Infectious agents cause most shellfish-associated illness.
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.