Development of a risk assessment model for Salmonella in shell eggs and processed eggs in Ireland

• Project start date: 1 November 2005
• Project status: Completed
• Project type: Food safety
• Discipline: Microbiology and food hygiene
• Author/s: Dr David McCleery & Dr Robert Madden, Agri-Food and Biosciences Institute, Belfast
• Collaborator/s: Prof Seamus Fanning & Dr Paul Whyte, University College Dublin, Dr Louise Kelly, University of Strathclyde

Research objective

Hens' eggs are the principal source of Salmonella Enteritidis. This project investigated the effect of vaccination of layers and application of Quality Assurance (QA) schemes on the risk of human infection with Salmonella spp. from the consumption of eggs and processed liquid egg produced on the island of Ireland. 

Research report

• Title: Unpublished report
• Date: 7 September 2006
• Summary: This research assessed the risk of Salmonella in eggs produced on the island of Ireland and found that contamination levels are very low, with both vaccination and test-and-cull control strategies proving equally effective in protecting public health.
• Findings:

  Very low prevalence of Salmonella in eggs

  • Over 30,000 eggs were tested across 5,018 samples from both jurisdictions.
  • Only two samples were positive for Salmonella (0.04%).
  • Contamination was found on shells only, not inside eggs.
  • No egg contents were positive.
  • The serovars found (Salmonella Infantis and Salmonella Montevideo) are not typically associated with internal egg contamination.

  No difference between control strategies

  • Northern Ireland uses vaccination of laying flocks.
  • The Republic of Ireland uses testing and culling, with vaccination prohibited.
  • There was no statistically significant difference in Salmonella prevalence between the two systems.
  • Both approaches are equally effective in controlling Salmonella in eggs.

  Low risk to public health

  Both the qualitative and quantitative risk assessments concluded that:

  • The probability of exposure to Salmonella from Irish-produced eggs is low.
  • The estimated probability of a contaminated serving was 0.00013.
  • The annual risk of illness linked to eggs is also low.

  Declining trend over time

  The prevalence of Salmonella in eggs has fallen steadily compared with earlier UK and Irish surveys.

  This aligns with the introduction of regulation, quality assurance schemes, and industry controls.

  Processed egg products are effectively controlled

  Long-term monitoring data for pasteurised liquid egg showed no Salmonella detected over more than 20 years.

  Pasteurisation was confirmed as an effective control measure.

  Data gaps identified

  Limited Ireland-specific data were available on:

  • Time and temperature conditions during distribution and storage.
  • Consumer handling, preparation, and cooking practices.
  • Some assumptions had to be taken from international studies.
• Recommendations:

  Maintain existing control measures

  • Continue current Salmonella control strategies in both jurisdictions.
  • No evidence supports changing policy in favour of one system over the other.

  Improve data collection

  Collect Ireland-specific data on:

  • Egg storage temperatures and durations across the supply chain.
  • Consumer and catering preparation practices.
  Use structured questionnaires with producers and retailers.

  Continue surveillance

  • Ongoing monitoring remains important, even at low prevalence.
  • Other Salmonella serovars could emerge if ecological conditions change.

  Use the risk model as a decision tool

  The quantitative model provides a baseline.

  It can be updated to:

  • Test future policy options.
  • Assess potential risk mitigation measures.

  Build on developed expertise

  • The project successfully built in-house capacity in microbiological risk assessment.
  • This expertise should be used for future food safety assessments.

Other outputs

Peer-reviewed articles

Murchie, L., Whyte, P., Xia, B., Horrigan, S., Kelly, L. and Madden, R.H. (2007). The prevalence of Salmonella spp. in grade A whole shell eggs in the island of Ireland. Journal of Food Protection, 70(5):1238-1240. https://doi.org/10.4315/0362-028X-70.5.1238

Murchie, L., Xia, B., Madden, R.H., Whyte, P. and Kelly, L. (2008). Qualitative exposure assessment for Salmonella spp. in shell eggs produced on the island of Ireland. International Journal of Food Microbiology. https://doi.org/10.1016/j.ijfoodmicro.2008.04.026