Public health implications of the infection of egg-laying hens with Salmonella enteritidis phage type 4

Abstract

In England and Wales in 1988, as in other European countries, eggs were identified as an important source of Salmonella enteritidis phage type 4 infection in humans. Unlike other non-host-adapted strains that can infect chickens, S. enteritidis PT4 is almost unique in that it can be isolated from the contents of intact, clean shell eggs. When present in the yolk of intact eggs it can survive certain forms of cooking, and analysis in England and Wales has shown that, in addition to outbreaks being associated with consumption of raw egg dishes and sauces, a number were also caused by cooked egg dishes. Although S. enteritidis PT4 is more heat-resistant than some other common poultry-associated strains, its ability to survive cooking is due more to the protective qualities of egg yolk.

In studies involving individually caged, naturally infected layers where the average prevalence was ≈1% of the output, the production of contaminated eggs was shown to be clustered but intermittent. This is in accord with observations made on other naturally infected flocks where the proportion of contaminated eggs has been observed to fluctuate. While salmonella-positive eggs were shown to be occurring during outbreak investigations involving human cases, some weeks later all eggs being produced by some flocks were found to be salmonella-negative even though the birds were shown to be carrying S. enteritidis PT4 at slaughter.

Although S. enteritidis PT4 has been shown to be highly virulent for young broiler chicks, it does not appear to behave in the same way in egg-laying hens. In three independent studies it was shown to be possible to infect mature hens with an inoculum as low as 100 cells. The organism was invasive and could be isolated from viscera and from egg contents. None of the birds, however, showed any signs of illness. Studies with naturally infected hens demonstrated that they have circulating antibodies against S. enteritidis PT4. There would also appear to be an association between antibody level and invasion by the organism. No such relationship exists with the production of contaminated eggs.

The ultimate control of this important human pathogen lies in the identification and eradication of infected flocks. Until that is achieved, however, significant improvements in egg quality and consumer safety can be made by storing eggs at 8°C or less. Fresh, naturally contaminated eggs contain small numbers of salmonella organisms and cool storage successfully prevents multiplication.

Keywords::

• Eggs
• infection
• poultry
• public health
• salmonella