Scientists have warned of an ongoing increase of Salmonella infections in the Netherlands linked to the egg laying hen sector.
Since mid-2023, the country has experienced a sustained increase in Salmonella Enteritidis infections.
Infections have gone up from an annual average of 281 from 2017 to 2019 to 427 in 2023, and 401 in 2024. In the first six months of 2025, 209 cases were reported, compared with 180 during the same period in 2024. The rise comes at the same time as a 2.5-fold increase in positive tests for Salmonella in laying hen flocks.
While the increase is currently limited to the Netherlands, spread to other European countries is possible because of trade, travel and movement of poultry products.
Started with an outbreak
On Dutch laying hen farms, culling is not mandatory after detection of Salmonella Enteritidis, but all eggs from positive flocks must go for processing, including a heat treatment. After the production cycle, the house must be thoroughly cleaned and disinfected before new hens can be introduced.
According to the study, published in the journal Eurosurveillance, the increase in 2023 was largely due to a major outbreak linked to eggshells from positive flocks inadequately treated before being added to poultry feed.
This led to widespread infection in the laying hen sector and resulted in human cases through the consumption of contaminated eggs. The outbreak, which began in the first half of 2023, resulted in 151 cases in 2023, 27 in 2024, and 31 in 2025 through June.
In line with EU rules, adult laying hen flocks are sampled at least every 15 weeks during the laying period. This monitoring showed an increase in Salmonella Enteritidis-positive flocks since May 2023, matching the rise in human cases. In 2023, 74 positive flocks were detected, followed by 81 in 2024, and 50 in the first half of 2025.
A causality analysis found the number of positive flocks significantly predicted the number of human cases at time lags of 1 month, as well as at 3 to 6 months.
Smaller clusters
Following the outbreak, most cases in 2024 and 2025 have occurred in smaller and more diverse clusters. Using whole genome sequencing, scientists identified 38 non-travel-related clusters in 2023, 42 in 2024, and 22 so far in 2025, with a median size of three cases for all years. This complicates outbreak investigations and the identification and control of infection pathways, said researchers.
The National Institute for Public Health and the Environment (RIVM) set up a response team in early 2025. The presence of many small clusters suggested a wide transmission pattern rather than a single point-source outbreak. As a result, the recommended control measures focused on reducing Salmonella Enteritidis prevalence and transmission throughout the industry.
Possible interventions include increasing testing frequency in laying hen flocks to reduce the period in which contaminated eggs are marketed, helping to limit human exposure or accelerating removal of positive flocks to lower infection pressure in the wider laying sector, reducing the risk of further farm-to-farm spread and re-infection.
Reasons for the sustained increase of Salmonella Enteritidis in the sector remain unclear. One possibility is the extended productive lifespan of hens, which may increase their vulnerability as vaccine-induced immunity fades over time.
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