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Blue-Green Algae

BOAH veterinarians advise animal owners to learn more about blue-green algae, often called "pond scum."  The algae is a health concern for both people and animals.  The algae grows best on hot, dry, calm days.

Veterinarians are asked to report incidents of blue-green algae exposure in animals. Form 55580 should be submitted to BOAH as part of a statewide blue-green algae surveillance program.

What is blue-green algae?

Blue-green algae, also known as cyanobacteria, is a microscopic bacteria found in freshwater lakes, streams and ponds where water is warm and stagnant.  Most people refer to the algae as "pond scum."

What's so bad about blue-green algae?

It's poisonous.  While some types of algae are harmless, the blue-green type produces a natural powerful toxin.  Some form toxins that affect the nervous system and others produce toxins that affect the liver.

Livestock, pets and wild animals can be poisoned by the toxins produced by some algal blooms.  Lighter weight animals can ingest a toxic dose quickly.  Dogs are particularly susceptible to blue-green algae poisoning because the scum can attach to their coats and be swallowed during self-cleaning.

What does the algae look like?

Blooms look like green paint floating on water, foam or scum, or mats on the surface of freshwater lakes and ponds. The blooms can be blue, bright green, brown or red.  Some blooms may not affect the appearance of the water but as algae in the bloom dies, the water may smell bad.  Blue-green algae is not the type that grows in mats of plant material along shorelines.  When you pick it up, the algae disperses in the water and does not hang together in a stringy mass.

How does your animal get poisoned by the algae?

Swimming or drinking from water that has been contaminated with blue-green algae can result in illness or death within hours or days.

What are the clinical signs of blue-green algae poisoning in animals?

Signs of poisoning may include:

  • Vomiting

  • Diarrhea

  • Decreased appetite

  • Black-tarry stool

  • Weakness

  • Lethargy

  • Pale gums

  • Jaundice gums

  • Shock

  • Death

Other, neurologic signs include:

  • Excessive drooling

  • Excessive eye tearing

  • Muscle tremors

  • Muscle rigidity

  • Inability to walk

  • Difficulty breathing

  • Blue gums

  • Death

What if your animal gets sick?

Contact a veterinarian immediately.  Animals may die within minutes of clinical signs appearing, but can survive for several hours or up to two or more days after exposed. If the animal has consumed a lethal dose, no antidote exists for the toxins.  Sometimes, animals are found dead with no signs observed.

What should you do if you see a blue-green algae bloom?

Don't let pets or livestock swim in or drink from areas where blooms are seen.  If pets swim in scummy water, rinse them off immediately with soap and water, to remove the toxin.  Do not let them lick the algae off their fur.  Follow these steps to assess the risk:

  1. Establish if animals are drinking water or eating dried algae mats from the area where a bloom has been identified.

  2. Have a qualified laboratory examine sample of the water.  They can identify the bacteria in the blooms and determine whether enough is present to be at risk.

  3. Livestock should be withdrawn from the water supply and an alternative source should be used.  If an alternative source is unavailable and the bloom is floating, you may be able to allow livestock to drink from an area on the upwind side of the bloom.

If I have blue-green algae blooms and the water is treated, can my livestock use the water?

Methods to prevent toxicity include restricting access to infested ponds and treatment of the pond using copper sulfate or algaecides.  Be sure to check the product manufacturer's safety precautions before allowing animals access to the water.  Livestock should be fenced away from the pond for several days after the treatment is complete.

What can you do to prevent blue-green algae poisoning?

  • Monitor and watch for algae blooms during hot weather.

  • Do not treat blooms with algaecides or herbicides as they break open the bacteria cells.  This will lead to a rapid release of toxins in the water.

  • Fence off downwind drinking areas and force animals to drink from areas where concentration of blue-green organisms is unlikely.

  • Pump water from several yards below the surface of a fenced-off pond to a nearby livestock tank.

  • Use other water sources, if available.

More information about blue-green algae, not specific to just pets, can be found on the Indiana Department of Environmental Management's website: