Certain foods need to be treated with caution while abroad: in the tropics, fish and shellfish pose a particular hazard.
Dr Elizabeth Driver is currently a medical adviser in a law firm in London. She has worked as a toxicologist at the Medical Research Council and in the Tropical Metabolism Research Unit in Jamaica.
One of the most enjoyable aspects of visiting a foreign country is the opportunity to sample the local cuisine—and provided such food is carefully prepared and cooked, this usually carries little risk.
Unfortunately, some foreign delicacies or even staple foodstuffs contain contaminants and biological toxins in their raw or uncooked form, and although local culinary methods have usually evolved for dealing with such contaminants prior to consumption, cases of poisoning still occasionally occur . This risk of poisoning is in addition to the hazards of food-borne infection considered elsewhere in this section.
Travellers should therefore know which foods carry a significant danger, and either avoid them completely or exercise extreme caution. The foods themselves range from cassava, eaten as a staple throughout the tropics, to ‘fugu’ or puffer fish, considered a delicacy in Japan and much of the Indo-Pacific.
Types of poisoning
The variety of potential food contaminants and toxins is vast and their effects range from the trivial and inconvenient to the frankly dramatic. In general, however, poisoning can be classified into ‘acute’ and ‘chronic’ varieties. Acute poisoning, is more or less immediate in onset and may follow consumption of a single portion of contaminated food. Chronic poisoning is a longterm process which generally follows the repeated consumption of small amounts of a toxic substance over an extended period— so it is unlikely to appear in travellers on a short trip.
Acute poisoning may be produced by foods of either plant or animal origin.
Local customs have generally developed to allow the consumption of potentially poisonous plant material by the use of particular methods of preparation which minimize the hazard. Travellers should be wary of preparing unusual foods for themselves, but may be reasonably confident that food prepared by local people will be innocuous. Poisoning is much more likely to result from eating wild berries or fungi. The two most toxic flowering plants are the castor bean (Ricinus communis) and the rosary pea (Abrus precatorius). The latter is sometimes encountered in bead necklaces, usually of African origin, and should be confined to this decorative use. If seeds with broken coats are swallowed, persistent diarrhoea, often with bloody mucus, begins after a latent period of up to three days. Death may result. Many mushrooms are toxic, but generally produce gastrointestinal symptoms, sweating or headache, which will resolve spontaneously after a few hours of discomfort. A few mushrooms have an alcohol-sensitizing effect, whilst others induce hallucinations. Only Amanita phalloides and similar types are likely to be fatal.
Cassava (manioc) is a dietary staple in many tropical countries. The tubers may be eaten whole, rather like sweet potatoes, or may be processed to produce a flour which is used to make bread. The problem is that cassava, particularly the ‘bitter’ varieties, contains poisonous hydrocyanic acid. When the tubers are damaged, an enzyme accelerates the release of the cyanide.
The poisonous nature of cassava has long been recognized in every country where it is used, and traditional ways of preparing the tubers minimize the amount of cyanide that reaches the table. These methods include soaking, washing in running water, boiling and pounding.
Consumption of improperly prepared cassava results in the dramatic onset of abdominal pain and vomiting, progressing to mental confusion, muscle paralysis, and ultimately to respiratory failure in fatal cases.
Ackee Travellers to Jamaica or Nigeria may be introduced to the
delights of the fruit of Blighia sapida, known as the ackee in Jamaica and isin in Nigeria.
In Jamaica, this strange fruit is served up with bacon or salt- fish as a kind of scrambled egg look-alike. Problems arise if the fruit is eaten unripe or has been improperly prepared, as it contains a potent toxin that rapidly lowers the blood sugar level. The victim succumbs rapidly to vomiting, followed by convulsions, coma, and death in the majority of cases. This is more likely to occur in the already malnourished, but unripe fruit can be lethal to anyone. Correct preparation involves boiling the fruit and then discarding the water.
The vast majority of poisonings are the result of eating fish or other forms of sea food. There are approximately 1200 marine species known to be poisonous or venomous. They are found throughout the world but only pose a medical or socio-economic hazard in a few areas.
Animal toxins fall into two categories. First, a normal constituent of the animal or one of its organs may be toxic. For example Eskimos have always known of the toxicity of polar bear liver, which contains immensely high levels of vitamin A. Second, the animal itself may be contaminated with toxins.
Puffer fish The puffer fish—known as ‘fugu’ in Japan—is a kind of culinary Russian Roulette. It is said to be sufficiently delicious to warrant the considerable risk attached to its consumption. Japanese chefs have to be specially licensed to carry out the delicate operation of removing the roe, liver, and skin which contain the lethal tetrodotoxin, but every year several deaths occur from eating fugu. About 40 per cent of those who develop significant signs and symptoms die. Death is said to be preceded by a tingling sensation of the lips . . .
Paralytic shellfish poisoning Rather less dramatic in onset, but nonetheless unpleasant, is the poisoning caused by eating fish, or more often shellfish, which have ingested plankton containing saxitoxin. In some areas of the world, such as the Caribbean, dinoflagellate protozoa may be present in such large numbers that the sea looks red and the amount of saxitoxin in the flesh of fish or shellfish will be correspondingly high. Fishermen in areas which are commonly affected know not to harvest the fish when there is a ‘red tide’. Symptoms of poisoning are slowing of the heart rate even to the point of heart failure, and muscle paralysis. Mild poisoning may result from ingesting just 1 milligram of the toxin, which could be found in a single clam. Without treatment 4 milligrams of the toxin would be fatal.
Ciguatera Another form of fish poisoning is ciguatera. Many fish, in particular the red snapper, grouper, and barracuda, in a broad circumglobal belt from 35 degrees North to 35 degrees South, particularly in the tropical Pacific and Caribbean, are capable of accumulating ciguatoxin from their foodstuffs. The toxin is present in the entire flesh of the fish. Unfortunately, the problem may suddenly appear in any one location after years of absence, and is not signalled by any visible change in the sea. Symptoms appear one to six hours after consumption of the fish and include abdominal pain, nausea, vomiting, numbness, and tingling of the lips and tongue, and shooting pain in the extremities. In severe cases, blood pressure falls and respiratory paralysis can lead to death. Treatment is difficult.
Scombroid poisoning Some fish of the mackerel or tuna varieties may be the cause of poisoning if they are inadequately refrigerated and preserved. Cooking does not destroy the toxin. A toxic substance, originally thought to be histamine, is formed by the action of enzymes on the muscle of the dead fish. Recent research suggests that another toxin, saurine, is also formed. This causes nausea, vomiting, diarrhoea, and epigastric pain. The face of the victim becomes flushed and burning and there may be numbness, thirst, and generalized urticaria. Fortunately all these signs and symptoms, which arise within about 2 hours of the meal, subside within 12-16 hours. No treatment is required. It is said that the flesh of the fish becomes rather peppery. This type of poisoning may occur anywhere in the world.
The long-term effects of food toxins are of rather less importance to travellers than acute poisons, but travellers should none the less be aware of the dangers of certain moulds and metals.
Mycotoxins, the toxins produced by moulds, have been implicated in several diseases both in tropical and in temperate climates. Improper food storage frequently causes problems, because moulds grow well in warm damp conditions. Mould-contaminated food may be consumed directly by man, or by domestic animals reared for food—pigs for example—which are capable of accumulating toxins in their flesh. Balkan nephropathy, a strange and serious disease of the kidneys confined to areas of Yugoslavia, Rumania, and Bulgaria, was first recognized thirty years ago. Only recently has it been linked to consumption of a mould that grows on maize.
Aflatoxin is the toxin of Aspergillus flavus, which grows on pea
nuts and other crops. The toxin is highly carcinogenic, and consumption of mouldy peanuts explains the high incidence of liver cancer in some parts of Africa—particularly West Africa. This has serious economic implications for many of the countries involved.
Eating mouldy cornmeal may be one of the factors involved in the high incidence of cancer of the oesophagus (gullet) found in areas of China. Excessive consumption of pickles and preserved foods containing nitrites may also be contributing factors.
Poor nutritional status, particularly combined with deficiency of vitamins A and C, probably increases susceptibility to myco- toxins. The well-nourished traveller may be at rather less risk than the indigenous population, but would still be well advised to avoid mouldy foods.
Various metals can be toxic. Lead from cooking pots is found in high levels in home-brewed beers in various parts of Africa, and can lead to chronic poisoning. Mercury present in seed dressing has caused poisoning in starving peasants in Iraq, forced by hunger to eat the grain intended for planting.