Oysters from different sources have been linked to illnesses in several countries in recent weeks.

There have been several product withdrawals and recalls plus the closure of production zones in France due to the detection of norovirus.

When announcing the closure of harvesting areas, French authorities reported illnesses but did not say how many and Santé publique France has yet to provide information on cases to Food Safety News.

The problem and the impact on producers has been raised in the French Senate with politicians saying 15 production areas have been closed. They asked what measures the government intended to take to support the sector in the current situation and to prevent future seasonal contaminations.

Recall notices of some oysters due to norovirus have been published by agencies in Belgium and Luxembourg.

Danish situation

Two outbreaks in Denmark have been caused by shellfish. The first with 19 people sick involves oysters from France but originating in Ireland. The second with 73 cases has been linked to oysters from Norway.

Nikolas Kühn Hove, head of crises management at the Danish Veterinary and Food Administration, said it was still early in the investigation.

“The majority of cases fell sick one to four days after consumption on 31 December. Age range and gender information is still pending. Based on the preliminary investigations, Norovirus is suspected as the agent for the two outbreaks but confirmation is pending,” he said. 

“People bought oysters either from retail chains or in a web shop. It is too early to point to any food business operator involved outside Denmark. Oysters related to one outbreak have been recalled from the Danish market. We have yet no information on actions in other countries directly related to the Danish case.”

People sick in Hong Kong

Risk factors for shellfish-related norovirus incidents include cold weather leading to low water temperatures, high prevalence of norovirus in the community, and high rainfall causing sewage system overflows.

Oysters from Ireland caused at least 16 illnesses in Hong Kong. Eight men and eight women, aged 25 to 36, developed abdominal pain, diarrhea, dizziness, fever, general weakness, nausea, and vomiting about 21 to 44 hours after attending a lunch banquet at a restaurant in late December. Import of raw oysters harvested in Carlingford Lough in Ireland has been suspended.

The Centre for Food Safety (CFS) of the Food and Environmental Hygiene Department has also told industry to stop importing raw oysters harvested in an area of France.

As oysters feed by filtering a large volume of seawater, pathogens can accumulate if they are grown in or harvested from contaminated water. Susceptible groups, such as pregnant women, young children, the elderly and people with weakened immune systems or liver diseases, should avoid eating raw oysters, said health officials.

Pathogens in mollusks in Poland

Meanwhile, researchers have evaluated the microbiological contamination of raw bivalve molluscan shellfish (BMS) on the Polish market.

A total of 1,000 raw bivalve mollusks were collected during 2009 to 2018 including clams, mussels, oysters, and scallops and findings were published in the journal Foods.

Salmonella was detected in 31 samples, and half of the bacterial isolates were Salmonella Typhimurium. Listeria monocytogenes was isolated from 18 BMS and more than 15 percent of bivalve mollusks were contaminated with Staphylococcus aureus.

Salmonella-contaminated BMS mostly originated from the Netherlands. Most of the Listeria contaminated samples came from the Netherlands and France.

Vibrio parahaemolyticus was identified in 261 or 242 samples depending on the method used. Most samples came from the Netherlands.

A total of 60 bivalve mollusks were contaminated with more than one pathogen simultaneously. Bacteria were more likely to be identified during the warmer period compared to samples analyzed in colder months. 

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