Frequently Asked Questions
What are marine toxins?
Marine toxins are naturally occurring chemicals that can contaminate certain seafood. The seafood contaminated with these chemicals frequently looks, smells, and tastes normal. When humans eat such seafood, disease can result.
What sort of diseases do marine toxins cause?
The most common diseases caused by marine toxins in United States in order of incidence are scombrotoxic fish poisoning, ciguatera poisoning, paralytic shellfish poisoning, neurotoxic shellfish poisoning and amnesic shellfish poisoning.
Scombrotoxic fish poisoning also known as scombroid or histamine fish poisoning, is caused by bacterial spoilage of certain finfish such as tuna, mackerel, bonito, and, rarely, other fish. As bacteria break down fish proteins, byproducts such as histamine and other substances that block histamine breakdown build up in fish. Eating spoiled fish that have high levels of these histamines can cause in human disease. Symptoms begin within 2 minutes to 2 hours after eating the fish. The most common symptoms are rash, diarrhea, flushing, sweating, headache, and vomiting. Burning or swelling of the mouth, abdominal pain, or a metallic taste may also occur. The majority of patients have mild symptoms that resolve within a few hours. Treatment is generally unnecessary, but antihistamines or epinephrine may be needed in certain instances. Symptoms may be more severe in patients taking certain medications that slow the breakdown of histamine by their liver, such as isoniazide and doxycycline.
Ciguatera poisoning or ciguatera is caused by eating contaminated tropical reef fish. Ciguatoxins that cause ciguatera poisoning are actually produced by microscopic sea plants called dinoflagellates. These toxins become progressively concentrated as they move up the food chain from small fish to large fish that eat them, and reach particularly high concentrations in large predatory tropical reef fish. Barracuda are commonly associated with ciguatoxin poisoning, but eating grouper, sea bass, snapper, mullet, and a number of other fish that live in oceans between latitude 35° N and 35° S has caused the disease. These fish are typically caught by sport fishermen on reefs in Hawaii, Guam and other South Pacific islands, the Virgin Islands, and Puerto Rico. Ciguatoxin usually causes symptoms within a few minutes to 30 hours after eating contaminated fish, and occasionally it may take up to 6 hours. Common nonspecific symptoms include nausea, vomiting, diarrhea, cramps, excessive sweating, headache, and muscle aches. The sensation of burning or "pins-and-needles," weakness, itching, and dizziness can occur. Patients may experience reversal of temperature sensation in their mouth (hot surfaces feeling cold and cold, hot), unusual taste sensations, nightmares, or hallucinations. Ciguatera poisoning is rarely fatal. Symptoms usually clear in 1 to 4 weeks.
Paralytic shellfish poisoning is caused by a different dinoflagellate with a different toxin, than that causing ciguatera poisoning. These dinoflagellates have a red-brown color, and can grow to such numbers that they cause red streaks to appear in the ocean called "red tides." This toxin is known to concentrate within certain shellfish that typically live in the colder coastal waters of the Pacific states and New England, though the syndrome has been reported in Central America. Shellfish that have caused this disease include mussels, cockles, clams, scallops, oysters, crabs, and lobsters. Symptoms begin anywhere from 15 minutes to 10 hours after eating the contaminated shellfish, although usually within 2 hours. Symptoms are generally mild, and begin with numbness or tingling of the face, arms, and legs. This is followed by headache, dizziness, nausea, and muscular incoordination. Patients sometimes describe a floating sensation. In cases of severe poisoning, muscle paralysis and respiratory failure occur, and in these cases death may occur in 2 to 25 hours.
Neurotoxic shellfish poisoning is caused by a third type of dinoflagellate with another toxin that occasionally accumulates in oysters, clams, and mussels from the Gulf of Mexico and the Atlantic coast of the southern states. Symptoms begin 1 to 3 hours after eating the contaminated shellfish and include numbness, tingling in the mouth, arms and legs, incoordination, and gastrointestinal upset. As in ciguatera poisoning, some patients report temperature reversal. Death is rare. Recovery normally occurs in 2 to 3 days.
Amnesic shellfish poisoning is a rare syndrome caused by a toxin made by a microscopic, red-brown, salt-water plant, or diatom called Nitzchia pungens. The toxin produced by these diatoms is concentrated in shellfish such as mussels and causes disease when the contaminated shellfish are eaten. Patients first experience gastrointestinal distress within 24 hours after eating the contaminated shellfish. Other reported symptoms have included dizziness, headache, disorientation, and permanent short-term memory loss. In severe poisoning, seizures, focal weakness or paralysis, and death may occur.
How can these diseases be diagnosed?
Diagnosis of marine toxin poisoning is generally based on symptoms and a history of recently eating a particular kind of seafood. Laboratory testing for the specific toxin in patient samples is generally not necessary because this requires special techniques and equipment available in only specialized laboratories. If suspect, leftover fish or shellfish are available, they can be tested for the presence of the toxin more easily. Identification of the specific toxin is not usually necessary for treating patients because there is no specific treatment.
How can these diseases be treated?
Other than supportive care there are few specific treatments for ciguatera poisoning, paralytic shellfish poisoning, neurotoxic shellfish poisoning, or amnesic shellfish poisoning. Antihistamines and epinephrine, however, may sometimes be useful in treating the symptoms of scombrotoxic fish poisoning. Intravenous mannitol has been suggested for the treatment of severe ciguatera poisoning.
Are there long-term consequences to these diseases?
Ciguatera poisoning has resulted in some neurologic problems persisting for weeks, and in rare cases, even years. Symptoms have sometimes returned after eating contaminated fish a second time. Amnesic shellfish poisoning has resulted in long-term problems with short-term memory. Long-term consequences have not been associated with paralytic shellfish poisoning, neurotoxic shellfish poisoning, and scombrotoxic fish poisoning.
How common are these diseases?
Every year, approximately 30 cases of poisoning by marine toxins are reported in the United States. Because healthcare providers are not required to report these illnesses and because many milder cases are not diagnosed or reported, the actual number of poisonings may be much greater. Toxic seafood poisonings are more common in the summer than winter because dinoflagelates grow well in warmer seasons. It is estimated from cases with available data that one person dies every 4 years from toxic seafood poisonings.
What can I do to prevent poisoning by marine toxins?
General guidelines for safe seafood consumption:
- Although any person eating fish or shellfish containing toxin or disease-causing bacteria may become ill, persons with weakened immune systems or liver problems should not eat raw seafood because of their higher risk of Vibrio infection (see Vibrio FAQ).
- Keep seafood on ice or refrigerated at less than 38° Fahrenheit to prevent spoilage.
Specific advice for avoiding marine toxin poisoning:
- Keep fresh tuna, mackerel, grouper, and mahi mahi refrigerated to prevent development of histamine. Don't believe that cooking spoiled or toxic seafood will keep you safe. These toxins are not destroyed by cooking.
- Do not eat barracuda, especially, those from the Caribbean.
- Check with local health officials before collecting shellfish, and look for Health Department advisories about algal blooms, dinoflagellate growth or "redtide" conditions that may be posted at fishing supply stores.
- Do not eat finfish or shellfish sold as bait. Bait products do not need to meet the same food safety regulations as seafood for human consumption.
What is the government doing about these diseases?
Some health departments test shellfish harvested within their jurisdiction to monitor the level of dinoflagellate toxins and asses the risk for contamination. Based on the results of such testing, recreational and commercial seafood harvesting may be prohibited locally during periods of risk. State and federal regulatory agencies monitor reported cases of marine toxin poisoning, and health departments investigate possible outbreaks and devise control measures. The Centers for Disease Control and Prevention (CDC) provides support to investigators as needed.
What else can be done to prevent these diseases?
It is important to notify public health departments about even one person with marine toxin poisoning. Public health departments can then investigate to determine if a restaurant, oyster bed, or fishing area has a problem. This prevents other illnesses. In any food poisoning occurrence, consumers should note foods eaten and freeze any uneaten portions in case they need to be tested. A commercial test has been developed in Hawaii to allow persons to test sport caught fish for ciguatoxins.
How can I learn more about this and other public health problems?
You can discuss any medical concerns you may have with your doctor or other healthcare provider. Your local city, county, or state health department can provide more information about this and other public health problems that are occurring in your area. General information about the public health of the nation is published every week in the Morbidity and Mortality Weekly Report, by CDC in Atlanta, GA (www.cdc.gov/mmwr). Epidemiologists in your local and state health departments are tracking a number of important public health problems, investigating special problems that arise, and helping to prevent them from occurring in the first place, and from spreading if they do occur.