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Occupational & Environmental Epidemiology

Algal Blooms

Algal Blooms

Algae often live in fresh bodies of water such as lakes, ponds and canals. Like plants, they can use the sun as an energy source. Small numbers of algae can explosively grow into large numbers very quickly – this rapid increase is called an algal bloom.

Conditions that can contribute to algal blooms include hot, dry weather and increased nutrients in a body of water. Sometimes an algal bloom can become harmful to people, pets, livestock, and aquatic plants and animals by producing toxins, shading light, and clogging gills in fish. It is hard to tell whether a bloom is harmful by just looking at it, so it is best to avoid water suspected of having an algal bloom. When in doubt, stay out!

Avoiding Recreational Water Illnesses

  • Cyanobacteria (Blue-green Algae)
    • One type of freshwater algae increasingly seen in North Carolina is cyanobacteria, also known as blue-green algae, which may actually look reddish-brown as well as bright green or blue-green. Many types of algae flourish in lakes and ponds with poor water flow, especially during the hot months of the year.
    • When conditions are right, blue-green algae can multiply and accumulate rapidly, causing a bloom. The algal bloom may turn the water neon green, blue-green, or reddish-brown; may cause a bad smell and taste in the water; and may form a foam or scum on the water's surface. The algae may periodically use up oxygen in the water, killing fish.
    • It can also affect other animals and people. By nature, dogs often play and wade in shallow areas of a pond or lake where algal blooms tend to concentrate due to wind and water currents, and where the toxins can become concentrated. Dogs usually become exposed to the toxins by drinking bloom waters or eating the algae. Algal toxins can be lethal to a dog.
    • Young children also play and wade in the shallow areas of ponds and lakes, and are exposed to toxins if they ingest bloom waters containing toxins. Due to their size, children may have adverse health effects from smaller amounts of toxins than would affect an adult.

Algal Toxins

  • Toxins associated with fish
  • Ciguatera fish poisoning, or ciguatera, is an illness caused by eating ocean fish that contain toxins produced by naturally occurring microscopic marine algae called Gambierdiscus toxicus.

    People who have ciguatera poisoning may experience nausea, vomiting and neurologic symptoms, such as tingling fingers or toes. They also may experience temperature reversal sensation, so that cold things feel hot and hot things feel cold. Some fish commonly associated with ciguatera toxins include snapper, jacks, sea bass, grouper, triggerfish and barracuda. These large fish accumulate the toxins when they eat smaller fish that have been eating the toxic algae.

    People who have ciguatera can be treated for their symptoms. However, ciguatera has no cure. Symptoms usually go away in days or weeks but can last for years.

  • Toxins associated with shellfish
  • Shellfish poisoning is a type of food poisoning people can get when they eat shellfish that have eaten toxin-producing algae. The 20 toxins responsible for paralytic shellfish poisonings (PSP) are all derivatives of saxitoxin produced by various species of algae including, but not limited to, Kerenia brevis, Alexandrium spp., Nitzchia, and Dinophysis. Paralytic Shellfish Poisoning (PSP), Diarrheic Shellfish Poisoning (DSP), Neurotoxic Shellfish Poisoning (NSP) and Amnesic Shellfish Poisoning (ASP) are the forms detected most frequently.

    Eating contaminated shellfish results in a wide variety of symptoms, depending upon the toxins(s) present, their concentrations in the shellfish, and the amount of contaminated shellfish consumed.

    In Paralytic Shellfish Poisoning (PSP), the effects are predominantly neurological and include tingling, burning, numbness, drowsiness, incoherent speech and respiratory paralysis, which requires prompt medical intervention.

    Diarrheic Shellfish Poisoning (DSP) is usually a generally mild gastrointestinal disorder involving nausea, vomiting, diarrhea, and abdominal pain accompanied by chills, headache and fever.

    Neurotoxic Shellfish Poisoning (NSP) causes both gastrointestinal and neurological symptoms, including tingling and numbness of lips, tongue and throat; muscular aches; dizziness; reversal of the sensations of hot and cold; diarrhea; and vomiting.

    Amnesic Shellfish Poisoning (ASP) causes gastrointestinal disorders (vomiting, diarrhea, abdominal pain) and neurological problems (confusion, memory loss, disorientation, seizure or coma). Elderly people are most at risk from this type of shellfish poisoning.

    Shellfish poisoning can be prevented by avoiding potentially contaminated bivalve (two-shell) mollusks. This is particularly important in areas during or shortly after "red tides." Travelers to developing countries should avoid eating all shellfish because they carry a high risk of viral and bacterial infections. Marine shellfish toxins cannot be destroyed by cooking or freezing.

    • CDC: Harmful Algal Bloom (HAB)-Associated Illness External link
    • FDA: Bad Bug Book External link - Primarily technical, this handbook also provides consumer-focused "snapshots" with basic information about the major known bacteria, viruses, parasites and natural toxins that cause foodborne illness and how to avoid getting sick.
  • Toxins associated with recreational swimming
  • For recreational activities in algae impacted waters the primary concern are exposures to cyanotoxins. These are produce by cyanobacteria which are also known as blue-green algae. Information for the most common cyanotoxins can be found below.

    • Microcystin - Symptoms of exposure include headache, sore throat, vomiting and nausea, stomach pain, dry cough, diarrhea, blistering around the mouth, and pneumonia. Additionally, exposure to elevated levels of microcystins can potentially lead to liver damage.

      Microcystins typically have a half-life of four to 14 days in surface waters or may persist longer, depending on factors such as photodegradation, bacteria, and the presence of organic matter. Microcystins can persist even after a harmful algal bloom is no longer visible.

    • Cylindrospermopsin – Symptoms of exposure include fever, headache, vomiting, bloody diarrhea with the kidneys and liver appearing to be the primary target organs for cylindrospermopsin toxicity.

      The breakdown of cylindrospermopsin in natural water bodies is a complex process that can be influenced by many environmental factors, including toxin concentration, water temperature, sunlight, and the presence of cell pigments and bacteria. Half-lives of 11 to 15 days and up to eight weeks have been reported for cylindrospermopsin in surface waters.

Protecting Your Family’s Health

Although there are no documented reports of people getting sick from algal blooms in North Carolina, people should follow these practical precautions around algal blooms:

  • Keep children and pets away from waters that appear discolored or scummy.
  • Do not handle or touch large accumulations of algae, also called “scums” or “mats”.
  • Do not water ski or jet ski over algal mats.
  • Do not use scummy water for cleaning or irrigation.
  • If you accidently come into contact with an algal bloom, wash thoroughly.
  • If your child appears ill after being in waters containing a bloom, seek medical care immediately.
  • If your pet appears to stumble, stagger, or collapse after being in a pond, lake or river, seek veterinary care immediately.
  • If you are unsure whether or not a bloom is present, it is best to stay out of the water.

For additional information regarding how to protect your health in recreational waterbodies from other exposures visit here.

For additional information regarding how to protect your pet's health in recreational waterbodies please review the following fact sheet.

Identifying and Reporting a Suspected Algal Bloom

The North Carolina Department of Environmental Quality (NCDEQ) is responsible for monitoring and investigating reports of algal blooms across the state. NCDEQ has the ability to collect algal bloom samples to analyze them for algae that potentially pose a health risk. More information on how to report an algal bloom to NCDEQ is provided below.

How to identify an algal bloom (PDF)

To report an algal bloom, please contact your regional NCDEQ office.


If you have questions regarding health concerns related to algal blooms please contact the Occupational and Environmental Epidemiology Branch at (919)-707-5900.

Additional Resources