Domoic acid is a potent, naturally occurring neurotoxin responsible for Amnesic Shellfish Poisoning (ASP) after consuming contaminated seafood. The toxin is odorless and tasteless, making it impossible to detect without specialized testing. Its presence is directly linked to harmful algal blooms in coastal waters.
Environmental Origins of Domoic Acid
Domoic acid is produced by specific species of microscopic algae, primarily diatoms belonging to the genus Pseudo-nitzschia. These single-celled organisms are a natural part of the marine phytoplankton community. Under certain environmental conditions, such as warmer water temperatures and nutrient availability, their populations can rapidly multiply to form a harmful algal bloom (HAB).
The toxin enters the food web through bioaccumulation, where it concentrates in the tissues of organisms that feed on the toxic algae. Filter-feeding shellfish, such as mussels, clams, oysters, and scallops, accumulate high levels of domoic acid. Small finfish, such as anchovies and sardines, also accumulate the toxin, transferring it up the marine food chain to larger predators and humans.
Symptoms and Neurological Effects
The illness is formally recognized as Amnesic Shellfish Poisoning due to its defining neurological symptom. The toxin is structurally similar to glutamate, the brain’s main excitatory neurotransmitter. It binds to and overstimulate specific receptors, causing excessive stimulation known as excitotoxicity. This causes damage to neurons, especially those concentrated in the hippocampus, the area of the brain responsible for memory formation.
The clinical presentation follows a dose-dependent continuum, beginning with gastrointestinal symptoms that appear within 30 minutes to 24 hours after consuming contaminated seafood. These initial signs include vomiting, nausea, abdominal cramps, and diarrhea, often resolving within a few days. In cases of higher exposure, severe neurological effects begin to manifest within 48 hours.
The severe symptoms include confusion, disorientation, headaches, motor weakness, and seizures. The most characteristic feature of the poisoning is the potential for permanent short-term memory loss (amnesia), a direct consequence of damage to the hippocampus. Life-threatening complications can also occur, such as respiratory difficulty, cardiac arrhythmias, and a progression to coma or death.
Diagnosis and Supportive Care
Diagnosing Amnesic Shellfish Poisoning relies on a patient’s clinical presentation combined with a detailed history of recent seafood consumption. The symptoms, especially the characteristic gastrointestinal distress followed by neurological signs, strongly suggest the diagnosis. While testing the suspected seafood for domoic acid can confirm the source, there is no routinely available clinical blood test to measure the toxin in a human patient.
There is no specific antidote available to neutralize domoic acid once it has been ingested. Treatment focuses entirely on supportive care, aiming to stabilize the patient and manage the severe symptoms. This includes administering anti-seizure medications to control convulsions and providing respiratory support for patients experiencing breathing difficulties or coma.
The gastrointestinal symptoms will resolve quickly with supportive measures. However, the prognosis for those who experience severe neurotoxicity is less certain. The neuronal damage caused by the excitotoxicity can lead to long-term or permanent disability. While some neurological symptoms may improve, the short-term memory loss is often irreversible.
Preventing Exposure and Public Safety
Prevention is the most effective defense against domoic acid poisoning, achieved through stringent monitoring and public advisories. Regulatory agencies, such as the Food and Drug Administration (FDA), continuously monitor coastal waters and shellfish beds for the presence of the Pseudo-nitzschia algae and the toxin. When domoic acid levels exceed regulatory action limits—typically 20 parts per million (mg/kg) in shellfish meat—authorities issue mandatory harvest closures and quarantines.
Consumers must strictly adhere to these official advisories and only harvest or purchase seafood from areas declared safe. Domoic acid is a heat-stable neurotoxin, meaning that cooking, steaming, or freezing contaminated seafood does not destroy the toxin. For crustaceans like crab and lobster, the toxin concentrates heavily in the viscera, often called “crab butter.” Thoroughly removing these parts before consumption can help reduce the risk of exposure.