RED TIDE FACT SHEET


What is Red Tide?

Red Tide is caused by a "population explosion" of toxic, naturally occurring microscopic plankton (specifically, a subgroup known as dinoflagellates). Explosions or "blooms" are coastal phenomena caused be environmental conditions which promote explosive growth. Factors which are especially favorable include warm surface temperatures, high nutrient content, low salinity and calm seas. Rainy followed by sunny weather in the summer months is often associated with red tide outbreaks. Organisms which cause theses blooms around the United States are: Does it really color the water?

Yes, water in coastal areas can be colored red by the algae, thus the term "red tide." Although toxic blooms often turn the water reddish brown, many nontoxic species or reddish brown plankton cause the same discoloration. Conversely, toxic plankton may be numerous enough to toxify shellfish, but not sufficiently abundant to discolor water. Discolored water should always be regarded with suspicion.

What seafoods are unsafe to eat from waters where red tide occurs?

Only a few marine animals accumulate these toxins. Shellfish, are particularly prone to contamination as they feed by filtering microscopic food out of the water. If toxic, planktonic organisms are present, they are filtered from the water along with other nontoxic foods.

Filter-feeding shellfish include quahogs, soft-shell clams, oysters, mussels and scallops. Since toxins are stored in the digestive tract (stomach) of these shellfish, scallops are safe to eat as long as only cleaned muscle meat (the only part generally eaten) is consumed. Whelks and moon snails can also accumulate dangerous levels of the toxin during red tide as they feed on contaminated shellfish.

Lobsters, crabs, shrimp and finfish do not accumulate toxin and are safe to eat from affected waters.

What happens if toxic shellfish are consumed?

Eating toxic shellfish can cause paralytic shellfish poisoning (PSP) in humans. PSP is caused by saxitoxin, which is produced by G. tamarensis, and is one of the most potent toxins known to scientists. After ingestion, this poison immediately affects the nervous system, with symptoms usually occurring within 30 minutes. Severity depends on the amount of toxin consumed. Initial reactions are tingling of the lips and tongue, which spreads to the face, neck, fingertips and toes. Headache, dizziness and nausea follow. These symptoms may be mistaken for drunken conditions and are further aggravated by alcohol consumption. In severe cases, muscular paralysis and respiratory difficulty may occur with 5 to 12 hours. Fatalities from respiratory paralysis have been reported.

How are symptoms treated?

Treatment varies with the severity of symptoms and can include pumping the stomach, inducing vomiting and charcoal hemoperfusion (a process involving the pumping of arterial blood through an activated charcoal filter to remove the poison.) Alkaline fluids such as sodium bicarbonate are also thought to be helpful in treating symptoms as the toxin is unstable in alkaline conditions. Artificial respiration may be required if patients exhibit respiratory stress.

What should one do if accidental ingestion of toxic shellfish is suspected?

Seek proper medical attention immediately. If a family doctor or other physician cannot be reached, the person should be taken to the nearest hospital emergency room or medical clinic. If none of these options are available, contact the nearest poison control center.

How is red tide monitored so that toxic shellfish are not marketed?

All shellfish producing states have monitoring programs which test water, sediments and shellfish for contamination. The established tolerance level for humans is 80 micrograms per 100 grams of raw edible meat. In Massachusetts, for instance, the Department of Environmental Quality Engineering (DEQE) is responsible for year-round testing of shellfish and shellfish growing areas. Monitoring efforts are more intensive in the spring, summer and fall. The DEQE notifies the Division of Marine Fisheries which, in turn, notifies affected city and town officials of closures. When blooms subside, shellfish purify themselves of the toxin, and when testing indicates a return to safe levels, the areas are reopened. Monitoring programs may differ in other states.

What precautions should recreational fisherman take?

Recreational shellfish gatherers should look for posted warnings and pay close attention to local media announcements. Also, it is advisable to contact appropriate state agencies for current news on closures.

Under no circumstance should individuals harvest shellfish from any area closed to shellfishing.

Toxic shellfish will taste and appear no different than nontoxic shellfish, and cooking does not destroy the red tide toxin. Testing is the only way to determine if shellfish contain unsafe levels of toxin.

Should consumers feel safe when purchasing shellfish at seafood markets?

Yes, consumers should feel confidant about purchasing shellfish from established markets. Harvesting regulations for shellfish are strictly enforced. Even during periods of red tide, clams, oysters, mussels and whelks in seafood markets are harvested from clean waters. The U.S. Food and Drug Administration visits state labs to observe routine testing procedures, and public health officials regularly monitor shellfish markets to ensure that only safe, nontoxic shellfish are available to consumers.

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Reprinted from the Mass Dept of Public Health