Isolation of Salmonella from urban wild brown rats (Rattus norvegicus) in the West Midlands, UK

A 6-month study was undertaken to investigate the prevalence of Salmonella enterica in wild urban brown rats (Rattus norvegicus) in the West Midlands. Samples were obtained of faecal droppings (n = 100) and from rectal swabs (n = 50) of rat carcases collected from active infestation sites. A subset of the rats (n = 25) had additional swab samples taken of the fur, paws and tail. Five (10%) of the rectal swabs were positive for Salmonella by direct plating onto XLD media. No further samples were positive following pre-enrichment and selective culture. A total of eight (8%) faecal samples were positive for Salmonella; two by direct plating and a further six following enrichment. All positive faecal samples were fresh or moist upon collection. None of the samples obtained from the outer surfaces of the rat were positive. Additionally, rat faeces were spiked with Salmonella and sampled periodically to determine survival in drying faeces exposed to a typical indoor environment. Salmonella could be recovered by direct culture up to 86 days. These results demonstrate a regional variability in the carriage of Salmonella in urban rats compared to other studies and that Salmonella longevity in faecal pellets is sufficient to present a potential contamination risk in the absence of an active infestation.