Bacterial pathogens and factors associated with Salmonella contamination in hybrid red tilapia (Oreochromis spp.) cultivated in a cage culture system

Microbial food safety in cultured tilapia remains a challenge to public health worldwide, due in part to intensive aquaculture leading to poor water quality and high organic matter deposition. This study aimed to determine the prevalence of indicator and potential pathogenic bacteria in hybrid red tilapia (Oreochromis spp.) and their cultivation water and to identify environmental parameters and other bacterial contaminants associated with Salmonella contamination. A total of 120 fish were sampled, which were partitioned into fish carcasses (n=120), muscle (n=120), intestine (n=120), liver and kidney (n=120), and cultivation water (n=120) from three commercial farms in western Thailand from October 2019 to November 2020. The prevalence of fecal coliforms and Escherichia coli (E. coli) in these 600 samples was 74.8% and 56.7%, respectively. The prevalence of Salmonella, Vibrio cholerae (V. cholerae), Aeromonas hydrophila, and Vibrio vulnificus (V. vulnificus) was 23.0%, 17.5%, 2.5%, and 1.7%, respectively. None of the samples tested positive for Streptococcus agalactiae. Cultivation water exhibited a high prevalence for Salmonella (58.3%). Among fish samples, Salmonella had the highest prevalence at 14.1%, which was mainly from fish intestine. There was a significant association of Salmonella with the presence of fecal coliforms, E. coli, V. cholerae, and V. vulnificus. The predominant serovars of Salmonella included Saintpaul, Neukoelln, Escanaba, and Papuana. Grazing ducks that were raised in proximity to these cultured tilapia shared the same isolates of Salmonella based on the similarity of their rep-PCR DNA fingerprints, suggesting that ducks may function as either a biological reservoir for tilapia or at minimum participate in the environmental replication of this strain of Salmonella. Taken together, the results suggest that the environment used for tilapia aquaculture may be contaminated with pathogenic bacteria; therefore, food safety precautions are needed during processing, transportation, cooking, and consumption.