A nationwide Salmonella Enteritidis outbreak in France sickened 81 people between May 3 and Aug. 12, and investigators have confirmed contamination in pumpkin seeds sold at Grand Frais supermarkets, according to a study published in Eurosurveillance.

The median age of patients was 35, 56 were female, eight were hospitalized and two visited emergency departments. On July 30, the National Reference Centre for E. coli, Shigella and Salmonella at Institut Pasteur, together with Santé publique France, flagged an increase in a genomic cluster of Salmonella Enteritidis — 49 isolates since May.

Santé publique France interviewed cases by phone using a 25-page trawling questionnaire covering food and environmental exposures. Hypothesis-generating interviews showed most patients shopped at Grand Frais, but no specific food item emerged at first. As the investigation continued, an unusually large proportion of patients reported eating different nuts and seeds.

Pumpkin seeds complicated the work. They have a long shelf life, are often eaten raw and appear in snack mixes, all of which hampers both epidemiological questioning and traceback.

The breakthrough came from outside France. In August, investigators posted a message on EpiPulse, the online platform where European health authorities exchange outbreak information. Austria reported a food isolate obtained in June from pumpkin seeds imported from Poland with a genomic profile identical to the French reference isolate. Follow-up interviews confirmed pumpkin seed consumption, alone or in mixes, for 21 of 24 patients.

Traceback work identified one pumpkin seed producer supplying Grand Frais. That firm shipped seeds in large bags bearing a lot number to two French contractors, which repackaged them into smaller bulk containers or pre-packaged them for retail sale — alone or in mixes — at the supermarket chain.

Home sampling sealed the link. Investigators collected pumpkin seeds in original Grand Frais packaging from the homes of three cases; all three samples tested positive for Salmonella Enteritidis, and sequencing confirmed clustering with patient isolates.

In late August, the supplier recalled and withdrew pumpkin seed batches that had been on the market since March. Onsite inspections found two surface samples at the processing plant positive for Salmonella Enteritidis; sequencing of those isolates is underway.

The implicated seeds were grown from seedlings imported from Austria and cultivated and processed in France. Traceback has not established any link between the French producer and the contaminated Polish pumpkin seeds identified by Austria.

The outbreak's reach extended across borders. Exports were traced to Belgium, Switzerland and Luxembourg through the EU's Rapid Alert System for Food and Feed. Switzerland and Belgium reported no related cases, but Luxembourg recorded a related human isolate on Sept. 3 — a person who fell ill in late August after eating mixed nuts and raisins containing pumpkin seeds. Sampling of leftover product confirmed contamination with the outbreak strain.

The United Kingdom and Portugal also reported isolates in the cluster. The UK patient had traveled to France from mid-July to early August and developed symptoms the week after returning, but investigators have no information on pumpkin seed consumption. The Portuguese isolate dates from May 2026, with no established link to France or pumpkin seeds.

The study also flags a structural weakness in outbreak detection: the delay between strain isolation at primary laboratories and sequencing results runs about four weeks, meaning case counts likely lag real-time transmission. With plant surface samples still being sequenced and the Polish connection unresolved, French investigators have yet to close the file on how contamination entered the supply chain.