A pilot study in France has pointed to a significant under-reporting of shellfish-related foodborne outbreaks.

The objectives of the research were to quantify and describe the health impact of shellfish-related outbreaks as well as the circumstances of occurrence in Brittany, with the end goal of reducing their frequency.

An online questionnaire looked at shellfish consumption between November 2021 and April 2022, the practice of shore fishing, and the occurrence of shellfish outbreaks over the past five years.

A total of 438 responses were received between May and July 2022. Most respondents were women, the median age was 46, and 21 percent practiced shore fishing. From November 2021 to April 2022, 93 percent of participants reported having eaten shellfish. Almost two-thirds ate shellfish raw and cooked, while a quarter only ate cooked shellfish. Scallops and mussels were the most consumed, but oysters were the top raw type of shellfish eaten.

Official statistics and survey feedback
Norovirus is behind most shellfish outbreaks, and the majority involve bivalves eaten raw, such as oysters. Outbreaks mainly occur from November to April, with contributing factors including increased consumption and heavy rains, which cause sanitation systems to overflow and contaminate production areas.

Six people fell sick in Norway from norovirus in oysters from France, according to a RASFF notification posted earlier this month.

38 respondents reported at least one outbreak linked to shellfish from 2017 to 2022. Between November 2021 and April 2022, nine outbreaks were reported by nine respondents, including eight involving raw oysters.

Official statistics show 58 shellfish outbreaks in Brittany between 2017 and April 2022, including 39 linked to oysters. From November 2021 to April 2022, the monitoring system identified five alerts for shellfish outbreaks — all linked to oysters — and four for norovirus.

The nine outbreaks identified by participants suggests under-reporting in official systems, said scientists.

Symptoms and incubation times reported by respondents were consistent with norovirus and included diarrhea, vomiting, and abdominal pain.

Respondents said no outbreak had been investigated by regional health agencies or the Departmental Directorate for Population Protection (DDPP). Many people do not seek medical attention because of the short duration and sometimes mild severity of symptoms. Most shellfish had been bought commercially, but some cockles and clams had been personally fished.

Because of the low number of outbreaks reported in the survey, no production location could be identified as at high risk of contamination.

“This pilot study supports the hypothesis of a significant under-reporting of shellfish foodborne outbreaks at the regional level and, therefore, probably at the national level. Further consideration should be given to better quantifying the health impact of shellfish outbreaks to inform the shellfish industry and stakeholders in charge of coastal waters,” said researchers.