Detection of antimicrobial resistant Salmonella in livestock: a chance event?

In Great Britain, monitoring the levels of antimicrobial resistant Salmonella in livestock occurs both as part of a passive surveillance system and as structured surveys. To provide insight into such surveillance activities, a probabilistic model has been developed to assess the probability of detecting resistance at the faecal, pen and farm level. Using this model, it is concluded that the probability of detecting resistant Salmonella is dependent upon the level of resistance within sample/pen/farm and the diagnostic power of the test used. The likelihood of detecting low level (e.g. emerging) resistance on individual farms was low and therefore the use of selective plating (antimicrobial present in the plate at the specified breakpoint concentration so growth confirms the presence of resistant Salmonella) is recommended. Importantly, the models provide an insight into the sampling and testing methods and could therefore be used to inform any future on-farm surveillance programmes or research projects.