It sounds more like the opening of a James Bond movie than a scene from a restaurant near you: A couple sits down to enjoy a fancy seafood dinner, during which they unknowingly ingest a toxin they can’t taste, see or smell. Within a few hours — sometimes minutes — they succumb to a variety of bizarre symptoms.

After dinner, the victims may have numbness in their fingers, toes or mouth. Difficulty breathing. Vomiting, diarrhea, muscle aches, drooling. Some may even be left with lingering neurological symptoms, such as the reversal of hot and cold sensations (imagine sipping hot coffee and having it feel like ice on your tongue), or the feeling that their teeth are falling out.

This is ciguatera, a nonbacterial foodborne illness from seafood affecting between 50,000 and 500,000 people each year. Although it’s seldom deadly, it can cause people to experience severe gastrointestinal and neurological symptoms. The poison has no antidote. And it can’t be neutralized by cooking, freezing or sterilizing the fish it’s hidden inside.

Ciguatera is caused by microalgae (dinoflagellates like Gambierdiscus and Fukuyoa) that bloom near coral reefs in tropical and subtropical seawater. These microscopic organisms produce ciguatoxins, which are apparently harmless for fish but bioaccumulate as they move through the food web. That means there’s a small amount found in herbivorous fish that eat the algae, more in predatory fish that eat the algae-eaters, and so on until humans eat a large, very toxic, predatory reef fish.

Eating reef fish like snappers may cause ciguatera food poisoning. (Credit: Rob Roberts)

Outbreaks on the Rise

Although many people in northern climes have never heard of it, ciguatera fish poisoning is an age-old affliction. Outbreaks were recorded centuries ago by Homer, Alexander the Great and Captain Cook. Traditionally, ciguatera occurred in the Pacific, Atlantic and Indian oceans between 35 degrees north and south latitude.

But now, due to climate change and the increased globalization of seafood, ciguatera poisoning is expanding outside of these boundaries. In recent years, ciguatoxin-producing algae have been found off Mediterranean islands, as well as Atlantic islands off Spain and Portugal. Japan has also reported several cases of ciguatera poisoning.

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The expansion of ciguatera to higher latitudes may be related to warming oceans, which allows the algae to expand its range and increase its growing season. Research in French Polynesia shows that a rise in sea temperature corresponded to an increase in Gambierdisus species density 13 to 17 months later, followed three months later by outbreaks of ciguatera fish poisoning.

“It’s a general trend that tropical species are moving to high latitudes as oceans warm, which indicates Gambierdiscus will move, too, and that we’re likely to have more ciguateric fish in the future,” says Lucia Solino, a researcher with the Portuguese Institute for Sea and Atmosphere.

This poses new risks for seafood consumers and new challenges for the global fishing industry. More than 400 species of reef fish can carry ciguatera, many of them high-value commercial fish like grouper, sea bass and red snapper. Since many fisheries’ stocks have been depleted in the Northern Hemisphere, tropical fish are increasingly exported to meet the growing consumer demand for seafood.

Various snapper species swimming near a coral reef in the Caribbean. (Credit: Rob Roberts)

Preventing Ciguatera Poisoning

To help people avoid eating toxic seafood, most countries’ health departments have guidelines or regulations aimed at avoiding certain species and sizes of fish. Large predatory reef fish are typically considered higher risk for ciguatera poisoning, like barracuda or Spanish mackerel. For instance, Spain banned the sale of amberjacks larger than 14 kilograms from the Canary Islands unless the fisherman first freezes the fish and sends in a sample for testing.

However, recent studies have cast doubt on the correlation between ciguatoxin concentration and fish size. This could be because a few species of Gambierdiscus are up to 1,000 times more toxic than others, so eating a small snapper laced with a particularly toxic strain might make a hapless consumer fall ill. Plus, ciguatoxins aren’t relegated to fish — popular seafood like crab, lobster, sea urchins and clams have all tested positive for ciguatoxins in the past. That means guidelines focused on fish species and size might not be adequately protecting consumers.

In addition, regulatory agencies like the U.S. Food and Drug Administration and the European Food Safety Authority set guidelines based on acute toxicity — the level of a single dose that would cause ciguatera poisoning. But it’s possible that chronic levels of toxicity may also accumulate in our bodies and eventually cause ciguatera if people repeatedly eat enough fish with low levels of ciguatoxins.

Parrotfish are herbivorous reef grazers and may accumulate less ciguatoxins than predatory species like snapper or grouper. (Credit: Rob Roberts)

That’s just one of the many unknowns facing scientists like Mike Parsons, a marine scientist at Florida Gulf Coast University, who is researching ways to prevent ciguatera poisoning.

“It’s extremely complex. These molecules bio-transform as they move through the food web, so you’re basically dealing with a chimera. It’s a moving target,” says Parsons.

Parsons believes that mitigating ciguatera impacts requires a three-pronged approach: remote sensing models that predict and monitor blooms of ciguatoxic microalagae; rapid-screening methods for easily testing fish; and treatments that deactivate the neurotoxin when people suffer from ciguatera. The first is the closest to becoming reality. Models are being developed that use satellite-based data on sea surface temperature to predict where “pulses” of Gambierdiscus might occur. Currently, though, we have no way to track the many reef fish that accumulate these toxins or cure the people who eat them. Until we do, it might be best to stick to steak when perusing the dinner menu.