Blue-Green Algae – How Toxins in Water Can Affect your Pet

Dog swimming in the water.

If you take your dog swimming in lakes or ponds during the summer, listen up! During hot weather in late summer and early fall, lakes and ponds can accumulate a dangerous type of algae that floats seemingly innocent on the surface. This species grows in “blooms” that give the water a blue-green color. 

What is it?

Blue-green algae (cyanobacteria) is a microscopic organism found in freshwater ponds, lakes, and streams. It can also accumulate in containers of stagnant rain water left outside. Not all types of algae are poisonous but blue-green algae is; this type produces toxins called microcystins and anatoxins. These toxins are very dangerous to a variety of different species such as humans, birds, cattle, horses, dogs, and cats. You should never let your dog swim in or drink water that could be potentially contaminated with this deadly toxin.


Symptoms of blue-green algae poisoning depend on which toxin (microcystin vs anatoxin) is present. Symptoms of blue-green algae that produce microcystins are:

  • Lethargy
  • Weakness
  • Vomiting
  • Diarrhea
  • Shock
  • Seizures
  • Death

Symptoms of blue-green algae that produce anatoxins are:

  • Excessive salivation
  • Inability to walk
  • Tremors
  • Muscle rigidity
  • Paralysis
  • Difficulty breathing
  • Death

Prevention is Critical

It’s impossible to know if the algae you see floating on top of a pond or lake is poisonous or benign without laboratory testing. For this reason, it’s in your dog’s best interest to play it safe and find a different body of water with no visible signs of algae on the surface. Even tiny amounts (1-2 mouthfuls) can be potentially fatal. Death can follow within thirty to sixty minutes of exposure due to respiratory paralysis.

Never let your dog swim in any lake or pond with visible algae on the surface and always provide fresh water for your pets so they are never tempted to drink out of containers of stagnant contaminated water.


Sadly, the prognosis for blue-green algae toxicity is poor. Some dogs die before they have time to receive medical attention. Aggressive and immediate treatment is essential to treat this fact-acting poison due to the rapid onset of symptoms. There is no antidote for blue-green algae poisoning so if you believe your dog may have been exposed, get to the nearest animal hospital ASAP. Like with any other poison, the sooner the dog is able to have treatment the better the prognosis.

posted in:  Pet Safety