Three patients were exposed in Serbia and took pork back to France where they shared it with relatives and friends. About 47 people were given the meat in France and Serbia and 20 cases of trichinellosis were reported, nine in France and 11 in Serbia. Nine of them were female.

While cases in Serbia were moderate, those in France were severe, as all patients were hospitalized but none had major complications. One reason for this could be cases in Serbia ate mainly smoked pork, while dried pork was consumed in France. The outbreak showed smoking and drying are not sufficient to inactivate the parasite, according to researchers in the journal Eurosurveillance.

In 2017, 15 European countries reported 224 cases of trichinellosis, of which 168 were confirmed. Incidence in France is low, with only 68 cases reported from 2001 to 2016. Among these, 38 were related to eating infected meat abroad or that was illegally imported. However, Serbia is a high-risk country for trichinellosis. During 2001 to 2016, there were 2,897 cases, including three deaths in 2005.

Suspected meat in the 2017 outbreak came from three backyard pigs bred on a family-owned small farm in a village near Belgrade. Meat from one pig was prepared as canned food — sausages and dried meat — the second was cooked and the third prepared as smoked food.

French patients
A woman in her 40s was hospitalized for two weeks on Feb. 7, 2017, near Paris. Her symptoms began 11 days before consulting a doctor. A relative, a man in his late 30s, and her friend, a woman in her 60s, were hospitalized around two weeks later for seven and nine days respectively.

Diagnosis of trichinellosis was delayed from the end of January to Feb. 24 because of the time in seeking treatment, consulting physicians not familiar with the rare disease in France and as antibodies usually appear a few days after onset of the acute phase of the disease, leading to frequent negative serology at the start of symptoms. Some patients of Serbian origin were not fluent in French, which could explain misunderstanding of symptoms.

During the Christmas holidays, the patients had travelled to Serbia where they ate pork from the farm on three occasions and took meat from the first two pigs back to France. They shared the meat near Paris with 19 people between Jan. 9 and 20, 2017. In Serbia, 25 people ate meat from three pigs during meals between Dec. 31, 2016, and Jan. 15, 2017.

Among the 19 others exposed in France, seven were asymptomatic and 10 had clinical signs compatible with trichinellosis from mid-January to end of February. Trichinellosis was excluded in four symptomatic people. Six additional patients identified by the survey had a confirmed infection. For French patients the incubation time was between 16 and 28 days and all needed hospital treatment.

Pork meat was positive for Trichinella larvae, identified as Trichinella spiralis. Sausages and dried meat contained 51 and 62 larvae per gram, respectively. The mouse bioassay was negative, suggesting the larvae were not infectious when the meat was analyzed – two months after first consumption.

Serbian investigation
Among 25 people exposed in Serbia, 11 had trichinellosis. Those infected were family members of the first two patients and eight family guests.

During examinations at a clinic in Serbia in early March, symptoms of Trichinella infection had passed. Described symptoms began between Feb. 10 and 18 and were mild. Incubation time was estimated between 34 and 42 days.

Serbian cases could have had previous contact with Trichinella parasites and developed an immunity, leading to a longer incubation and milder symptoms in cases of reinfections, according to the study.

Presence of Trichinella larvae was detected in samples of smoked and fresh meat from the third pig. The larval burden ranged from 21 to 61 larvae per gram.

According to the farmer, meat from the outbreak was sent after slaughter for analysis to a veterinarian who informed the family by telephone that it was not infected, but did not send official written confirmation. Although analysis was by trichinelloscopy, a non-recommended method, it would have been sensitive enough to detect the high larval burden of the meat.

When comparing trichinellosis in both countries, researchers found a few differences such as most French patients were older than Serbian ones as eight of nine compared to four of 11 were above 30 years old, so had more risk of complications.

In the outbreak, smoked meat was prepared in less than 20 days, sausages were exposed only to quick cold smoking and dried meat was prepared by quick dry salting. Drying time may not have been long enough to inactivate the larvae, while smoking may have inactivated more larvae by inducing a quicker dehydration. The longer incubation time of trichinellosis in Serbian versus French cases suggested the infectious dose was lower in smoked pork.

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