Seafood poisoning from marine toxins is an under-recognized hazard for travelers, particularly in the tropics and subtropics. Furthermore, the risk is increasing because of factors such as climate change, coral reef damage and spread of toxic algal blooms.
CIGUATERA FISH POISONING
Ciguatera fish poisoning occurs after eating reef fish contaminated with potent toxins that originate from small marine organisms (dinoflagellates) that grow on and around coral reefs and are ingested by herbivorous fish. The toxins are then concentrated as they pass up the food chain to large carnivorous fish and finally to humans. Toxins in fish are concentrated in the liver, intestinal tract, roe and head. The risk of ciguatera is likely to increase as more coral reefs die because of climate change, construction and nutrient runoff.
Risk for Travelers
More than 50,000 cases of ciguatera poisoning occur globally every year. Ciguatera is widespread in tropical and subtropical waters and is particularly common in the Pacific and Indian Oceans and the Caribbean Sea. Fish that are most likely to cause ciguatera poisoning are carnivorous reef fish, including barracuda, grouper, moray eel, amberjack, sea bass or sturgeon. Omnivorous and herbivorous fish such as parrot fish, surgeonfish and red snapper can also be a risk.
Typical ciguatera poisoning results in a gastrointestinal illness, followed by neurological symptoms and, rarely, cardiovascular collapse. The first symptoms usually appear one to three hours after eating contaminated fish and include nausea, vomiting, diarrhea, and abdominal pain. Neurological symptoms appear 3–72 hours after the meal and include pain in the teeth or the sensation that the teeth are loose, itching, metallic taste, blurred vision or transient blindness. These symptoms usually last a few days to several weeks. Chronic neuropsychiatric symptoms resembling chronic fatigue syndrome may be disabling, last several months, and include malaise, depression, headaches and fatigue. Cardiac manifestations include arrhythmias, and hypotension. Commercial kits are available to test for ciguatera in fish. These are sensitive but expensive and there is no test for ciguatera in humans.
- Avoid or limit consumption of the reef fish listed above, particularly when the fish weighs six pounds or more.
- Never eat high-risk fish such as barracuda or moray eel.
- Avoid the parts of the fish that concentrate ciguatera toxin.
Remember that ciguatera toxins do not affect the texture, taste or smell of fish, and they are not destroyed by gastric acid, cooking, smoking, freezing, canning, salting or pickling. Commercial kits can be used to check if the fish is safe to eat.
There is no specific antidote and treatment is generally symptomatic and supportive. Intravenous mannitol has been reported to reduce the severity and duration of neurological symptoms, particularly if given within 48 hours of the appearance of symptoms.
Scombroid, one of the most common fish poisonings, occurs worldwide in both temperate and tropical waters. The illness occurs after eating improperly refrigerated or preserved fish containing high levels of histamine and often resembles a moderate to severe allergic reaction. Fish that cause scombroid have naturally high levels of histidine in the flesh and include tuna, mackerel, mahi-mahi (dolphin fish), sardine, anchovy, herring, bluefish, amberjack and marlin. Histamine and other scombrotoxins are resistant to cooking, smoking, canning or freezing.
Symptoms of scombroid poisoning resemble an acute allergic reaction and usually appear 10–60 minutes after eating contaminated fish. They include flushing of the face and upper body (resembling sunburn), severe headache, palpitations, itching, blurred vision, abdominal cramps and diarrhea. Untreated, symptoms usually resolve within 12 hours. Rarely, there may be respiratory compromise, malignant arrhythmias and hypotension requiring hospitalization.
Preventive Measures for Travelers
Fish contaminated with histamine may have a peppery, sharp, salty, or bubbly taste but may also look, smell and taste normal. The key to prevention is to make sure that the fish is promptly chilled (below 38°F) after capture. Cooking, smoking, canning or freezing will not destroy histamine in contaminated fish.
Scombroid poisoning usually responds well to antihistamines.
Several forms of shellfish poisoning may occur after ingesting filter-feeding bivalve mollusks (such as mussels, oysters, clams, scallops, and cockles) that contain potent toxins. The toxins originate in small marine organisms that are ingested and concentrated by shellfish.
Risk for Travelers
Contaminated shellfish may be found in temperate and tropical waters, typically during or after dinoflagellate blooms or “red tides.” Poisoning results in gastrointestinal and neurological illness of varying severity. Symptoms typically appear 30–60 minutes after ingesting toxic shellfish but can be delayed for several hours.
Paralytic Shellfish Poisoning
This is the most common and most severe form of shellfish poisoning. Symptoms usually appear 30–60 minutes after eating toxic shellfish and include numbness and tingling of the face, lips, tongue, arms and legs. There may be headache, nausea, vomiting and diarrhea. Severe cases may show clinical features such as mental status changes, flaccid paralysis and respiratory failure. The death rate may be particularly high in children.
Neurotoxic Shellfish Poisoning
Neurotoxic shellfish poisoning usually presents as gastroenteritis accompanied by minor neurological symptoms. Inhalation of aerosolized toxin in the sea spray associated with a red tide may cause an acute respiratory illness.
Diarrheic Shellfish Poisoning
This produces chills, nausea, vomiting, abdominal cramps and diarrhea. No deaths have been reported.
Amnesic Shellfish Poisoning
This is a rare form of shellfish poisoning that produces a gastroenteritis that may be accompanied by headache, confusion and permanent short-term memory loss. In severe cases, seizures, paralysis and death may occur.
Preventive Measures for Travelers
Shellfish poisoning can be prevented by avoiding potentially contaminated bivalve mollusks. This is particularly important in areas during or shortly after “red tides.” Travelers to developing countries should avoid eating all shellfish because they carry a high risk of viral and bacterial infections. Marine shellfish toxins cannot be destroyed by cooking or freezing.
Treatment is symptomatic and supportive.
Be careful about your seafood-eating habits and make sure you know all the ingredients in any dishes where fish is mixed with other food elements.