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"Red Tide" and shellfish poisoning

Updated on July 12, 2011

If you love to eat hard or soft-shell clams,oysters, mussels, scallops, whelks, and moon snails, you should read this.

Recently, researchers from the Gulf of Maine Toxicity project (funded by NOAA) have observed a larger than usual "seed bed" lying off the coast of New England. Researchers fear that this finding is a signal for a major algae bloom during the spring and summer of 2010, caused by the toxic phytoplankton (a microscopic plant-like organism that lives in the ocean) called Alexandriumfundyense. This organism is responsible for harmful algal blooms which are called “Red Tide” and the resulting poisoning of shellfish.

Red Tide emerges from a "population explosion" of toxic, naturally occurring microscopic plankton (in this case, the Alexandrium Fundyense).These "Blooms" are coastal phenomena caused by environmental conditions which includes: sea low salinity, high nutrient content, warm surface temperatures and calm seas. Also, rain followed by sunny weather in the summer months is often associated with this natural event.

The term "Red Tide" is used to name this phenomenon because water in coastal areas can be colored red by the algae. Although toxic blooms often turn the water reddish brown, many nontoxic species or reddish brown plankton cause the same discoloration. But it is also possible that toxic plankton may be numerous enough to toxify shellfish, but not sufficiently abundant to discolor water. Discolored water should always be regarded with suspicion, but even during a red tide event caused by Alexandrium fundyense, there is no risk with regard to swimming in the water.

Why should I care?

Although these algae pose not direct treat to humans, shellfish are particularly prone to contamination because the toxins they produce can be accumulated as mollusks feed by filtering microscopic food out of the water. During "Red Tides" shellfish harvested from affected areas are not safe to eat. But shrimp, crabs, Lobster meat and most finfish, do not normally accumulate toxin and are safe to eat from affected waters. On the other hand, Lobster tomalley (the green part or liver) is not safe to eat in general, and particularly during red tide events because this part of the lobster can buildup high levels of toxins and other pollutants.

Since toxic shellfish will taste and appear no different than nontoxic shellfish, precautions must be taken in order to avoid consumption of contaminated mollusks. The only way to determine if shellfish contain unsafe levels of toxin is testing. In the United States, all shellfish-producing states have monitoring programs that test water, sediments, and shellfish for contamination.

Is important to know that cooking does not destroy the red tide toxin.

Recommended books about Red Tides and Food Poisoning

What happens if someone consumes shellfish from a Red Tide area?

Human and animal consumption of contaminated shellfish may generate severe poisoning effects because the Alexandrium fundyense, produces one of the most potent toxins known to scientists: saxitoxin. This toxin acts on the voltage-gated sodium channels of nerve cells, preventing normal cellular function and leading to paralysis. Therefore, eating toxic shellfish can cause a condition called Paralytic Shellfish Poisoning (PSP). This condition immediately affects the gastrointestinal and nervous system, with symptoms usually occurring within 30 minutes. Severity depends on the amount of toxin consumed.

Mild Symptoms include:

  • headache
  • dizziness
  • nausea
  • vomiting
  • rapid pain
  • anuria (non-passage of urine)

Moderate to Severe symptoms include:

  • tingling on the face and neck progressing to numbness
  • numbness and incoordination of the extremities
  • respiratory difficulty
  • difficulty swallowing
  • sense of throat constriction
  • speech incoherence or complete loss of speech
  • brain stem dysfunction
  • complete paralysis

What to do if someone present symptoms of PSP?

  1. Seek proper medical attention immediately.
  2. If medical attention cannot be reached contact the nearest poison control center.

For a First Aid for Victims of Paralytic Shellfish Poisoning guide, follow this link.

If you want to learn more about marine toxins and the diseases they may cause, visit the U.S. Center for Disease Control and Prevention.


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      sohaila 7 years ago

      it was very usefull information about oyster poisoning because of ingestion of poisoned oyster which i did and i suffered from it having nausea vomiting headache abdominal pain after 3 hours i acte 85 mg can of oister