Amnesic shellfish poisoning

Amnesic shellfish poisoning occurs from ingesting bivalve shellfish (such as mussels, oysters, and clams) that contain toxins.

Steamed mussels and razor clams have been associated with amnesic shellfish illnesses. ASP was first reported in 1987 in Prince Edward Island when three people died and over 100 people became ill after consuming cooked mussels. These toxins can cause severe and life-threatening neurological effects. Symptoms include muscle weakness, disorientation, and short term memory loss. These toxins can also cause gastroenteritis and may initially present as gastrointestinal with diarrhea, abdominal discomfort, vomiting and dizziness.

Shellfish harvested in BC coastal waters can sometimes be contaminated with this toxin. Self-harvesters of shellfish should check to see if the area they are harvesting from is open.

Information for Health Professionals

Definition

Confirmed Case

Gastrointestinal symptoms [1] within 24 hours OR neurological symptoms [1] within 48h of eating shellfish [2];
AND
Laboratory confirmation through:

Detection of domoic acid in ingested shellfish in edible tissues in excess of 10 mg/kg (10 ppm);
OR
Detection of domoic acid in urine collected within 24h of exposure and illness.

Probable Case

Neurological symptoms [1] within 48 hours of eating shellfish [2];
OR
Gastrointestinal symptoms [1] within 24h of eating shellfish [2] AND where the shellfish was:

also eaten by a confirmed case or a probable case with neurological symptoms;
OR
from an area with domoic acid in excess of 20mg/kg (20ppm) in shellfish.

Notes

Clinical illness defined as:
1. Gastrointestinal symptoms (nausea, vomiting, abdominal cramps and/or diarrhea).
2. Neurological symptoms (confusion and/or loss of memory).
Illness lasts hours to days. In severe cases: tachycardia, hypotension, arrhythmias, pulmonary edema, profuse respiratory secretions, mutism, unusual ocular movements with disconjugate gaze and grimacing, seizures, coma and/or death.
At risk shellfish include filter feeding molluscan bivalve shellfish: clams, mussels, scallops (digestive tissue), oysters, cockles and whelks. Other at risk species include the hepatopancreas of crab, and plantiverous fish eaten whole with digestive tissue such as anchovies and sardines.

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Symptoms

Within 48 hours, neurological symptoms including,

headache
confusion, disorientation
seizures
coma
permanent short term memory loss

Within 24 hours, gastroenteritis symptoms including,

diarrhea
nausea
abdominal cramps
vomiting
headache

ASP symptom onset develops within 24 hours after eating contaminated shellfish. Usually gastroenteritis symptoms start in less than 24 rs, and neurological symptoms within 48 hours. The recovery period is a few hours to a few days.

Causes

Amnesic shellfish poisoning is caused from the ingestion of toxin-contaminated bivalve shellfish and crustaceans. Toxins accumulate in filter feeding bivalves, like mussels, when algal blooms of diatoms, such as Pseudo-nitzschia occur – usually during the warmer months of June to October. The most common species affected and associated with amnesic shellfish illnesses are mussels, but the toxins may also enter the food chain through razor clams, scallops and crustaceans. The toxin binds to fatty cells in razor clams, and can persist for long periods after Pseduo-nitzia is no longer found in the growing waters.

Domoic acid is a neurotoxin that binds to glutamate receptors, and acts on the hippocampus, which is involved in memory function.

Complications

Symptoms usually resolve completely within a few hours to 3 days after shellfish ingestion. There are few recorded illnesses of ASP, however, permanent short term memory loss is a known complications (or chronic sequelae) from amnesic shellfish poisoning.

Tests and diagnosis

The detection of toxin in epidemiologically linked food confirms the diagnosis. Shellfish should not exceed established Canadian standards.

Treatment and drugs

There is no antidote. In severe cases, oral rehydration is recommended.

Prevention

In Canada, CFIA has a national biotoxin monitoring program that includes testing for PSP, ASP, and DSP in bivalve molluscan shellfish. Historically in BC, CFIA has been monitoring for ASP on the east and west coasts since 1988, when the outbreak in Prince Edward Island occurred.

Health Canada has established limits for ASP-causing toxins (Canadian Standards). When these regulatory limits are exceeded, effected harvest areas are closed to harvesting.

All shellfish in BC must be inspected by federally registered shellfish processing plants before going to commercial market – this is part of the Canadian Shellfish Sanitation Program (CSSP), the federal monitoring and prevention program in Canada. The CSSP classifies harvesting areas and controls the commercial and recreational harvesting and processing of shellfish for the consumer market. The CSSP is run by 3 federal government agencies (1) Environment Canada - responsible to monitor water quality in shellfish areas, (2) Canadian Food Inspection Agency - responsible for monitoring marine toxins in shellfish areas and for registering and inspecting shellfish processing plants, and, (3) Fisheries and Oceans Canada - responsible for opening and closing harvest areas, and prohibiting shellfish harvesting when bacteriological or toxin levels are unsafe. More on Shellfish Contamination.