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Ciguatera, a disease of the warm seas

A tiger shark has just been captured on the west coast of Reunion, the first since the green light was given to capture sharks on August 10. His entrails should reveal whether or not he was a carrier of ciguatera. After two attacks by surfers, including a fatal one on July 23, the hunt for large predators is open in order, officially, to screen for ciguatoxins – a real plague responsible for the most frequent food poisoning due to seafood products in the world – and to track other marine toxins, as well as any heavy metals.

It is in the name of this poisoner that the consumption of tiger sharks and bull sharks was banned on the island at the end of 2009. Fishing had lost its interest, which had undoubtedly allowed these two species, no protected, to thrive near the coasts. Highlighting the “ciguatera risk” to authorize shark hunting, as the prefecture does, therefore appears to be common sense.

Except that sharks are not legion on the Reunion coast. As proof, it turns out to be very difficult to capture them, whether for the “take” – according to the administrative terminology – before dissecting them, or to mark them using a probe which will make it possible to better understand their behavior. In nine months, of the 80 specimens planned, only 23 could be equipped with this acoustic equipment inside their stomachs.

In addition, ciguatera is hardly present in these coastal waters. On average, ten to twenty people are affected each year in the region, most of them from eating imported fish. This reduces to very few the number of sick victims of a locally caught seafood product. By way of comparison, there are between 50,000 and 100,000 cases per year in French Polynesia, an assessment probably below the reality.

POISONING WITH VARIOUS SYMPTOMS

Rarely fatal, ciguatera, for which there is no cure, is a form of poisoning with various symptoms: vomiting, diarrhea, reversal of sensations of hot and cold, violent itching – it is also called scratch – , cardiovascular problems. The body can take months to get rid of concentrations of toxins.

“In the atolls, some inhabitants will be affected seven or eight times during their lives and will not declare it systematically, but many others turn away from fish”, observes Dominique Laurent, biologist at the Research Institute for Development (IRD). He began to study ciguatera twenty years ago, in New Caledonia and then in Tahiti. “In the past, the Polynesians’ pantry was in the lagoon, they drew their food there for all meals. Now, they tend to rely on corned beef, chicken… and obesity is progressing”, he observes. Suffice to say that the public health problem is not identical in Saint-Denis-de-La-Réunion and in Papeete.

Mr. Laurent is at the origin of the start of a monitoring network between the former French DOM-TOMs. The Caribbean zone is also concerned, as are all the warm seas in intertropical latitudes. His initiative led in 1992 to the creation, in Réunion, of the Agency for Marine Research and Development (Arvam-Océanologie), whose program consists precisely in tracking down cyanobacteria and other marine toxins around Réunion and Mayotte. . “There is a significant endemicity of ciguatoxins on the Mascarene plateau, a vast area stretching from the Seychelles to Mauritius, and of which Reunion forms the southern tip, but we did not find any in the samples. carried out in the neighboring lagoons”says Jean Turquet, deputy director of Arvam.

The file is sensitive. On the one hand because the question of sharks heats the spirits locally. On the other hand because the absence of ciguatoxins in large fish at risk is perhaps also indicative of a phenomenon of overfishing near these coasts.

“NO WAY TO DETECT THEM”

Let’s go up the ecological chain. It all starts with the death of coral reefs under the action of man – through mass tourism or during the digging of a port, for example – or cyclones. The land will then be colonized by an algae itself conducive to the development of Gamberdiscus, a micro-alga of the dinoflagellate family. It is she who secretes ciguatoxins. The grazing fish feed on it, before being in turn eaten by larger congeners that end up in the stomachs of large predators: groupers, barracudas, moray eels, loaches, jacks, sharks…

Gradually, the concentration of toxic microorganisms increases, and so does the dangerousness. “Unless you give the house cat a taste of the fish, there’s no way to detect them,” slips mischievously Dominique Laurent.

With Mireille Chinain and her scientific team from the Louis-Malardé Institute in Papeete, Dominique Laurent often travels to remote atolls where food is closely linked to the sea. Their missions lead them both to take samples from the corals and fish and to cross-reference their knowledge with that of local populations who know which areas are infected. Biologists have come to take an interest in traditional remedies and to test, in association with the Louis-Pasteur Institute in New Caledonia, the active ingredients of around forty plants.

This is how the IRD filed a patent for a detoxifying ciguatera molecule, the rosmarinic acid contained in a shrub called false tobacco. “We couldn’t go to the experimental stage due to a lack of resources,” regret Dominique Laurent. A few months away from retiring, he deplores the few researchers who devote themselves to the study of the tropical toxin and the lack of interest from the pharmaceutical industry.

That could change. Probably favored by global warming, ciguatera, like other emerging biotoxins, is in the process of extending its range of action.

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