Nordic nations had a seasonal campylobacteriosis peak in mid- to late summer while most other European countries had a smaller rise earlier in the year, according to a study.

Researchers in Eurosurveillance examined how the seasonality of campylobacteriosis varied across Europe from January 2008 to December 2016, looked at associations with temperature and precipitation and compared results with past studies.

Infection with Campylobacter is the most commonly reported foodborne illness in the European Union. It causes more than 200,000 cases annually, but due to under-reporting, the actual number of infections may be closer to nine million. In the EU, the associated annual cost in terms of public health and lost productivity is estimated at €2.4 billion ($2.7 billion).

In many countries, campylobacteriosis has a striking seasonal peak during early to mid-summer. Factors that contribute to this include changing bacterial colonization patterns in broiler flocks, different food preparation practices, transmission through flies and elevated consumption of fruit and salad.

The analysis included 1,784,996 cases from 18 countries for 2008 to 2016 and six clusters were produced indicating seasonality is comparable and factors contributing to seasonality may also be similar. Cases reporting foreign travel (n = 135,178) were removed from the dataset. Other exclusion criteria removed Belgium, Cyprus, Estonia, Iceland, Latvia, Malta, Poland, Portugal, and Romania.

Austria, Czech Republic, and Germany are in cluster one, with a diffuse seasonal peak that occurs in late July, but also a smaller peak around mid-June.

The Nordic countries of Denmark and Sweden (cluster two), and Norway and Finland (cluster three) demonstrate a peak in early August and late July respectively.

Most other European countries are in cluster four (France, Italy, Lithuania, Luxembourg, the Netherlands, Slovenia), with a seasonal peak earlier in the year around mid-June.

Cluster five groups the United Kingdom and Ireland with the geographically separate Hungary and Slovakia; peak incidence occurs in early June.

Cluster six (Spain) is most dissimilar from the others, with incidence more constant throughout the year and a peak less distinct. This could be a consequence of its southern location or low level of reported cases.

In spite of several European countries’ interventions to control foodborne illness, the strength and timing of campylobacteriosis peaks have remained broadly similar during the past 10 years. This supports the important role of the environment and weather conditions, which are minimally affected by food-based interventions, according to the study.

Compared with past studies the peak weeks of campylobacteriosis incidence are mostly the same except for Ireland and the Netherlands. While the reason for the Dutch change to a peak several weeks earlier is unknown the Irish one could be due to small case numbers in previous studies.

Five of the six-country clusters demonstrate significant associations with temperature. There was also a statistically significant association with precipitation but this has less influence on campylobacteriosis incidence than temperature.

The associations between temperature, precipitation and Campylobacter infections were stronger in Nordic countries than other European nations.

Researchers used data from The European Surveillance System (TESSy).

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