The dangers of eating fish in the tropics were highlighted this week with news of a woman stricken with ciguatera poisoning after eating fish in Fiji.
This can be a serious illness, and can last for years. It is caused by a build-up of toxins in fish, similar to toxins that occur in shellfish around our coast, but is far more serious.
The first link in the chain are marine organisms called dinoflagellates, which are found in all aquatic systems. They are minute plankton in thousands of forms in both fresh and salt water.
Those found in warm, tropical seas cause the problem. They stick to algae, coral and weed and are eaten by small, herbivorous fish. These, in turn, are consumed by larger predators so the chain continues. The toxins accumulate in the flesh of consecutive fishes, and remain there permanently.
One dinoflagellate in particular, gambierdiscus toxicus, is the main culprit responsible for several similar toxins that cause ciguatera, so it is a gamble whether fish served in a meal contains the relevant toxins. Another complication is that the toxins are odourless, tasteless and heat-resistant so cooking does not kill them.
The toxins are more common in fish that live on and around reefs rather than those that pass through. So bass, barracuda, parrotfish, tropical red snapper, gropers, triggerfish and amberjacks are more likely to cause a problem than pelagic species such as tuna, wahoo, mahimahi and marlin.
Ciguatera toxin is found in 400 species of reef fish, so avoid all such fish. Ciguatera was first described by a surgeon's mate on Captain James Cook's ship, HMS Resolution, in 1774, and some researchers have suggested that outbreaks caused by cooling climatic conditions contributed to migratory voyages by Polynesians between 1000AD and 1400AD.
Folklore describes old-time Caribbean remedies for poisoning involving bed rest, an enema of guanabana juice, bleeding the gastrointestinal tract, "cleansing" the sick person with a dove, and drinking a tea made from mangrove buttons to flush toxins from the system. The success of such treatments in Cuba and Puerto Rico has not been substantiated.
In the Bahamas, Grand Cayman Islanders test barracuda by putting a piece of fish on the ground for ants to crawl over. If the ants survive the fish is safe to eat. In Northern Australia ciguatera can be a common problem and local lore includes two testing methods - the first says flies will not land on contaminated fish, and the second that cats will show symptoms after eating it. Another less common test involves placing a silver coin under the scales of suspect fish. If it turns black it is contaminated. The accuracy of these tests is not known.
An account by a linguistics researcher in Vanuatu indicates the local treatment: "We had to go with what local people told us: avoid salt and any seafood. Eat sugary foods. And they gave us a tea made from the roots of ferns growing on tree trunks. I don't know if any of that helped, but after a few weeks, the symptoms faded away."
It has also been suggested that sexual intercourse can transmit the problem to healthy individuals, and that affected breastfeeding mothers can pass it to infants. Symptoms of toxic poisoning from eating shellfish include numbness around the mouth, nausea and vomiting, but with ciguatera this spreads to more severe gastrointestinal problems including diarrhoea, headaches, muscle aches, hallucinations and a burning sensation. Symptoms last from weeks to years and in extreme cases long-term disabilities, but most people recover.
In many trips to the Pacific Islands and Australia, local people have routinely advised against eating reef fish. But other fish, like trevally, wahoo, sailfish, tuna and mahimahi are fine to eat safely. If in doubt, ask a local.