The Molecular Characterization of Salmonella from Cattle Feces in Chiang Mai, Thailand, Its Antibiotic Susceptibility, and Genetic Relatedness to Clinical Isolates

Aims: Salmonella is the most common pathogenic bacteria causing food-borne and zoonotic diseases in humans, and food-producing animals are considered the primary source of Salmonellosis. Cattle slaughtered for human consumption have been implicated as a source of human infection with antimicrobial-resistant Salmonella. The purpose of this study was to determine the prevalence of Salmonella in the feces of apparently healthy slaughtered cattle and characterize the circulating non-typhoidal Salmonella (NTS) serovars in cattle in Chiang Mai, Thailand as well as determine their antibiotic resistance profiles and genetic relatedness to human isolates.

Methodology: A cross-sectional study was conducted from June to September 2017 at the municipal abattoir in Chiang Mai, Thailand.One hundred and fifty fecal samples were collected from apparently healthy slaughtered cattle for Salmonella isolation. One hundred and ten blood samples were also collected from febrile patients at the Chiang Mai provincial hospital prior to any antimicrobial treatment. Circulating Salmonella serovars were identified using standard culture techniques and DNA sequencing, antimicrobial sensitivity was tested using a disc diffusion method, and 16S rRNA were used to determine phylogenic relationships.

Results: The results showed that the prevalence of Salmonella from cattle was 22.6%. According to the NCBI blast, three serovars were identified: S. Typhimurium (50.0%), S. Enteritidis (41.2%), and S. Weltevreden (11.8%); these serovars formed distinct clades on the phylogenetic tree. S. Typhimurium and S. Enteritidis isolates from cattle were closely related to human isolates. All isolates from cattle were susceptible to 10 selected commonly prescribed antimicrobials.

Conclusion: Our study revealed a high prevalence of Salmonella in healthy slaughtered cattle in Chiang Mai. S. Typhimurium and S. Enteritidis isolated from cattle and humans were closely related. The finding suggested that slaughtered cattle is a source of Salmonella infection in humans. This poses a major public health threat since slaughtered cattle are a potential source of food-borne salmonellosis. Therefore, the local administration should establish a system for the routine surveillance of infectious sources.