Health officials in a city in Finland are investigating several cases of illness caused by contaminated oysters.

Food poisoning in Helsinki is suspected to be related to eating in different restaurants and at an event since the beginning of February. This past week, officials reported at least 20 people were affected but updates in local media suggest there are around 100 illnesses.

Investigators have tested food from restaurants and taken patient samples and have found norovirus. Some of those sick reported eating oysters.

A few restaurants have already been inspected after suspected epidemics and oyster importers have started to issue withdrawals and recall.

Food safety officials in Helsinki asked people who had eaten oysters and then fallen sick to contact them.

Norovirus is the most commonly identified cause of foodborne outbreaks in Finland. Between 2017 and 2021, oysters caused 11 norovirus outbreaks in which more than 110 people fell ill, according to the Finnish Institute for Health and Welfare (THL).

In October 2022, five people fell sick in the country after eating oysters from France contaminated with norovirus.

Wider problem
Finland is the latest country to report illnesses caused by shellfish. Harvesting areas have been closed and outbreaks reported in France although it is unclear how many people were sick.

Twenty people were ill in Belgium in February due to norovirus in oysters from France. Another outbreak was linked to oysters from the Netherlands but authorities did not report how many people were affected.

Two outbreaks in Denmark from late 2022 to early 2023 were caused by shellfish. The first with 19 people sick involved oysters from France but originating in Ireland. The second with 73 cases were linked to oysters from Norway.

Oysters from Ireland caused at least 16 illnesses in Hong Kong earlier this year. Recall notices of oysters due to norovirus have also been published by agencies in Italy and Luxembourg.

The incubation period for illness caused by norovirus is 12 to 48 hours. Symptoms include sudden onset of cramping, abdominal pain, and nausea, followed by vomiting. Most people also have diarrhea. They usually last 12 to 72 hours.

Handwashing with soap and water is key to fighting the virus. Affected staff in the food industry should stay away from work for at least two days after symptoms have stopped to avoid spreading the infection. Norovirus can be transmitted directly from one person to another and through contaminated surfaces.