Fish may be “brain food,” but some seafood can harbor natural toxins that have potentially serious effects on the nervous system, researchers say.

Such fish-borne illness is most common in rural coastal communities in the tropics and subtropics, where fish species likely to accumulate the toxins are a mainstay of the diet. In contrast, people in Western nations, where fish consumption tends to be low, may be unaware of the risks that certain seafood can pose, according to Dr. Geoffrey Isbister of Newcastle Mater Misericordiae Hospital in Waratah, Australia.

Travelers to tropical locales, Isbister told Reuters Health, may want to exercise caution and forgo exotic shellfish or large tropical fish.

Reporting in the April issue of The Lancet Neurology, Isbister and colleague Matthew Kiernan, of the Prince of Wales Hospital, Sydney, describe several forms of seafood poisoning that deserve greater recognition among fish lovers.

Ciguatera poisoning, caused by eating toxin-bearing fish from certain coral reefs, is the most widespread form of marine poisoning globally, according to the researchers. The ciguatera toxin is produced by algae and accumulates in certain fish via the food chain; it is not destroyed by freezing or cooking.

Reef-dwelling grouper, red snapper, moray eels, barracuda and mackerel are among the fish species most frequently implicated in the illness, which is endemic to Australia, the Caribbean and South Pacific.

Ciguatera is rarely fatal, but can cause neurological symptoms such as tingling in the mouth, hands and feet, numbness, joint and muscle pain, problems with muscle coordination, and a condition known as cold allodynia - in which a person feels a burning sensation when in contact with cold.

Gastrointestinal woes, including abdominal pain, diarrhea, nausea and vomiting, are the other hallmark of ciguatera.

According to Isbister, there have also been reports of people suffering lasting effects from the illness, such as fatigue and chronic joint and muscle pain.

Fish accumulate ciguatera toxins sporadically, and not all coral reefs or fish from reefs present a hazard. Indeed, Isbister pointed out, the potential for foodborne illness should not scare people away from eating fish.

“We should be aware of marine poisoning, which rarely occurs with particular fish,” he said, “(but) it shouldn’t prevent us increasing fish in our diet.”

Two other common forms of seafood-borne illness worldwide are shellfish and puffer fish poisoning. Toxic shellfish poisoning - usually from mussels, clams, scallops or oysters - accounts for about 1 percent of food-related illness in the U.S., according to Isbister and Kiernan. As in ciguatera, these cases are caused by natural toxins that shellfish accumulate from algae, as opposed to the more common type of shellfish poisoning caused by bacterial or viral contamination.

Most often, shellfish toxins cause so-called paralytic poisoning, which triggers numbness and tingling in the face, arms and legs, headache, dizziness and problems with muscle coordination - typically developing within two hours of eating. Rarely, severe poisoning can lead to muscle paralysis, respiratory failure and death.

According to the Centers for Disease Control and Prevention, the toxin behind paralytic shellfish poisoning comes from a type of red-brown algae that, in large concentrations, causes red streaks to appear in the ocean. In the U.S., the toxin may concentrate in shellfish living in the colder coastal waters off the Pacific states and New England.

Puffer fish poisoning, a potentially fatal condition, is most common in Japan, where puffer fish filet is considered a delicacy. It’s mainly sold in restaurants where specially trained and licensed chefs prepare the fish in a way that reduces the risk of poisoning. Still, cases continue to be reported every year - mainly, according to Isbister, in instances where unskilled cooks prepared the fish.

The toxin behind puffer fish poisoning, known as tetrodotoxin, rapidly causes symptoms such as numbness and tingling in the face and extremities, muscle weakness, dizziness and coordination problems. In life-threatening cases, the illness leads to paralysis throughout the body, respiratory failure and heart rhythm disturbances.

Tetrodotoxin can also accumulate in certain other fish, such as the less-than-attractive toadfish, which is the main source of tetrodotoxin poisoning in Australia.

“In Australia,” Isbister noted, “we have a saying, ‘Don’t eat an ugly fish.’”

SOURCE: The Lancet Neurology, April 2005.