Ostreopsis: a toxic microalga causing health problems

Ostreopsis is a microscopic alga found in the Mediterranean and, more recently, along the coast of the Basque Country when the water temperature exceeds 20°C. It produces toxic compounds that can be carried by sea spray and cause poisoning, with symptoms such as cough, headache, fever and sore throat. You only need to be within a few metres of the sea to be affected. This article provides some explanations.

What is Ostreopsis alga and when do its blooms occur?

Ostreopsis is a microscopic alga of tropical origin. Ostreopsis blooms mainly occur in summer, when the water temperature rises above 20°C.

Where is Ostreopsis found?

After arriving in the Mediterranean some 20 years ago, its range gradually expanded and it has now been present in Basque coastal waters for several years.

Is Ostreopsis toxic and if so, what are the symptoms?

Ostreopsis blooms can cause poisoning, leading to flu-like symptoms, respiratory problems, headaches and fever. Signs appear a few hours after exposure by the sea, and then disappear within a few days.

The Regional Health Agency (ARS) and the Bordeaux poison control centre are currently centralising reports of poisoning cases.

How does Ostreopsis poisoning occur?

Anyone spending time on the seashore – residents, tourists, local workers, sports enthusiasts, etc. – can be affected.

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

Rare cases of food poisoning have been reported outside Europe, but the link with Ostreopsis remains uncertain. Based on current knowledge, the Agency believes that it is not possible to rule out the risk of oral exposure from eating seafood contaminated by Ostreopsis toxins, especially when it is not gutted before consumption.

What are the most at-risk population groups?

People with respiratory conditions are most at risk of developing symptoms.

People 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.

How do you know if there is an Ostreopsis bloom?

Ostreopsis can accumulate on the surface of the water to form "blooms", either as clumps or brown slicks covering an area of up to several square metres. However, these slicks can quickly disappear, and just because they are not visible does not mean there is no risk. If a bloom can be seen, this indicates a high risk of poisoning.

The only distinctive sign of the presence of this microalga is a metallic taste in the mouth or an unusual odour.

How can you avoid poisoning?

In the event of an Ostreopsis bloom, the Agency reiterates the good practices to be followed by anyone spending time at the seashore or in areas for supervised bathing, water sports and other aquatic activities, as well as by workers:

Take a shower as soon as possible after each swim. In the event of airborne exposure (i.e. without swimming), ensure that the mucous membranes of the face are thoroughly rinsed
Avoid activities on the beach or in the water near or within rocky areas, which are favoured sites for Ostreopsis blooms and accumulation.
If symptoms worsen or persist for more than three days, see a doctor or call the Nouvelle-Aquitaine poison control centre.
As a precaution, do not collect shellfish or other seafood.
People with respiratory conditions are advised not to spend time at the seashore or engage in water sports and other aquatic leisure activities. If this is unavoidable, the Agency advises wearing an FFP3 mask.
Residents living by the shore should keep their sea-facing doors and windows closed.

Visitors and workers are also asked to follow the instructions and recommendations issued by the local authorities, which are displayed at the entrance to the beaches.

Is there any monitoring of Ostreopsis?

The Basque Country Urban Community and Ifremer take regular water samples to monitor the spread of this microalga on beaches where blooms are reported on a recurring basis.

What has ANSES done to address this issue?

The Agency has provided guidance on managing Ostreopsis blooms on the Basque coast. It reiterates that its proposed recommendations are neither regulatory nor binding.

ANSES has assessed the risks to human health associated with Ostreopsis blooms on the Basque coast. In its work, published in 2023, the Agency proposed a strategy for monitoring Ostreopsis in supervised bathing areas and in the adjacent areas used for water sports and other aquatic leisure activities. This strategy was then updated in 2026. In its work, it also suggested a series of measures to protect the most vulnerable groups, particularly people working on or near the beaches: reorganisation of working hours, wearing of protective equipment, etc.

In addition, it recommended stepping up communication to raise awareness among anyone spending time on the Basque coast of the risks associated with Ostreopsis blooms, in particular by displaying signs at the entrance to sites affected by the blooms, and informing the public of the risks of Ostreopsis and who to contact in the event of poisoning. It also proposed introducing specific signage for Ostreopsis, whenever blooms and/or poisoning cases are reported.

To ensure a rapid response from those responsible for bathing waters in the event of an Ostreopsis bloom, the Agency recommended centralising reports of poisoning cases and algal blooms within a single body.

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