Cyanobacterial blooms in recreational water are dangerous for humans and animals

August 30, 2022
Brigitta Gruenberg, Division of Public Health

Several lakes and reservoirs have health advisories because of cyanobacterial blooms so far this summer. Most of the year, Idaho’s lakes and reservoirs are safe to enjoy. But when water temperatures increase and the right type of nutrients are available, some bodies of water can produce blooms of harmful bacteria that can be dangerous for humans, pets, livestock, and wildlife. Those blooms are called cyanobacteria harmful algal blooms, or HABs for short.

What causes a bloom?

The blooms are caused by bacteria that can produce toxins. The blooms are also referred to as cyanobacterial blooms. When weather conditions are calm and there is an increase in water temperature and nutrients, bacteria can rapidly increase and produce a bloom. Blooms can occur at any time, but they most often occur in late summer or early fall.

What do these blooms look like?

They can be blue, bright green, brown, or red. They may look like paint or anti-freeze floating on the water. As the bloom develops, it may look like foam, scum, or mats on the surface of lakes and ponds.

What are the symptoms of an exposure to a bloom?

The most common health effects are irritated skin and eyes. Other more severe effects can include:

  • Difficulty breathing
  • Stomach pain
  • Nausea
  • Vomiting
  • Diarrhea
  • Dizziness
  • Numbness and tingling in lips, fingers, and toes

If you experience mild irritation, rinse with clean water immediately. If you experience severe symptoms, seek medical attention as soon as possible.

Are the symptoms the same for animals?

Pets and livestock exposed to blooms may show symptoms such as:

  • Weakness
  • Staggering
  • Foaming at the mouth
  • Difficulty breathing
  • Convulsions/seizures

If your pet encounters a bloom, rinse them with clean, fresh water immediately. If they start to have symptoms, call a veterinarian. In severe cases, animals can die within minutes and up to several hours after they have been exposed to harmful blooms.

Is it safe to eat fish from a lake with a bloom?

If you choose to fish in water with a bloom, wear protective clothing such as gloves or waders, and wash your hands thoroughly with clean water. Information about the risk of eating fish from affected waters is limited. However, fish fillets are less likely to have toxins compared to other parts of the fish. If you decide to eat fish from affected waters:

  • Remove the skin, organs, and fatty deposits from the fish.
  • Avoid cutting into organs while you’re cleaning the fish.
  • Rinse the fillets with clean water before you cook the fish.

What should we know to avoid a bloom?

  • Be sure to look at the water before anyone gets in it. If it has an unusual color or looks different than you expected, do not go in or allow your children, pets, or livestock to go in or even near it.
  • Check current information on potential blooms by visiting Idaho Recreational Water Health Advisories.
  •  Avoid any water sports in areas with harmful algal blooms.
  • Do not use untreated water for drinking, bathing, cleaning, or cooking. Boiling water that has been contaminated by a bloom will not remove the toxins, and it may cause more toxins to be released.
  • Remember: When in doubt, stay out.


Brigitta Gruenberg is a health program specialist in the Environmental Health Program, Division of Public Health, Department of Health and Welfare.

The Idaho Department of Health and Welfare is dedicated to strengthening the health, safety, and independence of Idahoans. Learn more at

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