A gastroenteritis epidemic sweeping France has hit oyster farmers in Brittany after the virus was found in shellfish.

Health authorities have banned the fishing and selling of oysters in the bay around Mont-Saint-Michel and other shellfish farming areas on France’s north-western coast until further notice.

The ban follows the discovery of norovirus – also called the winter vomiting bug – in shellfish and is the fifth to be announced covering seven zones since mid-December.

Oyster farmers have blamed pollution along the coast on sewage that has not been fully treated.

The regional shellfish committee, the CRC, says almost half the oyster farms in the south Brittany region – 150 out of a total of 330 – are affected by the bans.

Philippe Le Gal, the president of the regional CRC said: “The gastroenteritis phenomenon has been around since Christmas. The sewage isn’t always properly treated by the purification plants and the virus ends up in the sea, especially after the rains we have had.”

He called for the local authorities to investigate the cause of the contamination.

In a petition launched on 1 January, an alliance of oyster farmers called for “immediate action to deal with the ecological emergency and substantial compensation for all oyster farmers”.

“This is a real economic and human disaster imposed on us by a state that is incapable of protecting our magnificent coastline,” it wrote.

Le Gal told Ouest-France newspaper: “The (fishing and farming) zones at potential risk have been closed. Suspect oyster batches have been withdrawn from sale. Those that remain don’t pose a risk. Now we have to understand what’s going on.”

The producers have blamed local officials who “boast of welcoming more and more residents and tourists, signing building permits with a vengeance while forgetting to take care of the management of human waste and sanitation.

“Astronomical quantities of polluted water are poured into the sea without raising the eyebrows of the authorities and are poisoning our coasts and oysters, which are now laden with the human gastroenteritis virus, norovirus. The water of the Morbihan coast is now unhealthy. Oyster farmers and their coastline are dying in total indifference,” they wrote.

The local police prefect said it was a temporary crisis caused by an unusual combination of low temperatures and heavy rains, which had caused sewage overflows into local rivers.

Patrice Faure told Agence France-Presse he would be meeting the agriculture minister to call for a speeding up of research to find a “rapid decontamination process” for the oyster farms and to discuss compensation.