Skip directly to: content | left navigation | search


Algae are vitally important to marine ecosystems, and most species of algae are not harmful. However, some species of microscopic marine algae produce powerful toxins that can harm animals and humans. People who come in contact with these toxins by eating fish that feed on harmful algae, swimming among harmful algae, or breathing air that contains toxins from harmful algae, may experience neurologic symptoms (such as tingling fingers and toes), respiratory, or gastrointestinal symptoms.

  • CDC Focus Areas
    • Ciguatera
      Ciguatera fish poisoning (or ciguatera) is an illness humans can get by eating fish that contain toxins produced by the microscopic marine algae Gambierdiscus toxicus. People who have ciguatera may experience nausea, vomiting, and neurologic symptoms such as tingling fingers or toes. They also may find that cold things feel hot and hot things feel cold. Ciguatera has no cure. Symptoms usually go away in days or weeks but can last for years. People who have ciguatera can receive treatment for their symptoms.

      Investigators at CDC, the Food and Drug Administration, the University of Miami, and Jackson Memorial Hospital in Miami are collaborating to develop a test that could be used to identify whether someone has been exposed to the toxins that cause ciguatera. For additional resources and information about ciguatera, including information about CDC's study, visit CDC's Ciguatera Web site at
    • Red Tide
      Red tides occur when the microscopic marine algae Karenia brevis (K. brevis) grow quickly, creating blooms that make the ocean appear red or brown. K. brevis produces potent toxins called brevetoxins, which have killed millions of fish and other marine organisms. Red tides have damaged the fishing industry, shoreline quality, and local economies in states such as Texas and Florida. In addition to killing fish outright, brevetoxins can become concentrated in the tissues of shellfish that feed on K. brevis. People who eat these shellfish may suffer from neurotoxic shellfish poisoning, a food poisoning that can cause neurologic symptoms (such as tingling fingers or toes) and severe gastrointestinal symptoms.

      The human health effects associated with eating shellfish that contain high concentrations of brevetoxins are well documented. However, scientists know little about how other types of environmental exposures to brevetoxin — such as breathing the air near red tides or swimming in red tides — may affect humans. For more information about red tide, and to learn what CDC is doing to study environmental exposures to brevetoxins, visit CDC’s red tide Web site at
  • Data
    Links to federal and non-federal web sites with databases related to harmful marine algae ...more
  • Resources
    Useful information about harmful marine algae from federal, state, international, non-profit organizations as well as universities and others ...more

Other Environmental Hazards & Health Effects Topics