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Blue-green algae: Why is it a danger to dogs?

Updated 30 June 2025
Read time: 6 mins
article author
Written by Corinne Homer
Copywriter

There’s nothing like walking your dog during the warmer spring and summer months. But, if you’re passing any lakes or ponds with a bright green film on the surface,  this is a warning to you and your dog to steer clear, instead of taking a cooling dip.

Blue-green algae form on still water and are extremely dangerous, or even fatal, to your pooch if they ingest it or it comes into contact with their skin. In this article, we’ll explain what blue-green algae is, how to identify it, and how to keep your dog safe from its toxic effects.

What is blue-green algae?

Blue-green algae looks like a paint-like film on top of water and wet surfaces, and it’s not actually algae at all, but Cyanobacteria

Usually it’s green but it can also be blue, brown or red. Sometimes there is foam, and often, an unpleasant smell.  

This bacterium gathers and breeds in wet places, particularly on still or stagnant water like lakes, ponds or lagoons. When you see a pond that is covered in green, it might look scenic, but you and your pooch shouldn’t go anywhere near it. 

Not all algae is blue-green algae, but the risk is that it’s often difficult to tell the difference. 

 

Where is blue-green algae most commonly found?

Bodies of freshwater that are still or slow-moving are a prime spot for the Cyanobacteria to thrive and create blue-green algae. 

Lakes, lagoons, ponds, brackish areas (where freshwater meets the sea), even coastal pools or marshes can be susceptible to breeding these toxic bacteria. 

Another risk worth noting is that you won’t always see the blue-green algae. It doesn’t only present as a green film on the surface of water, but is sometimes in the water. 

Still water that has a murky, milky quality is prone to having these bacteria swimming within its depths. This is why you should be especially vigilant and put your dog on a lead if they have a tendency to run,  jump or drink water whenever they see it.

 

 

How does blue-green algae affect dogs? 

Blue-green algae is severely toxic if your dog ingests it, but it can be poisonous even if it gets on your dog’s skin or fur.

This is because it releases two types of toxins that can be absorbed into the body, Neurotoxins and Hepatotoxins. Put simply, these toxins can make your dog severely unwell, and can prove fatal, sometimes immediately. 

Neurotoxins attack your dog’s nervous system. This can lead to:

 

  • Seizures
  • Tremors
  • Collapsing


Hepatotoxins attack the dog’s liver and can cause further damage around the body if untreated. 

 

 

Symptoms of blue-green algae poisoning

If your pooch has come into contact with blue-green algae, consider it an emergency and immediately watch out for the following symptoms: 

 

  • Vomiting or diarrhoea, as your dog’s body tries to fight and eject the toxins that are causing harm.

  • Drooling, due to the toxins producing excess saliva in your dog.

  • Lethargy or weakness which is more noticeable than regular tired behaviour.

  • Disorientation or confusion. If your pooch doesn’t seem as alert or switched on, this can be a sign that they need veterinary care.

  • At its most severe, blue-green algae poisoning can lead to difficulty breathing, seizures, or even immediate collapse

 

How quickly does blue-green algae affect dogs? 

Almost immediately.

You may see symptoms appear within five minutes to a few hours of your dog coming into contact with the algae. 

In some cases, there isn’t a slow onset of symptoms and the severe signs could trigger straight away. This makes contact with blue-green algae a medical emergency for your pooch. As soon as you know they have touched, licked or drunk it, call your vet.

 

 

Ways to keep your dog away from blue-green algae

Prevention is always better than a cure when it comes to these dangerous toxins, so be prepared to keep your dog safe from blue-green algae. 

 

  • Make sure your dog has excellent recall so that if you see them go near algae, you can call them back to you straight away.

  • Always bring fresh drinking water on walks, so that your dog isn’t tempted to drink from any suspect ponds, pools or puddles.

  • Deter your pooch from jumping into still water. It might be fun at the beach or in clean, freshwater lakes, but it’s helpful if your dog knows that they shouldn’t jump into water still without your command.

  • Keep your dog on a lead if you’re in damp, boggy areas to avoid the risk of impromptu pond-dips or curious puddle drinking.

  • Carry a towel with you on forest walks, just in case you need to wipe them down after a dip, before they try to lick themselves

  • Have distractions handy, such as tasty dog treats or favourite toys so they’ll come straight to you in risky areas.

What if my dog drinks blue-green algae? 

If you catch your dog drinking from a source of blue-green algae, stop them from doing so and immediately call your vet or take them straight to an emergency vets. Don’t try to induce vomiting, just keep them warm and safe, and watch out for the onset of any symptoms. 


What if my dog jumps into blue-green algae? 

If your pooch has jumped into an algae pond or body of water, directly and clearly order them to get out of the water (it helps if they have a strong call-back response). Do all you can to prevent them from licking or grooming themselves, and if possible, wipe them down with a towel. Call your vet or an emergency vets straight away and watch out for any symptoms that might appear. 

 

How do vets treat blue-green algae poisoning in dogs?

Dogs with blue-green algae poisoning will need emergency treatment as symptoms can be lethal. 

A vet will treat any symptoms that are currently presenting, and induce vomiting (but don’t do this yourself). 

 

  • It’s likely they will check your dog’s breathing, heart rate and temperature, and may provide oxygen or anti-seizure medication if necessary. Usually, the most important treatment is used to flush toxins out with fluid, and to give medication for any liver damage (caused by the Hepatotoxins) or impacts to the nervous system (caused by Neurotoxins). 

  • In the most severe cases, your pooch may need to stay under observation in the clinic to be sure the treatment is working. If your dog ingests a lot of toxins and survives, the effects of the poisoning can cause long term effects.

  • While your dog is recovering, you might want to switch to dog food for sensitive tummies and utilise restorative supplements to reduce the strain on their digestive system and replenish good gut bacteria.

 

 

FAQs

Is green algae in a water bowl bad for dogs?

Not all algae is toxic, but annoyingly, it can be hard to know the difference. Any green film that appears in your dog’s water bowl could either make your dog slightly unwell or extremely sick, so you should always clean out your pooch’s bowls with soap and water at the end of each day. Also, don’t forget to empty and clean bowls that may have been left outside, as these are particularly at risk of breeding bacteria.

Why does my dog have diarrhoea after drinking pond water?

There can be all kinds of bacteria in pond water that can cause grief in your pooch’s gut, especially with the diversity of their gut microbiome. Some are mildly harmful, and others are dangerous. If you suspect your pooch may have come into contact with blue-green algae (a green, soup-like film that gathers on the surface of still water), you should definitely call a vet straight away, as diarrhoea could be a symptom of dangerous blue-green algae poisoning. In any case, you might want to switch to dog food for sensitive tummies while they recover.

How to tell the difference between blue-green algae and green algae?

Regular algae isn’t toxic to dogs like blue-green algae is, but the danger lies in how hard it is to tell the difference. While natural algae might form in patches in freshwater lakes and streams, blue-green algae smells bad and appears more scummy; it has a soup-like quality, sometimes with foam, and gathers in stagnant, wet places. Generally, it seems more chemical than natural (like regular algae).

Can dogs drink dirty water?

Ideally, no - the older the water, the more likely bacteria has had time to develop. This includes days-old water in a dog bowl (which is probably the lowest risk), and water in puddles and ponds, which can be swarming with nasty parasites and harmful bacteria. Deter your dog from drinking dirty water by saying ‘No’, always having clean water on hand to quench their thirst, and putting them on the lead whenever you pass by still water, such as ponds covered in algae.

 

If your pooch gets sickness or diarrhoea from something they’ve ingested, consider switching to dog food for sensitive tummies. The gently digestible, natural ingredients and a burst of powerful probiotics help replenish the good bacteria in your pooch’s gut, so they feel back to their normal selves in no time.

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