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by the Centers for Disease Control and Prevention (CDC)

From Fall 2009 Forum

Cyanobacteria (blue-green algae):
Cyanobacteria are bacteria that grow in water and are photosynthetic (use sunlight to create food and support life). Cyanobacteria live in terrestrial, fresh, brackish, or marine water. They usually are too small to be seen, but sometimes can form visible colonies. Cyanobacteria have been found among the oldest fossils on earth and are one of the largest groups of bacteria. Cyanobacteria have been linked to human and animal illnesses around the world, including North and South America, Africa, Australia, Europe, Scandinavia, and China.

Cyanobacterial blooms and how they form:
Cyanobacterial blooms occur when algae that are normally present grow exuberantly. Within a few days, a bloom can cause clear water to become cloudy. The blooms usually float to the surface and can be many inches thick, especially near the shoreline. Cyanobacterial blooms can form in warm, slow-moving waters that are rich in nutrients such as fertilizer runoff or septic tank overflows. Blooms can occur at any time, but most often occur in late summer or early fall. They can occur in marine, estuarine, and fresh waters, but the blooms of greatest concern are the ones that occur in fresh water, such as drinking water reservoirs or recreational waters.

What a cyanobacterial bloom looks like:
Some cyanobacterial blooms can look like foam, scum, or mats on the surface of fresh water lakes and ponds. The blooms can be blue, bright green, brown, or red and may look like paint floating on the water. Some blooms may not affect the appearance of the water. As algae in a cyanobacterial bloom die, the water may smell bad.

Cyanobacterial harmful algal blooms (CyanoHABs):
CyanoHABs are algae blooms that threaten people, animals, or the environment. They are dangerous for many reasons:

  • Dense CyanoHABs can block sunlight and use up all the oxygen in the water, killing other plants and animals.
  • Some cyanobacteria that can form CyanoHABs produce toxins that are among the most powerful natural poisons known. These toxins have no known antidotes.
  • CyanoHABs can make people, their pets, and other animals sick. Often, the first sign that an HAB exists is a sick dog that has been swimming in an algae-filled pond.
  • Children are at higher risk than adults for illness from CyanoHABs because they weigh less and can get a relatively larger dose of toxin.

Other effects of fresh-water CyanoHABs:

  • CyanoHABs can make drinking water smell and taste bad.
  • They can make recreational areas unpleasant.

Species of cyanobacteria that form CyanoHABs in fresh water:

  • Microcystis aeruginosa
  • Anabaena circinalis
  • Anabaena flos-aquae
  • Aphanizomenon flos-aquae
  • Cylindrospermopsis raciborskii

Cyanotoxins are a diverse group of chemical substances that are categorized by their specific toxic effects as follows:

  • Neurotoxins affect the nervous system.
    • Anatoxin-a
    • Anatoxin-a(s)
    • Saxitoxin
    • Neosaxitoxin
  • Hepatotoxins affect the liver.
    • Microcystins
    • Nodularins
    • Cylindrospermopsin
  • Tumor promoters are chemicals that can increase tumor growth.
    • Microcystins
    • Lipopolysaccharides are chemicals that can affect the gastrointestinal system

How you could be exposed to CyanoHABs and cyanotoxins:

  • Drinking water that comes from a lake or reservoir with a CyanoHAB
  • Drinking untreated water
  • Engaging in recreational activities in waters with CyanoHABs
  • Inhaling aerosols from water-related activities such as jet-skiing or boating
  • Inhaling aerosols when watering lawns, irrigating golf-courses, etc. with pond water
  • Using cyanobacteria-based dietary supplements that are contaminated with microcystins
  • Receiving dialysis (this has been documented only in Brazil)

Types of illnesses people and animals can get from exposure to CyanoHABs:
Getting it on the skin may give people a rash, hives, or skin blisters (especially on the lips and unde swimsuits)

  • Inhaling water droplets from irrigation or water-related recreational activities can cause runny eyes and nose, a sore throat, asthma-like symptoms, or allergic reactions.
  • Swallowing water that has cyanobacterial toxins in it can cause
    • Acute, severe gastroenteritis (including diarrhea and vomiting)
    • Liver toxicity (i.e., increased serum levels of liver enzymes).
      • Symptoms of liver poisoning may takes hours or days to show up in people or animals.
      • Symptoms include abdominal pain, diarrhea, and vomiting.
    • Kidney toxicity
    • Neurotoxicity. These symptoms can appear within 15 to 20 minutes after exposure. In dogs, the neurotoxins can cause salivation and other neurologic symptoms, including weakness, staggering, difficulty breathing, convulsions, and death. People may have numb lips, tingling fingers and toes, or they mayfeel dizzy.

Testing for cyanobacterial toxins

  • Most of the toxins require specialized testing that can take weeks.
  • Some kits are available to test for microcystins on site.

How to protect yourself, your family, and your pets from exposure to CyanoHABs:

  • Donít swim, water ski, or boat in areas where the water is discolored or where you see foam, scum, or mats of algae on the water.
  • If you do swim in water that might have a CyanoHAB, rinse off with fresh water as soon as possible.
  • Donít let pets or livestock swim in or drink from areas where the water is discolored or where you see foam, scum, or mats of algae on the water.
  • If pets (especially dogs) swim in scummy water, rinse them off immediatelyódo not let them lick the algae (and toxins) off their fur.
  • Donít irrigate lawns or golf courses with pond water that looks scummy or smells bad.
  • Report any "musty" smell or taste in your drinking water to your local water utility.
  • Respect any water-body closures announced by local public health authorities.

How to treat people or animals that have been exposed to cyanobacterial toxins:

  • Get medical treatment right away if you think you, your pet, or your livestock might have been poisoned by cyanobacterial toxins.
  • Remove people from exposure and give them supportive treatment.

How to help reduce the occurrence of CyanoHABs:

  • Reduce nutrient loading of local ponds and lakes by using only the recommended amounts of fertilizers and pesticides on your yard.
  • Properly maintain your household septic system.
  • Maintain a buffer of natural vegetation around ponds and lakes to filter incoming water.

How to get more information about cyanobacteria:

  • Centers for Disease Control and Prevention (CDC)
    • Harmful Algal Blooms (HABs) site at
      This site defines HABs; describes CDCís HABs-related activities; and provides links to data, publications, and other HABs resources.
    • Cyanobacteria site at
      This site defines cyanobacteria; describes CDCís cyanobacteria-related activities; and provides links to data, publications, and other cyanobacteria resources.
  • Environmental Protection Agency (EPA)
    • Drinking Water Contaminant Candidate List site at
      This site provides information about EPAís list of contaminants that are not regulated, occur in public water systems, and may require regulation under the Safe Drinking Water Act. Algae that can be harmful are on this list.

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