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Blue-green algae and its dangers to dogs

  • Blue-green algae can be toxic to dogs. Call your vet immediately if you suspect poisoning.
  • Don’t let your dog drink from or swim in water containing blue-green algae 
  • Blue-green algae is most common during the hotter, drier summer months when there is less rain

What is blue-green algae and what does it look like?


They are not actually algae, but the organisms got this name because they often give the appearance of algae when they clump together in bodies of water.

Blue-green algae brown clumps
Blue-green algae may not always be harmful but you can’t tell simply by looking, so it's best not to let your dog near the water

The bacteria cannot be seen with the naked eye unless they clump together. When this happens, blue-green algae can look like green flakes, greenish bundles or brown dots in a pond, lake or stream.


Blooms of the organisms often build up around the edges of ponds and lakes, which may look like foam.

It is most common in non-flowing fresh water such as lakes and ponds during hot weather when there is less rainfall, but can also occur at other times of the year.

You may notice dead fish in ponds or lakes with a high concentration of the toxic bacteria. Don’t let your dog drink from water containing dead animals.

The algae may be present in a harmful form even if you cannot see it, so take note of any warning signs in the area.

Why is blue-green algae dangerous to dogs?

When cyanobacteria clump together, it can look like a blue-green scum, pea soup, green flakes, greenish bundles or brown dots in the water. Photo by Chris Tomlinson.




Concentrations of the algae vary throughout the year and may not always be harmful - but you can’t tell simply by looking at them whether or not they are dangerous, so it is best not to run the risk of allowing your dog to come into contact with water where the algae may be present.

What are the symptoms of blue-green algae poisoning?

a sign warns of blue-green algae in water
Take note of signs put up by authorities or land owners warning of the algae during dog walks and follow the information given


  • Vomiting/being sick
  • Diarrhoea
  • Seizures/fitting
  • Weakness/collapse/unconsciousness
  • Disorientation/confusion
  • Drooling
  • Breathing difficulties

There is no antidote for the toxins produced by the bacteria, but if caught early enough, your vet will likely try to make your dog sick and attempt to flush the toxins from the body before they take hold.

Sadly, blue-green algae poisoning often eventually causes fatal liver failure.

These symptoms are commonly seen with other illnesses too, which are often less serious, but you should always call your vet if you are worried your pet is sick.

How can I protect my dog from blue-green algae?

If your dog becomes ill after entering water, contact your vet urgently and tell them you are worried about blue-green algae. Photo by California Department of Fish and Wildlife.

Keep your dog away from lakes and ponds that you know, or suspect may, contain blue-green algae. 

Dogs should not be allowed to swim or paddle in water that contains blue-green algae.

Don’t let dogs drink from water that may have blue-green algae in. Because the wind often blows blooms of algae to the edges of ponds or lakes, higher concentrations of the toxin are more likely to be present in the parts of the water your dog may drink from.

Take note of signs warning of the algae during dog walks and follow the information given.

Can my cat be affected by blue-green algae?

Yes, blue-green algae is toxic to cats. Contact your vet immediately if your cat drinks from water contaminated with blue-green algae.

Is blue-green algae a health risk to humans?

Yes, blue-green algae can cause skin rashes, sickness, stomach pains, fever and headaches in humans. There have been some reports of more serious illnesses including liver and brain damage. Children are at greater risk than adults. Don’t drink from or enter this water.

If you think you have spotted blue-green algae in water and cannot see any signage, report it to the Environment Agency who can investigate.

— Page last updated 19/07/2018

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