trials showed symptoms consistent with haemorrhagic septicaemia, scale loss, fin rot, skin ulcer, Popeye, and eye loss. Phylogenetic analysis grouped sequences of each pathogen into a single cluster, regardless of their geographical origins, suggesting a common source and subsequent dispersal to various locations. The study calls for farmer training and harmonized efforts to strengthen disease control strategies, as farmers are likely dealing with the same bacterial strains.