Swedish public health officials have reported an increase in Yersinia infections in recent weeks.

Since the second week of January, more than twice as many people have fallen ill with yersiniosis compared to the same period in past years.

An increased number of patients has been observed in the regions of Stockholm, Västra Götaland and Halland, which account for 33 of 48 cases reported since Jan. 11.

Most of those sick are between the ages of 11 and 40 years old and are women. Five girls and one boy aged zero to 10 years old have also been infected.

Folkhälsomyndigheten, the Swedish Public Health Agency, and local infection control units in affected regions are trying to identify the source of infection by interviewing patients.

The agency is also collecting samples and has detected Yersinia isolates from patients in these regions. The isolates will be subject to whole genome sequencing to clarify if people have been affected by a common source of infection.

A recent Yersinia enterocolitica O3 outbreak in Norway that affected 10 people was traced to salad.

Between 200 and 300 Yersinia infections are reported annually in Sweden. In mid-2019, more than 20 people fell ill in the country in a Yersinia outbreak. A few months earlier, another outbreak, that also affected Denmark, sickened 37 people and was linked to imported fresh spinach from Italy or Spain.

Yersinia stats in Europe
Meanwhile, 29 countries reported 7,048 confirmed yersiniosis cases in Europe in 2019, according to officials.

The highest rates came from Finland, Lithuania and the Czech Republic, according to a report published by the European Centre for Disease Prevention and Control (ECDC).

Yersiniosis reporting is mandatory in most countries but voluntary in Belgium, France, Greece, Italy and Luxembourg and based on another method in the United Kingdom. No surveillance system exists in the Netherlands. The French, Italian and Spanish systems do not have national coverage.

Germany had the most cases with 2,154 followed by France with 1,135. These countries accounted for almost half of all confirmed Yersinia infections in Europe. Infants and children up to four years old made up almost one-quarter of confirmed cases in 2019.

The ECDC said that from 2015 to 2019, the trend for confirmed yersiniosis cases remained stable. Yersiniosis was the fourth most common foodborne disease in the EU in 2019.

Yersinia enterocolitica caused the majority of infections but 10 countries reported 74 Yersinia pseudotuberculosis cases in 2019. All 14 yersiniosis outbreaks reported to the European Food Safety Authority (EFSA) were from Yersinia enterocolitica. They involved 149 patients across seven countries.

Many the infections are thought to be related to eating undercooked contaminated pork or cross-contamination of other foods during handling and preparation of raw pork meat. Outbreaks also have frequently been linked to raw vegetables and ready-to-eat vegetable products such as lettuce and carrots that have been subjected to long cold storage.