Lethal fumes from decomposing seaweed have killed a horse in northern France.

The horse died after inhaling the fumes on a beach in Saint-Michel-en-Greve in Brittany last week. It suffered a fatal pulmonary oedema caused by the gases given off by the rotting seaweed.

The rider, a 27-year-old vet from Paris called Vincent Petit, was pulled from the beach after losing consciousness due to the fumes.

The rotting algae, known locally as lettuce seaweed, builds up a crust under which hydrogen sulphide grows. When the crust breaks it gives off lethal fumes that attack the respiratory system and can kill within minutes.

Several locals also been treated in hospital, including a council worker who slipped into a coma after clearing the beach from seaweed.

The beach has been closed and visitors are being warned to avoid beaches covered in seaweed.