UK salmonella survey of egg-laying farms
Results find that flock size is a risk factor for SE contamination
A recent publication in the Veterinary Record1 documents the results of a standardized survey conducted in accordance with an EU directive. An evaluation of 380 egg production units determined that flock size was the outstanding risk factor for infection with an odds ratio of 14.9 and a probability of 0.001 with respect to Salmonella enteritidis (SE) infection. Other risk factors included rodent infestation (odds ratio 8.17, probability <0.001) and non-caged systems (odds ratio 0.14, probability = 0.002).
It is noted that the survey involved both SE and also all other Salmonella serotypes, virtually none of which have any significance with regard to egg- borne infection. This is especially the case in the U.S. where eggs are washed and surface contamination is effectively removed or inactivated. With respect to the SE findings, it is noted that the prevalence rate in the U.K. based on the study was 5.8% of 380 farms (“holdings”) in 2004-2005. The authors noted that at the time of the survey the incidence rate of SE in UK consumers had decreased from the high levels recorded in the 1990s. As with the U.S. this is attributable to the implementation of the 1993 Poultry Breeding Flocks and Hatcheries Order (analogous to the NPIP) and the British Egg Industry Council, Lion Code of Practice (similar to the EQAPs).
The structure of the U.S. industry involving integrated operations, and the preponderance of white-shelled eggs derived from in-line caged flocks is completely different from the U.K. It is noted that at the present time the U.S. has a low prevalence rate of SE in commercial flocks and complexes. This is due to the reality that the Industry has operated in accordance with egg quality assurance programs for at least a decade. Vaccination using both live, mutant Salmonellatyphimurium vaccines in pullets and inactivated multivalent oil emulsion vaccines at transfer have markedly reduced the incidence rate of confirmed egg-borne SE infection in consumers.
Additionally maintaining an effective cold chain from production through to sale, education of food preparers and consumers as to the need to cook eggs thoroughly and the use of pasteurized product for high risk groups and in institutions have all contributed to a reduction in the incidence in confirmed cases and outbreaks of SE in the U.S.
Results of the field survey have been carefully evaluated with respect to experimental design, execution, microbiological procedures and results. Bias, confounding factors can produce statistical aberrations which may lead to incorrect conclusions as to risk factors and their epidemiologic significance. For example, in the U.K. study, decreased risk of SE was attributed to purchasing feed from an independent source and the presence of cats and dogs on the farm. It is counterintuitive that these factors could be beneficial in the context of the U.S. dogs and cats can serve as reservoirs ofsalmonella and would obviously negate the benefits of all-in/all-out placement programs. U.S. regulations relating to biosecurity imposed according to the FDA Final Rule as well as individual company operating procedures forbids the presence of dogs and cats on farms producing commercial table eggs.
Although studies such as the 2004-2005 U.K. study provide information on the specific risk factors associated with defined industries, correlation and extrapolation requires judicious consideration of the circumstances pertaining in the industries compared. It is axiomatic that good science can frequently be misinterpreted and distorted to support a preconceived standpoint and for political advantage.
1Snow, L.C., Davies, R.H., Christiansen, K.H., Carrique-Mas, J.J., Cook, A.J.C. and Evans, S.J. Investigation of risk factors for Salmonella on commercial egg-laying farms in Great Britain, 2004-2005. Veterinary Record 166: 579-586 (2010).