Ciguatera is a toxin produced by marine microalgae in the tropics, particularly in the Pacific Islands, Caribbean, and Indian Ocean Islands. Tropical marine fish can accumulate this toxin through their diet by eating algae or smaller fish. As big fish eat smaller fish, this toxin moves up the food chain, and can reach toxic levels in large fish.
When humans eat contaminated fish, they can develop a condition called ciguatera fish poisoning. The symptoms may include nausea, vomiting, tummy pain, diarrhoea, muscle cramps, and tingling around the mouth and in the fingers and toes. Some people also develop abnormal temperature sensation, where hot things feel cold, and cold things feel hot. These symptoms can start within 10 minutes of eating toxic fish, but sometimes take up to 24 hours. They usually last for a few days, but can occasionally persist for weeks or months.
Diagnosis is usually made based on the history of eating fish in ciguatera risk areas, and on the above symptoms. If portions of the fish are available, they can be tested for ciguatera toxin. In places where this condition does not occur, doctors may not be familiar with it, and the diagnosis is often missed. If you think you might have ciguatera fish poisoning, you should tell your doctor that you have eaten tropical fish in a risk area.
There is no specific treatment for ciguatera fish poisoning, but your doctor can help you manage symptoms such as nausea and vomiting. The majority of people recover spontaneously after a few days, and it is unusual to have any long-term problems.
You can't tell whether fish is toxic by looking at it or tasting it. Ciguatoxin is very heatstable, so cooking WILL NOT make the toxic fish safe to eat. The risk of a fish being toxic depends on where it is caught rather than on its species, and local fisherman may be also able to tell you which areas to avoid or which species of fish to not eat. If in doubt, it is best to avoid large reef fish such as barracuda and grouper - they are more likely to be toxic because they can accumulate toxins by eating lots of smaller fish.
The US government web site CDC has more information (external link).