More people have fallen sick in one of the Netherlands’ largest Salmonella outbreaks ever recorded.
As of mid-September, 171 laboratory-confirmed patients have been identified, including 151 in 2023 and 20 in 2024. Of these, 85 are male and 86 are female.
The National Institute for Public Health and the Environment (RIVM) said that because not all infections are diagnosed or reported, the number of sick people is estimated to be 20 times higher.
There has been a nationwide outbreak of Salmonella Enteritidis since June 2023. The latest case was reported in July 2024, so the outbreak is thought to have ended.
The peak in infections was in September 2023, after which the number of patients gradually decreased until the end of the year. However, several new patients were identified between May and July 2024 after almost three months without cases in this cluster.
Tracing the source
The Dutch Food and Consumer Product Safety Authority (NVWA) and COKZ, the Dutch supervisory authority for dairy, poultry, and eggs, investigated the outbreak with RIVM.
It was identified using whole genome sequencing (WGS), which found that sick people were part of two genetically closely related clusters with 117 and 54 patients, respectively. The age range was 1 to 98.
A questionnaire confirmed the suspicion of two clusters: one cluster was mainly related to the consumption of barn eggs, while the other was related to organic eggs.
Eggs from Salmonella-infected barns cannot be sold for direct consumption. Instead, they are supplied to the food industry, where they undergo an additional heating step to eliminate the pathogen.
Based on WGS, Salmonella isolates from 14 laying farms were genetically related to the outbreak. This indicated that there were multiple sources. A breakthrough came when four Salmonella isolates from eggshells were found to belong to the outbreak cluster. Eggshells can be fed to hens for a source of calcium.
Further monitoring of the animal feed chain by NVWA revealed a problem in the processing of eggshells, which resulted in Salmonella not being eliminated. Contaminated eggshells could end up in poultry feed. Investigators said this could explain why several laying hen farms were infected with a genetically similar strain.
After being informed of these findings, stakeholders in the egg and animal feed chain took action to improve hygiene and prevent further contamination, including tightening the heating process and stricter controls on cross-contamination.
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