Estimation of Salmonella prevalence in UK egg-laying holdings

As part of an EU-wide programme to reduce the prevalence of Salmonella in commercial egg-laying holdings, the EU has set for the UK an annual target of 10% reduction in the prevalence of Salmonella Enteritidis and Salmonella Typhimurium in commercial egg-laying holdings. To assist in demonstrating such a reduction, it is very important to obtain an accurate as possible baseline prevalence for Salmonella. The objective of this study was to provide a baseline estimate of the Salmonella prevalence in egg-laying holdings in the UK. Data from an EU baseline survey for Salmonella in UK commercial egg-laying flocks were therefore analysed using Bayesian methods, taking into account the sampling of only 1 flock per holding and estimates of the test sensitivity of the methods used in the EU baseline survey. In addition, in the UK the majority of the eggs come from farms which have participated in voluntary monitoring programmes for Salmonella since the early 1990s, and this data was also used, along with a prior estimate of the test sensitivity of voluntary surveillance. Results indicated that a true prevalence 14% for Salmonella Enteriditis and Typhimurium, and 18% for all serovars, both of these estimates being higher than has previously been reported from the EU baseline survey data. It is also shown that the sensitivity of voluntary surveillance is low, and it will therefore be important to compare results from "official" and "non-official" samples to check that the sampling performed in the National Control Plan is as sensitive as expected.