Amnesic shellfish poisoning

Amnesic shellfish poisoning (ASP) is one of the most common syndromes caused by the consumption of shellfish that contain the marine toxin, domoic acid. Many shellfish contain marine toxins, which are naturally occurring chemicals that can contaminate bivalve mollusks such as mussels, clams, oysters and scallops. The toxins are produced by microscopic algae in the form of dinoflagellates and diatoms. They are accumulated through the food chain and are ultimately deposited in higher predator fish or, in this case, filter-feeding bivalves.

ASP is a rare syndrome caused by a toxin made by a microscopic, red-brown, salt-water plant, or diatom called Pseudo-nitzschia. The toxin produced by these diatoms, domoic acid, is concentrated in shellfish and causes disease when the contaminated shellfish are eaten. The chemical can bioaccumulate in marine organisms that eat phytoplankton, such as shellfish, anchovies, and sardines. In mammals, including humans, domoic acid acts as a neurotoxin, causing permanent short-term memory loss, brain damage, and death in severe cases.

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Symptoms and treatment

In the brain, domoic acid especially damages the hippocampus and amygdaloid nucleus. It damages the neurons by activating AMPA and kainate receptors, causing an influx of calcium. Although calcium flowing into cells is a normal event, the uncontrolled increase of calcium causes the cell to degenerate.

Gastrointestinal symptoms can appear 24 hours after ingestion of affected molluscs. They may include vomiting, nausea, diarrhea, abdominal cramps and haemorrhagic gastritis. In more severe cases there are neurological symptoms that can take several hours or up to three days to develop. These include headache, dizziness, disorientation, vision disturbances, loss of short-term memory, motor weakness, seizures, profuse respiratory secretions, hiccoughs, unstable blood pressure, cardiac arrhythmia, and coma.

People poisoned with very high doses of the toxin or displaying risk factors such as old age and renal failure can die. Death has occurred in 4 of 107 confirmed cases. In a few cases, permanent sequelae included short-term memory loss and peripheral polyneuropathy.

There is no known antidote available for domoic acid, so if symptoms fit the description, it is advised to quickly go to the hospital. Cooking or freezing affected fish or shellfish tissue does not lessen the toxicity.

Discovery

ASP was first discovered when, in late 1987, a serious outbreak of food poisoning occurred in eastern Canada. Three elderly patients died and other victims suffered long-term neurological problems. Because the victims suffered from memory loss, the term "amnesic" shellfish poisoning is used.^[1] The story made front-page newspaper headlines.

Epidemiologists from Health Canada quickly linked the illnesses to restaurant meals of cultured mussels harvested from one area in Prince Edward Island, a place never before affected by toxic algae. Mouse bioassays on aqueous extracts of the suspect mussels caused death with some unusual neurotoxic symptoms very different from those of paralytic shellfish poison and other known toxins. On December 12, 1987, a team of scientists was assembled at the National Research Council of Canada laboratory in Halifax, Nova Scotia. Integrating bioassay-directed fractionation with chemical analysis, the team identi­fied the toxin on the afternoon of December 16, just 4 days after the start of the concerted investigation.

Possible animal effects

On June 22, 2006, a California brown pelican, possibly under the influence of domoic acid, flew through the windshield of a car on the Pacific Coast Highway. The acid is found in the local seas.

Domoic acid poisoning may also have caused a 1961 invasion of thousands of frantic seabirds in Santa Cruz, California. Director Alfred Hitchcock heard about this invasion while working on his adaptation of Daphne du Maurier novella "The Birds" for his feature film The Birds (1963).

Since March 2007, marine mammal and seabird strandings and deaths off the Southern California coast have increased markedly. These incidences have been linked to the recent and dramatic increase of a naturally occurring toxin produced by algae. Most of the animals found dead tested positive for domoic acid.

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