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Marine Toxins in Bivalve Shellfish: Paralytic Shellfish Poisoning, Amnesic Shellfish Poisoning and Diarrhetic Shellfish Poisoning

Consumers should be aware of some potential food safety issues associated with bivalve shellfish (i.e. clams, oysters, scallops, mussels, cockles), other molluscan shellfish (i.e. whelks and periwinkles) and the tomalley of lobster and crab.

These animals are highly sensitive to the quality of their marine environment. Because they feed themselves by filtering microscopic organisms from the water, harmful bacteria, viruses and biotoxins from their surroundings can build up in their tissues and cause illness in people who consume them.

Eating shellfish with high levels of certain toxins can lead to serious and potentially fatal illnesses such as: Paralytic Shellfish Poisoning (PSP), Amnesic Shellfish Poisoning (ASP) and Diarrhetic Shellfish Poisoning (DSP).

Paralytic Shellfish Poisoning (PSP)

PSP is an illness that may have serious and potentially fatal effects. It is caused by eating bivalve shellfish and other molluscan shellfish that have been contaminated by toxins produced by certain species of microscopic marine algae found in coastal waters. Lobster and crab tomalley (also called hepatopancreas, which is the soft green substance inside the body cavity) can also accumulate the toxins which cause PSP.

The toxins that cause PSP are not destroyed by cooking.

Symptoms of PSP

Symptoms of PSP could begin within a few minutes and up to 10 hours after consumption.

Symptoms of PSP can include:

  • a tingling sensation or numbness around the lips that gradually spreads to the face and neck,
  • a prickly sensation in the fingertips and toes,
  • drowsiness,
  • headache and dizziness, and
  • difficulty swallowing.

In more severe cases one may also experience:

  • incoherent speech,
  • a prickly sensation in the arms and legs,
  • stiffness and non-coordination of limbs,
  • weakness, and
  • a rapid pulse.

Respiratory difficulty, salivation, temporary blindness, nausea and vomiting may also occur.

In extreme cases, paralysis of respiratory muscles may lead to respiratory arrest and death within two to twelve hours after consumption. Seriously affected people must be hospitalized and placed under respiratory care. There is no known cure for PSP.

If you suspect you have PSP you should immediately seek medical attention.

Health Canada advises Canadians to limit their consumption of lobster tomalley to the equivalent of one lobster tomalley daily for adults, due to the possible presence of PSP. Health Canada has recommended that children not consume lobster tomalley.

Amnesic Shellfish Poisoning (ASP) and Domoic Acid

Amnesic Shellfish Poisoning (ASP) is an illness caused by domoic acid, a naturally occurring acid which is produced by some marine algae. Domoic acid can accumulate in a number of filter-feeding bivalve molluscan shellfish such as clams, mussels, scallops and oysters.

ASP was unknown in Canada until November 1987, when an outbreak in Eastern Canada resulted in four deaths.

The toxins that cause ASP are not destroyed by cooking.

Symptoms of ASP

The symptoms of ASP can include:

  • nausea,
  • vomiting and diarrhea,
  • muscle weakness,
  • disorientation, and
  • memory loss.

Symptoms usually occur 30 minutes to 6 hours after consumption.

If the poisoning is not severe, the symptoms disappear completely within a few days in an otherwise healthy person. However, in extreme cases, death can occur.

If you suspect you have ASP you should immediately seek medical attention.

Diarrhetic Shellfish Poisoning (DSP)

Diarrhetic Shellfish Poisoning (DSP) is an illness caused by toxins that are produced by certain microscopic plants. Generally, DSP is often short-lived and non life-threatening; however, for some people, especially young children, the elderly, pregnant women and those with weakened immune systems, DSP can be very serious.

The toxins that cause DSP are not destroyed by cooking.

Symptoms of DSP

The symptoms of DSP can include:

  • diarrhea,
  • nausea,
  • vomiting,
  • headache,
  • abdominal cramps, and
  • chills.

Symptoms usually occur 30 minutes to six hours after consumption.

The symptoms disappear completely within a few days in an otherwise healthy person. If you suspect you have a severe case of DSP or should problems persist you should immediately seek medical attention.

The Federal Government's Role in the Safety of Shellfish for Human Consumption

The Canadian Shellfish Sanitation Program (CSSP) was established to ensure that shellfish harvested in Canada are safe to eat and is a key component of the Government of Canada's commitment to protecting the health and safety of Canadians. Three federal government agencies work together to deliver this program:

  • Environment Canada analyzes water quality in shellfish harvesting areas and identifies waters that do not meet sanitary standards.
  • The Canadian Food Inspection Agency monitors for biotoxins in shellfish in harvesting areas and is responsible for registering and inspecting fish and shellfish processing plants.
  • Fisheries and Oceans Canada patrols and closes harvest areas, and bans the harvesting of shellfish whenever bacteria or toxin levels exceed safety standards.

Food safety guidelines for bacteria, biotoxins and other contaminants are set by Health Canada.

Red Tide

  • Under specific marine conditions, certain species of microscopic algae can multiply rapidly causing a "bloom" in population. This phenomenon is commonly referred to as a red tide because, at times, seawater can become discoloured from the dense accumulation of algae.
  • Red tide events can include biotoxin-producing algae that can contaminate bivalve shellfish in the area.
  • Algal blooms are most common in the spring and summer months when sunlight, temperature and precipitation favour algal growth. However, algal blooms can occur at other times of the year as well.
  • It is also important to note that many toxin-producing algal blooms are not "red" or do not cause discolouration of the water and cannot be readily seen from shore. This is why you should always check whether an area is open before you harvest

Visit DFO's website for more information on Shellfish Contamination and Red Tide.

What can you do to protect yourself and your family?

  • Caution is required when harvesting shellfish. It is your responsibility to contact your nearest Fisheries and Oceans Canada (DFO) office to find out which areas are assigned as "open" for shellfish harvesting.
  • An "open" area refers to a safe harvest area that is subject to monitoring and testing, and where harvesting is a legal activity. When an area is officially "closed," it is illegal to harvest shellfish in that area for any purpose.
  • Only shellfish harvested from harvest areas designated as "open" should be consumed.
  • Purchase shellfish only from suppliers you trust and those who have harvested from "open" areas.
  • Once you get them home, shellfish should be refrigerated or frozen until they're ready to be eaten. Be mindful that cooking bivalve shellfish does not destroy the toxins that cause illnesses such as PSP, ASP, DSP. Properly cooked shellfish can still be toxic.
  • Anyone who feels ill after eating bivalve shellfish should immediately seek medical attention.

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