Beware of amnesic shellfish poisoning

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I work in an ED in a coastal area where fishing is a popular pastime. What's amnesic shellfish poisoning? What are the signs and symptoms? Is any treatment available?—N.W., CALIF.
Michelle Wooten, BSN, BS, RN, and Bridget Parsh, RN, EdD, CNS, respond: Amnesic shellfish poisoning (ASP) was first identified in 1987 after an outbreak in which over 100 people became ill following ingestion of affected mussels.1 ASP is caused by a heat-stable neurotoxin called domoic acid (DA), which is produced by red algae that can rise to unsafe levels in shellfish that consume the algae.2
Because DA is produced by algae, outbreaks of ASP are associated with algal blooms, in which algae grow out of control in large quantities. The proliferation of algae occurs most commonly in warm summer months along the coasts.3,4
The incidence of shellfish poisoning is rare in humans due to increased education and regulation by public health officials.5 After the initial outbreak, officials looked at the lowest concentrations of DA in mussels that produced symptoms, and divided that by 10 to produce a limit of 20 micrograms of DA per gram of shellfish. These limits are linked with a cessation of large-scale occurrences such as the initial 1987 outbreak.1
When DA is ingested, it can bind to and stimulate glutamate receptors, leading to neuronal depolarization.5 This causes an influx of calcium, which can contribute to the neurologic signs and symptoms associated with ASP. Older adults and patients with renal disease are at higher risk for serious outcomes, including death.6 Renal disease increases the risk because the kidneys are responsible for removing toxins such as DA from the body.7
Signs and symptoms of ASP include abdominal cramps and diarrhea after consuming contaminated shellfish. Because these signs and symptoms are nonspecific, further investigation is needed. Be sure to obtain a recent food history and ask if the patient has recently consumed any shellfish. If it's available, leftover shellfish can be tested for increased levels of the toxin.4
Neurologic signs and symptoms include headache, dizziness, disorientation, and permanent short-term memory loss.8 In more severe cases, patients can experience seizures, paresis or paralysis, coma, and death.9
Treatment of ASP is limited to supportive care because no antidote to the neurotoxin exists.4 In the presence of seizures, neurologic damage can be mitigated or prevented with use of benzodiazepines. Activated charcoal may also be administered to bind to any contaminated foods that haven't yet been digested.1
Prevent ASP by avoiding consumption of affected shellfish. Advise patients to check with local health departments and follow posted signs for possible warnings.4 Contaminated shellfish appear, smell, and taste normal, and cooking or freezing the shellfish doesn't eliminate the neurotoxin.4 Commercially harvested shellfish operations follow strict federal guidelines and are regularly tested for biotoxins, so crabs and other shellfish purchased from commercial operations or ordered from restaurants are generally safe to eat.10 Always know where shellfish intended for human ingestion have come from. If patients catch their own crabs, tell them to clean them thoroughly and remove the whitish yellow fat inside the back of the shell where DA can accumulate.
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