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Ciguatera fish poisoning

From A Passion for the Sea, by Jimmy Cornell.

As in much of the Caribbean there is either little fish left or spearfishing is prohibited, so it was with great anticipation that I looked forward to the Tuamotus, where fish are still abundant and the underwater scenery is almost without equal.

En route from the Marquesas to Tahiti we stopped at various atolls, most attractive among them the uninhabited atoll of Tahanea. The pass into the large lagoon had beautiful coral formations and was teeming with fish.

Although aware of the possibility of ciguatera fish poisoning I couldn’t resist the temptation but only speared a couple of smaller fish. Gwenda marinated one of them in lime juice, which we ate raw in Polynesian style, and pan-fried the fillets of the other.

After 30 years of catching and eating tropical fish, I was now on the receiving end

Halfway through the afternoon, while beachcombing on a nearby motu, all hell broke loose as we were both overcome by terrible stomach cramps. By the time we got back to the boat, other symptoms were telling me clearly that after thirty years of catching and eating tropical fish, I was now on the receiving end.

The first twenty-four hours were the worst but not severe enough to risk sailing the 300 miles to Tahiti for help. We chose to deal with the matter ourselves and although we felt very bad for a couple of days, we very slowly
recovered although it was one month before all symptoms disappeared. The last to go was the tingling and a queer reversal of sensations when a hot drink felt cold… and ice cream burnt my mouth.

There are many different kinds of seafood poisoning, but the most prevalent type found throughout the tropics is what is now known as ciguatera. Known by the ancient Chinese, reported by Columbus on his first visit to the Caribbean, and accurately described by Pedro de Quiros during his Pacific voyage in 1606, ciguatera fish poisoning is endemic in all tropical areas and occurs regularly between latitudes 35°S and 35°N.

It is estimated that every year there are about 60,000 cases worldwide, while in the Caribbean, where most cases occur north of Martinique, around 100 cases per 10,000 people are reported each year. The situation is possibly even worse in the tropical Pacific Ocean, particularly in French Polynesia and the Marshall Islands.

There is no way of telling which fish may be poisonous and which safe.

Over 400 different species of fish have been incriminated at one time or another, a species being toxic in one area but not in another, even within the same lagoon. This is the essence of the problem as there is no way of telling which fish may be poisonous and which safe.

However, the actual source of the toxin is now known: a dinoflagellate, Gambierdiscus toxicus, a unicellular plant similar to a micro-algae. A creature of the ocean depths, normally only a few of these algae live on the reef but under certain circumstances their number can increase dramatically.

Fish feeding on coral ingest these toxic algae and so the toxin enters the food chain. These grazing fish usually live and feed in one small area so they gradually build up larger and larger amounts of toxin.

A predatory snapper, grouper or barracuda then swims into the area and with one bite acquires all the toxicity its herbivorous victim had spent a lifetime collecting. These larger fish then carry the toxicity to other parts of the reef or lagoon.

Much of the early research work had been conducted in French Polynesia, where ciguatera still causes hundreds of cases every year. In the late 1970s Dr Raymond Bagnis, head of the Medical Oceanographic Research Unit in Tahiti, finally identified the cause of ciguatera.

He also established that in every outbreak of ciguatera some damage to the coral reef had occurred in the preceding year or two. Sometimes this was a naturally occurring event, such as a bad storm or cyclone but more often it was caused by man’s interference, from blasting a pass into a lagoon, building quays or breakwaters on live coral, to the dumping of metallic materials or other polluting debris into lagoons. The toxic algae thrive on newly exposed coral surfaces multiplying very rapidly and so the cycle leading to ciguatera poisoning begins.

Not everyone eating ciguatoxic fish has the same symptoms although diarrhoea, nausea, abdominal pain or vomiting usually occur within a few hours. Prickling in the fingers and toes as well as tingling around the mouth are other symptoms accompanied by an alteration of sensation causing cold objects or drinks to feel hot or plain water to taste like soda and a shower to feel like pin pricks of electric shocks.

Other symptoms are extreme tiredness and lethargy, itching, muscle and joint pain, a weakened pulse and falling blood pressure. In very severe cases respiratory paralysis leads to death but this is very rare and mortality rates are under one per cent of cases. One of the first reported deaths was that of the Bounty’s surgeon, who passed away in Tahiti after a large feast.

In most instances the symptoms subside after a few days, although the itching and alteration of sensations can last for several weeks. The symptoms are caused by the toxin acting on the body’s sodium channels thereby causing changes in the electrical potential and permeability of the cells.

The risks can be minimised by gutting fish as soon as it is caught and by not eating the head, liver, roe and viscera.

Recently ciguatera has been treated successfully with Mannitol IV. The usual dose is one gram mannitol per kilogram of body weight. The medication is infused intravenously as a twenty per cent Mannitol solution. It is believed that Mannitol acts at cell level by rendering the toxin inert. The earlier a victim is diagnosed and treated with Mannitol IV, the more likely its success. A large number of patients treated in the Marshall Islands successfully responded to this treatment.

Antihistamines, calcium gluconate, atropine and vitamin B have also been used to ameliorate the symptoms.

Untreated, ciguatera can last between one to two months, although some symptoms may persist for longer. Recovery usually begins in a couple of days, but the disease does not produce any immunity so the next poisoning is usually more severe than the previous one.

People repeatedly exposed to tiny amounts of ciguatera poisoning, which are usually too small to cause an outbreak, gradually become sensitised to the toxin and consuming a toxic fish will trigger an attack, while other non-sensitised people, who have consumed the same fish, may not be affected.

The risks can be minimised by gutting fish as soon as it is caught and by not eating the head, liver, roe and viscera as the toxin is concentrated in these organs.

All very large fish caught inside a lagoon, or close to a reef, should be treated with suspicion, especially snappers, groupers, barracuda, jacks and moray eels.

Local fishermen are the best source of advice on which lagoon fish are safe to eat.

It pays to take local advice as most islanders know only too well which fish and which areas of their lagoon have to be avoided. Freezing, drying, cooking or marinating the fish does not destroy the poison and affected fish looks, smells and tastes normal.

Various traditional tests have been recommended but none is truly effective. Dr Hokama of Hawaii University has perfected a test kit capable of identifying the presence of ciguatoxin in fish flesh. When used properly Cigua-Chek™ will test ciguatoxin at levels generally below the level that can cause clinical symptoms in humans.

One message is abundantly clear: every outbreak of ciguatera indicates that something is wrong with the reef. For interfering with the fragile ecosystem of living coral nature exacts a high price.


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