Seafood and Aquatic Life Group

RED TIDE INFORMATION

Neurotoxic Shellfish Poisoning (NSP) Physician Alert

K. brevis produces at least two major heat-stable, lipid-soluble toxins known as brevetoxins A and B. These toxins appear to affect sodium transport in the autonomic nervous system and cause inhibition of neuromuscular transmission in skeletal muscle. Many species of fish are sensitive to brevetoxins and effectively "drown" in red tide waters when the toxin causes paralysis of the gills. No form of fish or shellfish that has washed ashore sick or dead (or was found floating "belly up" in the water) should ever be eaten.

Which Seafoods Are Safe to Eat When K. brevis is Present

Since brevetoxins are not concentrated by fish, shrimp, or crabs, these forms of seafood are safe to eat as long as they are caught alive and act normally when caught. Also, any molluscan shellfish (oysters, clams, mussels and scallops) legally available in markets and restaurants also should be safe to eat since they would have been harvested from areas that were not affected by the bloom. For current status of shellfish harvesting areas, call toll-free 1-800-685-0361 or contact the Seafood and Aquatic Life Group at (512) 834-6757.

Neurotoxic Shellfish Poisoning (NSP)

While fish are easily killed by high brevetoxin levels, oysters, clams, and mussels are not susceptible to these toxins and they may appear perfectly healthy. However, when these shellfish feed for a time in waters with high concentrations of K. brevis, they accumulate brevetoxins in their body tissues, making them toxic to humans who ingest them. The toxin is only slowly cleared from shellfish after the red tide disappears. Consequently, consumption of oysters, clams, or mussels caught during (or even months after) a red tide can cause neurotoxic shellfish poisoning (NSP) in humans. Any bays in Texas affected by a K. brevis bloom would be closed to harvesting by the Texas Department of State Health Services (DSHS). For current status of shellfish harvesting areas, call toll-free 1-800-685-0361. If the public is not aware of these closures, we may see cases of NSP occurring in individuals who harvest and eat shellfish from these waters despite the DSHS ban on harvesting.

Characteristic Symptoms of NSP: Gastrointestinal and Neurologic

There is currently no specific diagnostic laboratory test to identify NSP, so the diagnosis is based on clinical evaluation of cases with relevant exposures. Onset of NSP symptoms usually occurs within three hours of ingestion of contaminated shellfish (range 15 minutes to 18 hours after exposure). Initial complaints typically include abdominal pain, nausea, vomiting, and diarrhea accompanied by progressive paresthesias, which first affect the area around the mouth and later the pharynx, trunk, and limbs. Other common symptoms include vertigo, malaise, generalized muscle weakness, ataxia, incoordination, chills, headache, and myalgias. A reversal of hot and cold temperature sensation (similar to that seen in ciguatera poisoning) has also been reported. In cases of severe poisoning, dilated pupils, bradycardia, and (rarely) convulsions requiring respiratory support may also be seen.

Differential Diagnosis

To some extent, the geographical origin of the affected shellfish can help identify the probable toxigenic dinoflagellate. Alexandrium catenella (formerly Gonyaulax catenella) is the leading toxigenic dinoflagellate found along the Pacific coast of North America, while Alexandrium tamarense-excavatum (formerly Gonyaulax tamarensis) primarily affects the northern Atlantic coast of North America. K. brevis is the dinoflagellate most often responsible for red tides in the Gulf of Mexico and along the southern Atlantic coast of North America. NSP is a relatively mild illness and should not be confused with the much more serious condition known as paralytic shellfish poisoning (PSP). PSP is caused by a saxitoxin elaborated by one of the Alexandrium species of dinoflagellates. The symptoms of PSP are very similar to those of NSP, but the neurotoxic effects of PSP can progress rapidly to respiratory paralysis and death if the patient is not placed on a respirator.

NSP symptoms are also very similar to those of ciguatera fish poisoning caused by ciguatoxin produced by the dinoflagellate Gambierdiscus toxicus. During blooms of G. toxicus, this single-celled organism attaches to marine algae which are eaten by various herbivorous fish. These fish are then eaten by carnivorous fish, and the ciguatoxin is thereby passed up the food chain into a variety of larger tropical reef fish such as red snapper, barracuda, amberjack, and grouper. Humans can be affected if they catch and eat any fish that have accumulated large amounts of the toxin. Because NSP is associated with eating shellfish and ciguatera poisoning with eating fish, a dietary history is generally sufficient to distinguish these two types of poisoning.

Treatment for NSP

Since NSP is a relatively innocuous form of shellfish poisoning, and since no specific antitoxin is available, treatment is supportive in nature. Patients should be monitored closely for hydration status, particularly if the signs and symptoms include a significant amount of vomiting or diarrhea. Patients who have ingested large quantities of the affected shellfish and who are experiencing more severe symptoms may require brief hospitalization for observation. In general, the illness is self-limiting; symptoms usually subside in less than 24 hours with supportive therapy.

Other Symptoms Associated with K. brevis Blooms

Classically, NSP produces gastrointestinal and neurologic symptoms associated with ingestion of shellfish containing high levels of brevetoxins. However, some individuals have experienced respiratory, mucous membrane, and skin irritation simply by walking on the beach during this and other K. brevis red tides. Unlike the Alexandrium species of red tide organisms, which affect the North Atlantic and North Pacific coasts, Karenia species possess an outer shell that is very fragile and easily disrupted by the vigorous mechanical action of the surf. On disruption, Karenia organisms release cellular endotoxins into the surrounding water. Wind and surf action produce a fine aerosol that generally travels only a short distance from the beach. Thus, in areas with a great deal of surf action, airborne exposure to the toxins can be a problem for some individuals. Alexandrium species have a hard outer shell that is not easily disrupted, so airborne exposure is generally not a problem in red tides along the Pacific coast or the northern Atlantic coast.

Exposed individuals frequently report an acute but rapidly reversible syndrome consisting of conjunctival irritation, rhinorrhea, sneezing, cough, and (rarely) respiratory distress similar to an asthma attack. Persons swimming or wading in K. brevis red tides may experience eye and skin irritation accompanied by redness and itching. Following short-term exposures, the symptoms usually subside when the individual leaves the immediate vicinity of the beach. Prolonged exposure, however, can cause symptoms that linger for hours or days after the person leaves the affected area. Consequently, DSHS recommends that visitors to the affected areas avoid swimming or wading in red tide waters.

Prepared by Richard A. Beauchamp, M.D., TDSHS Bureau of Epidemiology; Kirk Wiles , TDSHS Seafood and Aquatic Life Group; and Kate Hendricks, M.D., M.P.H. & T.M., TDSHS Infectious Disease Epidemiology & Surveillance Division

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Texas Department of State Health Services, Environmental and Consumer Safety Section,
Seafood and Aquatic Life Group, 1100 West 49th Street - Austin, Texas 78756 - 512-834-6757

Last Updated November 2, 2007