Estimation of Salmonella prevalence in UK egg-laying holdings

doi:10.1016/j.prevetmed.2010.01.004

Preventive Veterinary Medicine

Volume 94, Issues 3–4, 1 May 2010, Pages 306-309

https://doi.org/10.1016/j.prevetmed.2010.01.004 Get rights and content

Abstract

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.

Introduction

Salmonella Enteritidis (SE), followed at a distance by S. Typhimurium (ST) are responsible for the majority of cases of human salmonellosis in the UK and elsewhere in Europe (Anon., 2009). Control programmes for these two Salmonella serovars have been in operation in the UK since 1989, and these have been associated with a reduction in the number of human incidents of these serovars. Epidemiological investigations have demonstrated that contaminated eggs produced by infected laying hens are the main source of human infection of SE (Coyle et al., 1988, Gillespie et al., 2005, De Jong and Ekdahl, 2006) and therefore considerable efforts have focused on monitoring programmes of commercial laying hens at the farm level. In order to provide the scientific basis for setting targets for reduction of prevalence of Salmonella at the EU level, an EU-wide baseline survey was carried out during 2004/2005 on a randomised selection of commercial scale (>1000 hens) laying farms in all member states (MS). The EU has set targets for each member state for annual reductions in the prevalence of SE and ST according to the prevalence found in the EU Baseline Survey. The target for the UK and other MS with <10% SE/ST prevalence was a 10% reduction per annum (EEC, 2006).

To demonstrate that there is such a reduction over time in the UK egg-laying farms in a statistically robust manner could prove difficult, especially as the prevalence declines, due to the uncertainty in the prevalence estimates. Therefore it is very important to obtain an as accurate as possible baseline prevalence for Salmonella. The sampling method used in the EU baseline survey was developed to be a cost-effective method that could be implemented by all MS and so provide comparable results. As such, it is not 100% sensitive and it is important to take this into account, in terms of both the loss of holding-level sensitivity from the sampling of only 1 flock per holding (Carrique-Mas et al., 2008a) on multi-flock holdings and in terms of the lower than 100% flock-level sensitivity of the sampling method (Carrique-Mas et al., 2008b).

Furthermore, in the UK the majority of the egg industry have participated in voluntary monitoring programmes for Salmonella in laying holdings since the early 1990s. These data on voluntary surveillance represents a potentially useful source of information on the prevalence of Salmonella in holdings of laying hens, since large numbers of flocks are tested, and therefore might be useful in reducing the width of the confidence intervals of the prevalence estimate from the EU Baseline Survey, thus enabling a statistically significant 10% annual reduction in Salmonella prevalence to be demonstrated with greater certainty.

The objective of this study was therefore to provide an estimate of the Salmonella prevalence in commercial egg-laying holdings in the UK, taking into account the sampling of only 1 flock per holding and the test sensitivity of the methods used in the EU Baseline Survey, and to explore the combined use of voluntary surveillance and the EU Baseline Survey data.

Section snippets

Data

Data on Salmonella occurrence in flocks of laying hens was obtained from 2 sources: (1) targeted and (2) scanning surveillance.

The targeted surveillance data was taken from the EU Salmonella baseline layer survey, conducted over a 12-month period from October 2004. Full details of the survey are published (Snow et al., 2007), but in summary 54 positive holdings were found from 454 sampled and tested, of which 36 were positive for either SE or ST.

Scanning surveillance data was obtained from the

Results

The estimated Salmonella prevalence of infected holdings for all Salmonella serovars from the EU survey data was 18% (95% Credibililty interval (CI) 12–25%). This increased when the voluntary surveillance data was included in the estimation to 19% (95% CI 13–28%), but the prevalence of the voluntary surveillance is so uncertain that its impact is small, reducing the standard deviation of the estimate from 3.4 (EU survey data only) to 3.2 (EU survey plus scanning surveillance data). The

Discussion

The Bayesian methods used in this study have been shown to provide a framework in which surveillance data from different sources can be combined to provide a single estimate of prevalence. The methods used were also able to characterise the uncertainty in the prevalence and in the parameters used to derive the prevalence. In this case the uncertainty in the sensitivity of voluntary surveillance has meant that the voluntary surveillance data is not able to provide any meaningful reduction in the

Acknowledgements

This study was funded by the Department for Environment, Food and Rural Affairs through projects FZ2000 and ED1039.

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Observations related to the Salmonella EU layer baseline survey in the UK: follow-up of positive flocks and sensitivity issues
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Preventive Veterinary Medicine

Abstract

Introduction

Section snippets

Data

Results

Discussion

Acknowledgements

References (8)

Salmonella enteritidis phage type 4 infection: association with hen's eggs

Lancet

The Community summary report on trends and sources of zoonoses, zoonotic agents, antimicrobial resistance and foodborne outbreaks in the European Union in 2007

EFSA J.

Code of Practice for Lion Code Eggs

Observations related to the Salmonella EU layer baseline survey in the UK: follow-up of positive flocks and sensitivity issues

Epidemiol. Infect.

Cited by (19)

Whole genome sequencing analysis of Salmonella enterica serovar Weltevreden isolated from human stool and contaminated food samples collected from the Southern coastal area of China

Designing a biochip following multiplex polymerase chain reaction for the detection of Salmonella serovars Typhimurium, Enteritidis, Infantis, Hadar, and Virchow in poultry products

Of Mice and Hens-Tackling Salmonella in Table Egg Production in the United Kingdom and Europe

Prevalence of and examination of exposure factors for Salmonella on commercial egg-laying farms in Barbados

Serotyping and Antimicrobial Resistance Profiling of Multidrug-Resistant Non-Typhoidal Salmonella from Farm Animals in Hunan, China

Study of Salmonella detection in laying hens using a Bayesian model

Short communicationEstimation of Salmonella prevalence in UK egg-laying holdings

Abstract

Introduction

Section snippets

Data

Results

Discussion

Acknowledgements

Lancet

The Community summary report on trends and sources of zoonoses, zoonotic agents, antimicrobial resistance and foodborne outbreaks in the European Union in 2007

EFSA J.

Code of Practice for Lion Code Eggs

Observations related to the Salmonella EU layer baseline survey in the UK: follow-up of positive flocks and sensitivity issues

Epidemiol. Infect.

Short communication
Estimation of Salmonella prevalence in UK egg-laying holdings