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

Despite government initiatives in East African countries promoting cage fish farming to compensate for income losses from declining capture fisheries, disease-related mortalities have become a major challenge, causing significant economic losses for farmers. Due to inadequate screening of pathogens causing these mortalities, farmers have been left to rely on guesswork, frequently using treatments that are either ineffective or inappropriate. Therefore, this study aimed to identify and characterize the etiological agents responsible for infectious diseases in cage-farmed fish in Lake Victoria, Tanzania. Fish samples were collected from six districts in the Lake Victoria Basin (LVB), which are known for their extensive cage fish farming operations. A total of 81 samples of blood, liver, and kidney tissues were collected from morbid Nile tilapia and enriched in Buffered Peptone Water then cultured on various agar media to isolate pathogens. Gram staining and a series of biochemical tests were used to identify the bacterial isolates. Similarly genomic DNA were extracted from each isolate and fragments of the 16S rRNA gene amplified and sequenced. Three pathogenic bacteria namely Citrobacter freundii, Pseudomonas aeruginosa, and Streptococcus agalactiae were identified isolated and identified. Pathogenicity trials demonstrated that P. aeruginosa exhibited the highest mortality rate (86.7%), followed by C. freundii (66.7%) and S. agalactiae (40%). Clinical and post-mortem findings from the trials showed symptoms consistent with hemorrhagic septicemia and septicemia. 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 harmonized efforts to enhance disease control strategies and reduce the impact on aquaculture operations, as farmers likely deal with the same bacterial strains.