The prevalence, serotypes and antibiograms of <i>Salmonella</i> isolates on Thoroughbred stud farms in New South Wales and Victoria

Salmonella enterica subsp. enterica is responsible for substantial morbidity and mortality within horse populations, worldwide. The prevalence of Salmonella shedding in faeces has largely been reported in hospital settings, with limited information from general horse populations. Further, there is little understanding of the serotypes and antibiograms of Salmonella isolates from horses in Australia. The objectives of this study were to (1) estimate the prevalence of Salmonella in stud farm horse populations, (2) determine serotypes and antibiograms for isolates, and (3) determine whether seasonal, animal, farm or management factors are associated with Salmonella shedding. A prospective longitudinal cohort study was performed on 11 Thoroughbred stud farms in New South Wales and Victoria between 1^st April 2023 and 30^th March 2024. Faecal samples were collected every 3 months (spring, summer, autumn, winter) and data for each animal and farm were obtained. Standard bacteriological culture methods were used to isolate Salmonella and serotype and antimicrobial susceptibility patterns were determined. A total of 1476 faecal samples from 1330 horses were submitted. Salmonella was isolated from 5 horses (0.4%, 95% CI 0.2-0.9) on four farms during summer and autumn: S. Mbandaka (2), S. Typhimurium (2) and S. Wandsbek (1). No isolate was multidrug-resistant (MDR). Five stud farms reported a history of a horse shedding Salmonella on the property (NSW: 4, Victoria: 1) and four farms reported vaccination for Salmonella. The prevalence of Salmonella on Australian Thoroughbred stud farms is low with varied serotypes involved and without evidence of MDR.