When swimming can be dangerous: Blue-Green Algae

Cyanobacteria (also known as blue-green algae) can bloom in lakes, streams and ponds, most often during periods of hot weather in mid- to late-summer months but can be present in lower concentrations throughout the year. The algae take on the appearance of blue or green paint on the surface of the water. These algae can produce microcystins and anatoxins which are toxins that affect the liver and nervous system respectively. Even a very small amount can be deadly!

Types of toxins:

Microcystins cause liver damage or failure, potentially leading to death. Symptoms can include some or all of vomiting, diarrhea, digested blood in stool, weakness, pale gums, juandice, seizures, abnormal behaviour, and shock.

Anatoxins are toxic to the nervous system and symptoms include excessive drooling, tearing, shaking, muscle rigidity, paralysis and suffocation due to increased secretions into the lungs. Death occurs within minutes to hours of exposure as a result of respiratory paralysis.

Not all cyanobacteria will contain these toxins, unfortunately without testing the algae itself you cannot tell the difference. Therefore, anytime your pet has come into contact with blue-green algae, it needs to be treated as an emergency.

What can you do to prevent this?

  • Investigate swimming areas prior to allowing your dog to swim and prevent your dog from swimming in any bodies of water that looks to have blue-green algae floating on its surface. See photos below to familiarize yourself with its appearance.
  • If accidental exposure has occurred, contact your veterinarian immediately. As prognosis is very poor, immediate medical attention before clinical signs appear is your pets best chance of survival. Treatment would include induced vomiting, liver protectants, IV fluids, muscle relaxants and bathing to remove excess algae from coat. There is no antidote to blue-green algae. All treatment is supportive to try and lessen the effects of the toxins.
  • If your dog has swallowed algae and vomited it up, save it for analysis.
  • Bathe your animals and yourself if any physical contact has occurred.
  • Protect yourself. When handling an animal covered in blue-green algae be sure to wear protective clothing. These toxins can be a danger to you as well if accidentally ingested.


While swimming is an excellent way for your dog to exercise and have fun this summer, take the few extra seconds it would take to scan bodies of water for blue-green algae before letting your dog run in. It could save their life!

Written by: Dr. Megan Haines, DVM


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Last updated: January 7, 2022.

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