Study of Salmonella detection in laying hens using a Bayesian model

As part of the measures to reduce the prevalence of Salmonella in poultry in the UK, National Control Programmes (NCPs) have been implemented. These involve regular statutory testing of poultry holdings to monitor and estimate the prevalence of Salmonella in the national flock population and to control Salmonella on holdings with positive flocks, especially those serovars most identified with human illness: Salmonella Enteritidis (SE) and S. Typhimurium (ST). It is very important to ensure that the level of testing is appropriate so that it is sufficiently effective to identify positive flocks and to monitor prevalence, but also efficient in the use of resources. The aim of this study was to estimate the sensitivity of both the Operator and Competent Authority (CA) Official sampling used to detect infected flocks, and to also estimate the true proportion of infected holdings of commercial laying flocks in GB each year of the NCP, along with the trend of any changes in prevalence for both SE/ST and non-SE/ST. A Bayesian model was developed to estimate the sensitivity of both Operator and CA Official sampling from the NCP data 2009-2018, and to estimate the true prevalence of infected holdings. The model estimate for the prevalence of infected holdings for the first complete year of the NCP was 3.9% (95% Credible Interval (CI) 2.8-6.2%) for non-SE/ST and 0.8% (95% CI: 0.4%-1.5%) for SE/ST. Prevalence had reduced to 1.6% (non-SE/ST) (95% CI 1.0%-2.5%) and 0.2% (SE/ST) (95% CI 0.1%-0.4%) in 2018. Results indicated a very low sensitivity of Operator sampling (~9%), but a much higher sensitivity of CA Official sampling (~44%). The true prevalence of Salmonella infected holdings in the UK had a mean average reduction of 10.6% (95% CI: 6.3%-15.1%) per annum (non-SE/ST) and 15.9% (95% CI: 6.0%-19.8%) annual reduction for SE/ST. This has shown the effectiveness of the NCP for Salmonella in commercial laying flocks, with reductions in Salmonella overall more or less equal to the target reduction for regulated serovars of 10% per annum. The true prevalence of SE/ST was estimated to be below the final target of less than 2% in every year and was below 0.5% at the end of the 10 year period.