Dr. Charles J. Rennie III
Adjunct Curator of Marine Mammals, Department of Vertebrate Zoology
Domoic acid is the result of an extreme proliferation of marine algae, usually (but not always) of the genus Pseudonitzschia (see below). This compound affects particularly the hippocampus and amygdala in the brain, where it causes both degeneration and scarring (gliosis). It acts like glutamate, the main excitatory neurotransmitter in the brain; an excess of which can cause cell death.
Domoic acid toxicity was first recognized on Prince Edward Island in Canada in 1987, when there was an outbreak of neurological symptoms among humans who had eaten mussels. Four people died, and a number of others suffered severe short-term memory impairment as well as other neuropathies. Domoic acid toxicity has been postulated as a cause of mortality in marine birds, although major die-offs have not been well enough investigated to confirm it as the definitive cause.
Domoic acid was first pinpointed as a problem in marine mammals in 1998, when many California sea lions (Zalophus californianus) died along the Central California coast. Seizures were the most common symptom, although abnormal scratching, head-bobbing, and ataxic ("drunken") movements were also seen. High levels of domoic acid were documented in anchovies, but not in local shellfish. Domoic acid toxicity may be responsible for multiple previous die-offs of pinnipeds, although studies were not sufficient to pinpoint it as the definitive cause.
In the current die-off of common dolphins (Delphinus sp.), domoic acid toxicity was first postulated by colleagues in Northern California and at the Los Angeles Museum of Natural History. Most of the affected dolphins appear to have been in otherwise excellent health: most were young otherwise healthy looking males, with few parasites. Tissue samples have been collected, as well as blood, urine, and feces, wherever possible. Several animals have tested positive for domoic acid in various fluids and tissues. The deaths are continuing. What is now needed is more information about the current Pseudonitzschia bloom and domoic acid levels in species preyed upon by marine mammals for food. Close monitoring of local shellfish and fish caught for human consumption is also needed; there may be some risk to humans here!