In many parts of the tropical oceans fish are poison. That might seem an unduly alarmist statement, but it is true of some fish. Ciguatera Fish Poisoning (CFP) affects tens of thousands of people every year. One authority estimated that 3% of all visitors to tropical destinations are affected. In this short piece, I will try to describe the problem, and what you can do to reduce the chances of being a victim. I am not a doctor, or a public health expert. I am relating what we do and the rational behind the decisions we have made.

If you have a technical, scientific, or medical background, this literature review article from 2008 is the best I have found in summarizing the current state of knowledge for treatment. If you are traveling in an area where CFP is known to exist, having a copy of this article to show the local physician can’t hurt.

After doing a lot of research, I came to the conclusion that the information in Wikipedia is seriously flawed. Looking at that source, there are several gross errors, confusions, and contradictions.

What is CFP?

Unlike most other kinds of “food poisoning” CFP is not caused by a germ growing in the fish, not by spoilage, not by improper handling, not by undercooking, nor by contamination. It is caused by chemicals the fish has eaten. The fact that those chemicals are 100% natural and “organic”, does nothing to make them any less nasty. CFP is actually caused by a number of different and closely related chemicals that have different toxicity levels and cause slightly different symptoms. Fish from different areas have different mixtures of these compounds, some of which are toxic to humans at extraordinarily small doses. As little as 1 millionth of a gram of some of them can cause symptoms.

It all starts with a microscopic brown algae that grows on coral reefs all over the world. These tiny plants produce a group of chemicals that are poisonous to animals, even in exceedingly tiny amounts. It is widely reported that the CFP toxins have no effect on fish. This is actually not true, fish that are fed a high enough dose of ciguatera toxin DO become ill. In the natural world however, the amount they typically consume in a single feeding is below what produces symptoms. The problem is the fish neither digest, nor excrete these poisons, but they accumulate them in their bodies. Over time the fish itself becomes toxic to people who eat it.

The Nasty Part…

What happens if you eat fish that contains a toxic level of the chemicals that cause CFP? Within 24 hours you are likely to experience an unpleasant set of digestive upsets. Vomiting, diarrhea, abdominal pain, nausea. If these are severe, medical care might be needed for maintaining hydration. In rare cases the patient can develop dangerously low blood pressure and/or slow heartbeat. Either of these should be cause for immediate emergency medical treatment. The good news is that these symptoms usually resolve themselves in 1 to 4 days, and are rarely fatal. The bad news is that a whole new set of unpleasant symptoms are just getting started. The secondary symptoms are mostly neurological, and cross a wide range.

Numbness and tingling in the feet and hands and teeth. Pain in the teeth, or a feeling that the teeth are loose. Generalized itching, muscle pain, joint pain, and fatigue. A distinctive symptom reported by many patients is an alteration in temperature perception in which cold surfaces are perceived as hot, or produce an unpleasant, abnormal sensation. This is sometimes not entirely accurately described as temperature “reversal.” This temperature-related effect is considered characteristic of CT, although not all patients report experiencing this symptom. This class of peripheral neurological symptoms is reported more often in patients in the Caribbean.

Unfortunately, we are not done with our list of symptoms yet: Anxiety, depression and memory loss. More marked mental status changes such as hallucinations, giddiness, poor coordination, and even coma are possible. This set of symptoms, more central nervous system and physciatric, are, for some reason, more often reported in from patients in the Indian and Pacific Oceans.

After the initial or acute illness, feelings of weakness generally last a few days to several weeks. Some patients experience chronic symptoms lasting weeks to months, in particular peripheral neurologic symptoms such as numbness and tingling in the extremities, generalized itching, and neuropsychiatric symptoms such as malaise, depression, generalized fatigue, headaches.

On the whole, a nasty business. One we should do our best to avoid.

Avoiding CFP

There is only one sure fire way to NEVER get CFP. Don’t ever eat any fish. Ever. Kind of like saying you will never get food poisoning if you never eat. It might be true, but…

Without a complete biochemical laboratory there is no way to determine if a fish is contaminated. Period. Full stop. There was a consumer test kit marketed a few years ago, but its very high cost and lack of accuracy drove it off the market. Do NOT believe any of the stories about pennies changing color, or flies avoiding fish flesh, or the taste of the fish’s blood, or anything else. They are all 100%, totally, completely, bogus–no matter how completely some people might believe them.

In general, the bigger the fish is, the older it is, and the higher up the food chain it is, the riskier it is to eat. Areas where the reef has been “disturbed” by storms or human activity seem to be more likely to have fish that are dangerous to eat. Barracuda larger than 5lbs or so are one of the most likely fish to cause CFP. Moray eels are another fish that should always be on your “to be avoided list.” Other fish certainly CAN, with probabilities varying greatly from place to place. Locals will frequently know that some kinds of fish from one side of the island are safe, while the other side is risky.

If a fish contains a toxic level of CFP toxin, there is nothing you can do to remove it. Cooking, freezing, washing, marinating, are all useless.

What we need to learn is how to avoid the fish that are most likely to be contaminated. This gets really hard because when you do research you frequently get lists of ALL fish that have tested positive for CFP toxin EVER. An example of why this is not helpful: ONCE someone got sick after eating Mahi-mahi. It was reported in the literature, and forever after mahi-mahi got “on the list” from that single report, despite the fact that literally millions of people eat mahi-mahi every year without getting sick. We eat mahi-mahi every chance we get.

So we find the lists of all potentially toxic fish to be not very helpful. Since we catch our own fish, and we know where they come from, that is what we use to decide what to eat. We do not fish on shallow reefs (<150 feet deep). The algae that cause this problem live at rather shallow depths, and we look for bottom feeding fish that do not range up and down the water column.

We do not worry about pelagic (open ocean) fish like Mahi-mahi, wahoo, tuna and the like. The probability of them having consumed enough reef fish to accumulate a dangerous level of the CFP toxin is very, very small.

We avoid most jacks since, even if caught deep, they move up and down the water column a lot. A Crevalle Jack caught at 150 feet off the edge of the reef this afternoon might have been feeding at 20 feet last night. Grouper and snapper we eat, if they are caught deep. If we are above 35º latitude we feel safe eating anything. Some broad areas of the ocean have historically always been free of CFP such as the west coast of North and Central America.

Do NOT assume that fish you are served at a restaurant is safe. Do NOT assume that any fish you are served at any restaurant or purchase in the market already filleted is the kind of fish you are told it is, or that it came from the place you are told.

Do not eat any parts of the fish except the fillet. Do not use the head or bones to make soup, and never eat the roe or any internal organs. All of those can contain much higher levels of ciguatera toxin than the muscle of the fish.

We know there is no strategy for eating fish that is 100% guaranteed to avoid CFP if you are in areas where it is endemic. With modern supply chains almost no place is safe. There have been reported CFP cases from imported fish in St Louis, Missouri. By knowing where our fish come from, and talking to the locals about what fish they avoid, we reduce our risk to a level we are comfortable with.

AND FOR YOU UP NORTH…

Least you feel all superior and vastly safer than your tropical fish eating brothers… have you heard of anisakiasis?