ASP Biotoxins

Eating contaminated Shellfish can be life threatening:
take a look at the areas that are closed due to PSP & ASP!

Agent: Domoic Acid (ASP)

Classification: Marine Biotoxin

Fish Products Most Affected: Filter feeding molluscan shellfish:

• clams, hard and soft-shell

• mussels, blue and red (horse)

• scallop, not adductor muscle (meats); and

• anchovies

Properties of Agents Relevant to Fish Products or Illness:

Filter feeding shellfish live off the small bits of organic matter which they filter from the sea water, including small plants or animals. Some of the small plants (phytoplankton) contain toxins. The primary sources of domoic acid on the east coast of Canada are the diatoms Pseudo-nitzschia multiseries and P. pseudodelicatissima. Additional Pseudo-nitzschia species (including P. australis) are sources of domoic acid on the Canadian west coast. It appears the Pseudo-nitzschia species blooms are periodic; they do not occur every year even though the organisms are apparently frequently present in the seawater.

When the mollusc digests the toxic organism, the toxin is retained largely in the digestive gland, but if the toxin accumulation is sever enough other tissues are contaminated as well. The toxin is water soluble and relatively rapidly cleared from most shellfish, such as blue mussels (Mytilus edulis) and soft-shell clams (Mya arenaria). It is not rapidly cleared by others, such as, red (horse) mussels (Volsella modiolus) or Atlantic scallop (Placopecten magellanicus). The toxin appears to be accumulated in the digestive gland of the Atlantic scallop over long periods of time.

Chemical Assay and Reported Units:

To routinely determine domoic acid contamination in situations where no major outbreak appears to be occurring, samples prepared for PSP determination by the mouse bioassay (using all of the soft tissues of enough shellfish to give at least 100 g), are evaluated chemically by HPLC (high performance liquid chromatography). Since this does not result in highly precise estimates because the PSP extracts do not give good recovery of domoic acid, an extract specific for domoic acid is prepared when serious levels of the toxin are encountered (5 µg/g or higher) and a precise estimate is required, such as when an area may need to be closed.

Domoic acid results are given as ug domoic acid/g soft tissue.

Action Level:

The Canadian action level for acceptance of product and closure of shellfish areas, is 20 ug domoic acid/g soft tissue.

Characteristic Illness:

The ingestion of domoic acid-contaminated shellfish can cause severe nausea, vomiting and diarrhea within ½ to 6 hours. If the poisoning is not severe and the person is not otherwise compromised (e.g., kidney problems), the person normally recovers completely within a few days. Unfortunately, if the poisoning is severe enough or the individual is not able to excrete domoic acid readily, temporary or permanent brain damage may occur. The syndrome is complex but the most notable simple characteristic is a loss of short term memory, hence the toxin is sometimes called Amnesic Shellfish Poison (ASP). In the most severe cases, as the 1987 outbreak in P.E.I., deaths may occur.

Control Measures:

Molluscan bivalves must come only from open harvest areas.

Samples of product from susceptible species from suspect areas must be monitored all year. The intensity of the monitoring activity reflects the amount of toxin found to be present and the likelihood of the accumulation of toxic levels. In general, the problem is most severe in the late summer and fall in P.E.I., the Bay of Fundy and the Canadian west coast.  The greatest contaminations experienced with P.E.I. shellfish have been in November and December. The shellfish clear the toxin (depurate) rapidly (about 48 hours) when the toxin source is removed and if the accumulation levels are not very high (e.g., 40 µg/g or less). If the accumulation is very severe and the temperature low (as has happened with P.E.I. mussels) then several weeks may be required to clear the toxin to acceptable levels.

If unacceptable products is detected it must be destroyed except in instances involving shellstock where absolute control can be assured, relaying of the shellstock for future harvesting may be permitted. If a shellfish area can be identified as the source of unacceptably contaminated shellfish then it is closed. The area can only be opened if three samples over a period of two weeks have had levels of the toxin less than 20 µg/g.