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Blue-green algae poisoning in dogs reading-time-icon 3 min read

Blue-green algae is not actually algae.

The term refers to a cluster of bacteria - known as cyanobacteria - that’s so small, it can’t be seen by the naked eye unless it groups together.

Blue-green algae can cause dogs to suffer liver failure and is even fatal in severe cases.

You should contact your vet immediately if you think your dog has ingested blue-green algae, or they’ve been in contact with water containing it. 

What does blue-green algae look like?

If you spot a thick green scum or brown substance forming blooms on the surface of the water, it may well be blue-green algae. There are lots of forms of blue-green algae and they all look different; you’ve probably seen it many times - perhaps without realising it. 

Blue-green algae is most common in the summer months, especially dry periods where there’s no rain. It lingers on stagnant water sources, like lakes and ponds, but can even be found closer to home, in our gardens and in water fountains! 

More often than not, it will have an off-putting appearance. In some cases where the toxicity level is really high, you may see dead fish floating in the water. 

If your dog is a keen swimmer (Springer Spaniels and Labradors - among others - love a good swim) make sure you steer clear of water that looks like it could be contaminated with blue-green algae.

Why is it dangerous to dogs?

Not all types of blue-green algae are dangerous but sadly, there’s no way of differentiating between the harmful and the harmless, so all algae-contaminated water is best avoided. 

Certain blue-green algae produce toxins that can cause severe liver damage if they’re ingested by dogs. This can be fatal in extreme cases - some forms of blue-green algae can cause death in just 15 minutes, so always contact your vet immediately if you think your dog has ingested it. 

Is it poisonous to cats too?

Yes! Cats don’t run as high a risk as dogs, mainly because they’re not big on swimming, but blue-green algae will have the same effect on cats that ingest it. Humans too! If you think your cat has eaten a suspicious substance, contact your vet immediately. 

If you have a fountain in your garden and your pet spends a lot of time outside, especially during warm summer days, make sure they have access to clean fresh water at all times. This will deter them from drinking dirty water. 

Read more: 11 summer pet care tips.

pond with greenish water

Symptoms of blue-green algae poisoning in dogs

How to tell if your dog has ingested a poisonous substance? Look out for any of these symptoms:

  • Vomiting
  • Diarrhoea
  • Confusion or disorientation
  • Loss of appetite
  • Drooling
  • Seizures
  • General unrest or seeming a ‘bit funny’

If you’ve just been out for a walk on a sunny day, you've gone past the local pond and your dog has drank from it, perhaps even swam in it, their symptoms are probably linked to blue-green algae. 

Note: These symptoms can belong to other illnesses. If you’ve been nowhere near blue-green algae and your dog is unwell, they could have ingested another harmful substance. Always contact your vet immediately if your dog shows signs of poisoning. 

How to keep your dog safe

Don’t take any chances! If a pond or lake appears dirty, or if there’s anything on there that even looks like toxic blue-green algae, keep your dog well away from it! Don’t let your dog swim in dirty water, and make sure they don’t turn to dirty ponds or lakes to quench their thirst. 

Keep your dog’s coat nice and clean. If they go for a swim (in crystal-clear water of course!), dry them thoroughly when you get home. Bathing your dog regularly will also help keep their coat free of toxins and other foreign bodies. 

Make sure you read the signs when you’re out walking. If a pond or lake is often plagued with blue-green algae, there may be signs telling you to keep your dog away from the water. 

Read more: Dog vaccinations: keeping your puppy happy and healthy.


Sadly, there’s no antidote for the toxins found in poisonous blue-green algae. Treatment will usually involve oxygen therapy, or placing your dog on a drip. 

Treatment will depend on how severe your dog’s symptoms are. In some cases, your vet may need to make your dog sick to eject the toxins; other more advanced cases may need more intensive treatment. 

As with most dangerous substances, it’s best to prevent your dog from coming into contact with blue-green algae, so take care when you’re out and about with your dog and always contact your vet right away if you’re concerned.

Read more: Getting a new puppy: the ultimate guide.

Find your nearest vet using our Find a Vet page, or speak to a vet online using Online Vets.

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