Expert assessment
1 min

Preventing cases of Ostreopsis poisoning on the Basque coast

Blooms of Ostreopsis microalgae have caused almost 900 cases of poisoning on the Basque coast since 2021. ANSES has recommended a series of measures to protect the populations most at risk, in particular professionals working on or near the beaches.

Being just a few metres from the beach is enough to be affected by toxic microalgae blooms

Ostreopsis is a microscopic alga of tropical origin that arrived in the Mediterranean around twenty years ago. The range of this microalga is expanding in France, with its presence being reported on the Basque coast in recent years. Ostreopsis blooms mainly occur in summer, when water temperatures exceed 20°C. The toxins produced by Ostreopsis can cause poisoning leading to flu-like symptoms, skin irritation and stomach problems. Signs appear a few hours after contact with the microalga or its toxins, then disappear within a few days.

The main route of exposure is via inhalation of sea spray. You only need to be a few metres from the sea to be affected.

How can I tell if there is a Ostreopsis bloom?

Ostreopsis can accumulate on the surface of the water to form "blooms". These can create brown slicks covering several square metres. However, these slicks can quickly disappear, and just because they are not visible does not mean there is no risk. The only distinctive sign of the presence of microalgae is a metallic taste in the mouth, even if they have only been inhaled and not ingested.

Exposure is greatest among workers

Professionals working on or near beaches, such as lifeguards and restaurant workers, are particularly affected. Prolonged exposure to Ostreopsis can cause the symptoms to persist for longer. Workers should be encouraged to report these symptoms to their occupational physicians. One general preventive measure could be the reorganisation of working hours, to limit the time professionals spend on the beaches during Ostreopsis blooms. ANSES also recommends the wearing of protective equipment, mainly masks and gloves, by the people responsible for taking water samples.

A tool to help local authorities manage risk situations

To protect the health of professionals and visitors to the beaches, the Agency has made available a decision tree for local authorities, particularly regional health agencies. This tool recommends measures that can be taken depending on the concentration of microalgae in the water or the number of poisoning cases reported. These measures range from informing the public, particularly at-risk populations and professionals, through to banning water sports and closing beaches. "People with respiratory problems are most at risk of developing symptoms," explains Carole Catastini, who coordinated the expert appraisal. "They should avoid being near the coast when there is an Ostreopsis bloom."

Monitoring should be stepped up

ANSES further recommends adapting the frequency of water sampling according to the microalgae concentration. Checks of bathing water quality are usually carried out every 15 days. This frequency could become weekly or even daily when Ostreopsis is detected, in order to closely monitor the situation, which can change in just a few days. "Similarly, we should not only be checking supervised beaches, as is currently the case, but also any beaches where there are water sports, whether swimming, surfing or sailing," explains Carole Catastini. "We also know that the presence of Ostreopsis can be very localised: it may be found at one end of the beach but not the other." As microalgae develop on macroalgae (seaweed) before floating to the surface, the Agency also advises taking samples from both the water and the seabed.

What about in food?

Rare cases of food poisoning have been reported outside Europe, but the link with Ostreopsis remains uncertain. As a precaution, the experts advise against collecting shellfish or other seafood when Ostreopsis blooms occur. "Fish must be gutted before being eaten, as the toxins accumulate in the digestive tract," explains Nathalie Arnich, who also coordinated the expert appraisal. 

To protect consumer health, ANSES has set a guideline value in shellfish, i.e. a concentration of toxins produced by Ostreopsis that must not be exceeded in these products