Why are shellfish a concern?
Shellfish are marine animals that have a shell. The Food and Drug Administration defines shellfish as clams, mussels, oysters, and scallops. [Crabs, crayfish, lobsters, and shrimp are defined as crustaceans, according to the National Oceanic and Atmospheric Administration, although they are often also called shellfish.] Shellfish provide food for humans and many other species. Some are found close to shore in tidal flats where people may dig or gather them for personal consumption. Offshore shellfish beds or reefs are more likely to be harvested by commercial fishermen using boats equipped with dredges or tongs.
Shellfish must grow and live in clean water to be safely eaten. Shellfish, as defined by the Food and Drug Administration, are "filter feeders," which means they pump and filter large amounts of water through their bodies every day as they eat. As the water filters through, the shellfish strain out particles for their food. These particles can include harmful chemicals, waste, bacteria, viruses, and marine toxins which can contaminate the shellfish.
Shellfish can become contaminated because of poorly treated sewage from wastewater treatment plants, cesspools, and septic systems. Contamination may also come from polluted water runoff from marinas, farms, and wildlife waste. They can be contaminated by marine toxins that are produced by algae blooms. Some marine toxins are dangerous and can cause extremely serious shellfish poisoning and even death in humans who eat contaminated shellfish.
Local and state health departments monitor shellfish for contamination and will ban harvesting in contaminated shellfish beds and coastal areas. Health departments also warn of the increased health risks from eating raw shellfish and may advise cooking shellfish to kill bacteria.
This description is based on the information found in the Web links listed with this topic.
Web Links from MedlinePlus (National Library of Medicine)
Harmful Algal Bloom (HAB)-Associated Illness (Centers for Disease Control and Prevention)
Marine Biotoxins (National Oceanic and Atmospheric Administration)
Shellfish Allergy (Nemours Foundation)
Shellfish Facts (Thurston County [WA] Environmental Health Commission) (PDF — 21 KB)
The Risk of Eating Raw Oysters or Clams (American Liver Foundation)
Last Updated: August 22, 2017