Researchers have looked at some of the factors potentially behind a stalling in the fall of a type of Salmonella in two European countries.

Salmonella Enteritidis incidence had seen a long-term decline but this trend stabilized in 2012. Exploring reasons behind the stagnating trend is important to identify opportunities to re-establish the declining pattern, said researchers in the journal Eurosurveillance.

The study aimed to identify factors that could explain the trend of infections from 2006 to 2019 in two neighboring EU countries. It showed that increased incidence was associated with season, younger individuals, travel-related cases, and the occurrence of potential outbreaks. 

Salmonella Enteritidis causes about 30 percent and 20 percent of all salmonellosis cases in Belgium and the Netherlands, respectively. Notification of salmonellosis is not mandatory in the Netherlands. The surveillance system covers 62 percent of the Dutch population.

In total, 5,377 Salmonella Enteritidis cases from 2006 to 2019 were reported in the Netherlands, of which 188 were excluded because of missing data on age and sex. In Belgium, 8,819 cases were reported, with 541 omitted because of missing data.

Differences by country
The number of patients was highest in summer in both countries. In the Netherlands, most patients were 15 to 59 years old, while in Belgium, the age groups 0 to 4 and 15 to 59 had the most cases.

In the Netherlands, the proportion of reported cases with known travel history was 17 percent and in Belgium, it was 5 percent, based on data from 2013 onwards.

In both countries, the incidence of Salmonella Enteritidis infection decreased significantly until 2015, when an upward trend started. Potential outbreaks also increased after 2015.

In the Netherlands, the rate of cases with invasive infection was significantly higher from 2015 to 2019 than from 2006 to 2014.

In Belgium, there were more infections in the age groups 5 to 14 and 15 to 59 in 2015 to 2019 compared with 2006 to 2014.

One of the largest Salmonella Enteritidis outbreaks linked to eggs from Poland was reported in 2016, which heavily affected the Netherlands and Belgium. Two more outbreaks occurred in 2019: one related to eggs from Spain which impacted both countries and the other linked to Lahmacum, a Turkish-style pizza, which was only in the Netherlands.

Before 2016, there were no detected large outbreaks in the Netherlands and Belgium. The introduction of whole genome sequencing (WGS) in 2016 allowed the identification of outbreak clusters among cases that would otherwise have been missed.

Future research should focus on areas such as urbanization degree and socioeconomic status. A next step would be to explore factors at the primary animal production and strain pathogenicity levels that could potentially play a role in the observed trends, said scientists.