The role of contaminated eggshells used in poultry feed in a diffuse nationwide outbreak of Salmonella Enteritidis, the Netherlands, 2023 to 2025

We describe a large and prolonged outbreak of <i>Salmonella</i> Enteritidis in the Netherlands. Between June 2023 and September 2025, we identified 227 outbreak cases (110 males, 114 females, three with missing information of sex; median age 43 years). Outbreak cases were individuals whose isolates belonged to the outbreak cluster based on whole genome sequencing (WGS) using single-linkage clustering with a threshold of ≤ 5 allelic differences, since June 2023. A case-control study focussing on egg consumption was conducted, alongside trace-back and trace-forward investigations. Findings of the case-control study confirmed the existence of two WGS subclusters: subcluster A linked to barn eggs (adjusted odds ratio (aOR) = 5.8; 95% confidence interval (CI): 2.11-15.99) and subcluster B linked to organic eggs (aOR = 63.6; 95% CI: 6.04-670.55). Isolates from 14 laying hen farms and eggshells were linked to the outbreak, suggesting the outbreak had multiple sources. Inadequate processing of contaminated eggshells before their use in poultry feed was most probably contributing to the spread and length of the outbreak. Measures to improve raw material control for animal feed were implemented, contributing to a decline in case numbers. However, since the outbreak likely had multiple sources, new cases continue to be detected, especially in subcluster B.