Presence and Absence of Escherichia coli in Wild, Caged and Market Fish (Oreochromis niloticus) from Selected Markets in Kisumu, Kenya

Fish is a critical protein source in Kenya, with Nile tilapia (Oreochromis niloticus) accounting for over 75% of aquaculture production. The fish supply chain - spanning wild capture, cage aquaculture and retail markets - presents multiple points of potential microbial contamination that threaten public health. Escherichia coli contamination along the fish supply chain in Kisumu, Kenya poses a significant food safety risk, with limited systematic data available to guide targeted interventions across the national fish value chain. Foodborne illnesses burden Kenya's health system, with diarrheal diseases among the leading causes of under-five mortality. Evidence-based data on contamination patterns is essential to protect consumers, particularly in informal markets serving low-income populations. To assess E. coli prevalence in wild, caged and market fish; compare contamination across aquaculture operations and markets; evaluate the impact of processing methods; and identify critical control points for intervention. A total of 104 O. niloticus samples were collected between November–December 2024 from wild sources, three cage operations and six Kisumu markets. Standard ISO 16649-2 microbiological methods were applied, with confirmatory IMViC biochemical tests. Data were analysed using Chi-square tests and visualised using Python. Overall E. coli prevalence was 39.4%. Wild fish recorded 0% contamination, caged fish 31.1%, and market fish 49.1%, with Obunga Market reaching 100% - including in cooked products - confirming post-cooking contamination as a critical failure point. Star Market achieved 0%, proving safety is attainable. Immediate handwashing infrastructure, vendor training and post-cooking hygiene enforcement are urgently needed, supported by medium-term market upgrading, good aquaculture practice guidelines and a long-term national fish safety surveillance system.