Bad dive or bad fish?
By Nathan Welton
Rodale's Scuba Diving
October, 2002

If you’ve ever burned yourself with an ice cube after eating barracuda, you may have been a victim of ciguatera, which is an illness caused by eating predatory reef fish. It is unfortunately prevalent in some of the world’s most popular dive locations, and to confound matters, some of its symptoms are easily confused with decompression sickness.

A dinoflagellate called Gambierdiscus toxicus that grows with tropical algae produces ciguatoxin, the chemical that causes ciguatera. Ciguatoxin concentrates in the tissues of tropical predators – specifically barracuda, snapper, groupers, amberjack, sea bass, and Spanish mackerel – when they eat grazers that feed on the algae.

Fisherman can't identify dangerous fish because the poison has no effect on infected creatures, and the toxin itself is odorless, tasteless and unaltered by cooking. And although various chemical test kits exist, Woods Hole Oceanographic Institute scientist Dr. Don Anderson says their accuracy is questionable.

According to Dr. Lora Fleming, a ciguatera expert at the University of Miami, problems usually begin with symptoms of classic food poisoning: diarrhea, vomiting, and nausea. However, without treatment, the symptoms progress into bizarre neurological disorders. “Generally, your nerves start acting up and sending mixed information,” says Fleming. “You feel like bugs are biting your legs or your teeth hurt.” Other ciguatera symptoms include confusing hot and cold, pain on urination, and painful sexual intercourse. But what can be most perplexing is that victims suffer some combination of vertigo, weakness, fatigue, numbness, and tingling, which are reminiscent of decompression sickness.

To differentiate between ciguatera and DCS, divers need to be aware of when and what they last ate and when they last dove. While both illnesses’ symptoms can be delayed for up to a day, decompression sickness usually appears within an hour of surfacing and ciguatera usually begins with 30 minutes to four hours after a meal. Ciguatera-specific symptoms include diarrhea and no headache; decompression sickness is the other way around. According to Fleming, sensory problems like temperature reversal are highly indicative of ciguatera poisoning, and she adds that because ciguatera’s attack rate is 100%, victims should look for afflicted dining buddies. Of course, be sure to ask dive operators which fish to avoid in the area, and if you become sick, seek medical attention promptly and tell the doctor your dive profile and your eating history.

Fleming explains the best treatment is a drug called mannitol, which has been known to revive victims within minutes who have been comatose for days. But it’s better not to get sick in the first place. Abstinence from carrier species is possible, and some regions have prohibited the sale of certain fish, but this has its drawbacks: French Polynesia alone suffers an estimated burden in the millions of dollars each year because of such prohibitions.

Currently, the simplest solution scientists can offer is to avoid larger individuals of suspect species. Predators heavier than four pounds are generally considered unsafe, since they’ve had time to acquire dangerous amounts of toxin. Avoiding the viscera and roe – usually the most tainted parts – of suspected fish is also a helpful way to avoid problems.