Nordic countries may experience a doubling of Campylobacter cases by the late 2080s, according to researchers.

Scientists used national surveillance data to analyze the relationship between climate and campylobacteriosis in Denmark, Finland, Norway, and Sweden and estimate impact of climate changes on future disease patterns.

They found nearly 6,000 excess Campylobacter cases per year in these four countries could be linked only to climate changes, according to the study published in the journal Scientific Reports.

Domestic cases of campylobacteriosis are commonly linked to contaminated food or drink such as poultry or unpasteurized milk. However, recently environmental and behavioral factors such as recreational water contact, occupational exposure at poultry farms and abattoirs and contact with household pets have emerged as important transmission routes.

Climate impact
A total of 64,034 reported cases of Campylobacter were included in the final database. However, it featured only domestic patients for Norway and Sweden but both domestic and cases of unknown origin from Denmark and Finland.

During the baseline period from 2000 to 2015, the average annual number of cases per 100,000 people in the four countries was 42, ranging from 25 in Norway to 60 in Denmark. This was predicted to rise to 117 in 2080 to 2089. The database also included per municipality per week and year from 2000 to 2015 precipitation and temperature, the number of heat waves and days with heavy precipitation.

Researchers calculated the excess number of cases caused only by climate change. Results showed climate changes can result in an average 145 more annual cases of Campylobacter by 2040 to 2049 and almost 1,500 by the late 2080s in each country per year. The effect was less pronounced in Sweden.

Models for Campylobacter and climate showed the amount of cases in any week during the summer rose significantly with increasing temperature and heavy rainfall in the previous week, suggesting a non-food transmission route. A rise in heat waves in any week during summer as well as increases in precipitation during winter decreased the amount of Campylobacter cases reported one week later.

Researchers estimated the effects of arbitrary climate changes in models by changing the different variables. For instance, a 1 millimeter increase in precipitation with all other variables unchanged in any municipality in any week during the summer will result in a 38 percent increase in Campylobacter cases in that municipality the following week.

Changing seasonal occurrence
Predictions indicate that Campylobacter cases in the four Nordic countries combined can increase by 25 percent by the end of the 2040s and 196 percent by the end of the 2080s compared to the predicted baseline of 2000 to 2015. The impacts vary with country and time period with the highest increases predicted in Denmark and Norway during the late part of the period.

Models also predict a change in future seasonal distribution of cases. At present, Campylobacter increases during spring and summer and almost half of the annual total is reported between July and September.

During 2040 to 2059, this pattern will remain similar although the high season extends until November. For later scenarios, the seasonal variation has become less pronounced with cases increasing from April and remaining higher until November. This means only a third of cases will be reported in July to September.

Campylobacter disease transmission reflects chicken flock infection rates and human behavior such as barbecues and outdoor activities, both of which are dependent on weather and likely to alter in a changing climate.

Researchers said results likely over-estimate the future number of cases as public health systems will adapt to higher incidences by taking stronger measures to reduce incidence.

“Establishing how extreme weather events and climate changes affect campylobacteriosis can form the basis of well-guided early warning systems in vulnerable areas and better targeting of prevention and control measures, potentially reducing the public health and economic impact of Campylobacter in these areas.”