A longitudinal study of faecal shedding of Yersinia enterocolitica and Yersinia pseudotuberculosis by Merino lambs in south-eastern Australia.

A prospective longitudinal study was conducted to investigate potential risk factors for faecal shedding of Yersinia enterocolitica and Y. pseudotuberculosis by Merino lambs in four flocks in south-eastern Australia. The primary aims of the study were to determine the seasonal patterns of shedding of pathogenic Y. enterocolitica and Y. pseudotuberculosis, and to evaluate putative risk factors for faecal shedding of these organisms, including worm egg count, live-weight and growth rate. The risk of shedding varied markedly between Yersinia spp., farms, seasons and years. Shedding of Y. pseudotuberculosis occurred predominately in winter, whereas Y. enterocolitica was commonly isolated from faeces throughout the year. Moderate to high prevalences of shedding of each organism occurred in the absence of outbreaks of yersiniosis. In general, for shedding of Y. pseudotuberculosis, animals with moderate or high worm egg counts were at increased risk of shedding compared with animals with low worm egg counts. Sheep with higher average daily weight gains were at decreased risk of shedding Y. enterocolitica but at increased risk of shedding Y. pseudotuberculosis. Live-weight was not significantly associated with risk of shedding either species. This study highlighted that exposure to determinants of shedding Y. enterocolitica and Y. pseudotuberculosis differ between farms and over time within farms. Shedding is likely influenced by environmental, animal and management factors. Our results indicate that different or additional risk factors are required for yersiniosis over those that cause faecal shedding of Yersinia spp., because moderate to high prevalences of shedding were not always associated with outbreaks of clinical disease.