Investigation of risk factors for Salmonella on commercial egg-laying farms in Great Britain, 2004-2005

In 2004/05, all European Union member states were required to carry out standardised prevalence surveys to establish the baseline prevalence of Salmonella in commercial laying flocks. As part of the survey in Great Britain, additional data were collected from 380 of the enrolled laying hen holdings to investigate risk factors for Salmonella at farm level. Stratified, simple random sampling was used to select holdings from which dust and boot swab samples were collected and tested for Salmonella using a modification of ISO 6579:2002. Using a nnultivariable logistic model weighted to account for the survey design, several factors significantly associated with Salmonella and Salmonella Enteritidis status were identified. Larger holdings (>= 30,000 birds) were found to be at higher risk of Salmonella (odds ratio [OR] 4.79, P=0.025), while vaccination (OR 0.28, P=0.013), providing foot dips with brushes (OR 0.27, P=0.042), washing and disinfecting the house at depopulation (OR 0.19, P=0.003), having a clean car park away from house (OR 0.14, P=0.001), using an independent (OR 0.19, P=0.007) or other non-company (OR 0.40, P=0.049) source of feed, being over 1 km from the nearest neighbouring farm (OR 0.45, P=0.021) and the presence of cats and dogs on the farm (OR 0.26, P=0.002) or on contiguous farms (OR 0.44, P=0.030) reduced the risk of any Salmonella serovars being present. Factors found to be associated specifically with an increased risk of S Enteritidis infection included holding size (OR 14.88, P=0.001) and frequent sightings of rats (OR 8.17, P<0.001) or mice (OR 5.78, P=0.006). Non-caged systems (OR 0.14, P=0.002), vaccination (OR 0.08, P=0.001), the use of a non-company feed source (OR 0.11, P=0.003), running the site as all-in/all-out (OR 0.06, P<0.001) and the presence of cats and dogs on the farm (OR 0.14, P=0.002) were associated with a reduced risk.