Hepatitis A, Norwalk virus, Vibrio parahaemolyticus and Vibrio vulnificus have all been transmitted through shellfish ingestion. Toxic illness caused by shellfish has been recognized for several hundred years. Native Americans are known to have warned early settlers to avoid shellfish during the summer months. These shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae in the form of dinoflagellates and diatoms. The saxitoxins act by blocking sodium ion movement through voltage-dependent sodium channels in nerve and muscle cell membranes.
Conduction block occurs principally in motor neurons and muscle.
Brevetoxins are polycyclic ethers that, like ciguatoxin, bind to and stimulate sodium flux through voltage-gated sodium channels in nerve and muscle. Brevetoxins are made by the dinoflagellate, Ptychodiscus brevis. Okadaic acid binds to intestinal epithelial cells and increases their permeability. This toxin is made by dinoflagellates of the species Dinophysis and Prorocentrum. Domoic acid is structurally similar to the excitatory neurotransmitter glutamate.