Estimation of the sensitivity of environmental sampling for detection of Salmonella in commercial layer flocks post-introduction of national control programmes

Abstract

A key element of national control programmes (NCPs) for Salmonella in commercial laying flocks, introduced across the European Union, is the identification of infected flocks and holdings through statutory sampling. It is therefore important to know the sensitivity of the sampling methods, in order to design effective and efficient surveillance for Salmonella. However, improved Salmonella control in response to the NCP may have influenced key factors that determine the sensitivity of the sampling methods used to detect Salmonella in NCPs. Therefore the aim of this study was to compare estimates of the sensitivity of the sampling methods using data collected before and after the introduction of the NCP, using Bayesian methods. There was a large reduction in the sensitivity of dust in non-cage flocks between the pre-NCP studies (81% of samples positive in positive flocks) and post-NCP studies (10% of samples positive in positive flocks), leading to the conclusion that sampling dust is not recommended for detection of Salmonella in non-cage flocks. However, cage dust (43% of samples positive in positive flocks) was found to be more sensitive than cage faeces (29% of samples positive in positive flocks). To have a high probability of detection, several NCP-style samples need to be used. For confirmation of Salmonella, five NCP faecal samples for cage flocks, and three NCP faecal boot swab samples for non-cage flocks would be required to have the equivalent sensitivity of the EU baseline survey method, which was estimated to have an 87% and 75% sensitivity to detect Salmonella at a 5% within-flock prevalence in cage and non-cage flocks, respectively.

Fig. 1.

The per-sample sensitivity of dust in cage (solid black lines) and non-cage (solid grey lines) flocks, and faeces in cage (dotted black lines) and non-cage (dotted grey lines) flocks relative to the within-flock prevalence for (a) NCP sampling; (b) EU sampling; (c) AHVLA sampling.

Fig. 2.

(a) Comparison between the model and the observed number of pools of five positive for Salmonella at each tenfold dilution from 41 flocks sampled; and (b) the estimated c.f.u./g of Salmonella in individual samples.