Blue-green algae (or cyanobacteria) occur in fresh, brackish and sea water throughout the world; in the UK they can sometimes appear in large blooms during the summer months in ponds, lakes, reservoirs and old gravel pits.

While usually green, or blue-green in colour (hence the name), they may be khaki, blue, black, dark brown or red. Blooms of blue-green algae can form thick scums on the surface of the water and are often cause for concern because as they start to break down they can release a range of toxins. If these toxins are ingested in large quantities they can present risks to human health and long-term exposure of human skin to the toxins can cause irritation. The effect on humans in the UK has been limited to illness rather than death and there are a number of steps that you can take to minimise the risk associated with blue-green algal blooms.

Blue-green algae can produce potent toxins that can result in a range of health effects in people and animals. Effects on people coming into contact with toxic scums include skin rashes, eye irritations, vomiting and diarrhoea, fever and pains in muscles and joints. Blue-green algae have caused the deaths of dogs, horses, cattle, birds and fish across the UK

According to reports, at least four dogs have died in the UK after venturing into to waterways known or thought to have be home to the algae. It has also been causing harm this summer in the US, where the Great Lakes have been particularly badly affected, according to scientists. Three dogs reportedly died in North Carolina after swimming in a contaminated lake.

Thanks to the climate emergency, those conditions are being met more often. “There’s pretty strong evidence that climate change is leading to increased problems, more widespread and intense blooms, right across the globe. We expect with warming that things will get worse.

Blue-green algae look a “pea soup” scum that floats on the water … but it’s actually bacteria. In fact, it’s bacteria that can photosynthesize. The bacteria grow and develop into large colonies that form blooms. And as a surface scum its intuitively attractive to many dogs!

Not all blue-green algae blooms produce toxins

Humans can become very ill when our skin comes into contact with toxic blooms and the same is also true for your dog … but it can be deadly if they lick or drink any of it.

Remember wild animals will be affected too … so if you‘re out near water and you see dead animals (even fish or birds) … don’t let your dog in the water! 

What Makes Some Blue-Green Algae Toxic?

The toxins produced that cause illness are Microcystins and Anatoxins.

Microcystins can cause:

  • liver damage and possible liver failure
  • vomiting
  • diarrhoea and or bloody diarrhoea
  • pale or jaundice gums ( mucous membranes)

Anatoxins can cause these neurological issues:

  • muscle tremors
  • seizures
  • paralysis

Symptoms Of Blue-Green Algae Poisoning in Dogs

Both toxins can cause symptoms in your dog and in as little as 30 to 60 minutes after your dog is exposed to the toxins.

Symptoms associated with microcystins may take up to 12 to 24 hours to appear … but it’s still critical to take your dog to a vet as soon as possible.

In a perfect world, you will be able to avoid these algae but if your dog is out of sight with access to water … you need to look out for these symptoms:

  • Diarrhoea
  • Vomiting
  • Blood in stool or black, tarry stool
  • Pale mucous membranes
  • Seizures, muscle tremors, muscle rigidity, or paralysis
  • Disorientation
  • Coma
  • Excessive salivation or tearing
  • Blue discoloration of the coat, skin and mucous membranes
  • Difficulty breathing

What to Do If You Suspect Blue-Green Algae Poisoning

  1. Rinse your dog off with fresh water as soon as possible to remove as much bacteria as you can.
  2. If you’re not sure if your dog licked or drank any water … follow the instructions below. Watch your dog carefully for any signs of illness.
  3. If your dog did lick or drink any water ... you need to get him to the vet as soon as you can. But follow the instructions below for some things you can do right away that can help!

Caution: If your clinic is closed, go to a 24-hour emergency clinic. Don’t wait till morning. That may be too late. Call ahead to let the clinic know you’re on your way and why. 

But right away … there re things you can do to start helping your dog. 

Supplies To Carry With You (Shopping List) 

  1. Hydrogen peroxide. Buy 3% hydrogen peroxide at any pharmacy.
  2. Activated charcoal.
  3. A syringe or small baster to give either hydrogen peroxide or charcoal, as needed. 

Remedies To Help With Blue-Green Algae Poisoning

Note: Follow these recommendations if you see any rapid onset symptoms when your dog’s been in the water – even if you didn’t see blue-green algae! It can hide for a long time in corners and other less visible places.

If your dog has no symptoms but you think he might’ve drunk some water immediately give a dose of activated charcoal. This is best used within an hour or two of exposure.

Charcoal binds to toxins and can help remove them from his body. If he has swallowed some toxic water, it could help him eliminate the toxins.

If he hasn’t drunk any water, the charcoal won’t hurt him. 

Dosing Charcoal

Depending on which charcoal option you bought, here’s how to give it:

Amount: 1 ml per lb of body weight.  It’s a good idea to calculate how much you’ll need ahead of time. 

If you know your dog has swallowed some water that you suspect is toxic …

Caution: If your dog has a seizure or is unconscious, don’t give either hydrogen peroxide or charcoal. He needs to be able to swallow the liquids.  

1 Immediately give hydrogen peroxide to induce vomiting. 

Only do this if it’s been less than 2 hours since he drank the algae water. If it’s been more than 2 hours go straight to 2, activated charcoal.

  • Amount: using a syringe, give 1ml (or cc) per pound of body weight of hydrogen peroxide.
  • The easiest way to do it is to hold his head level or slightly tilted … then use the syringe to squirt the peroxide into the side pouch near the corner of his mouth. Don’t tilt his head back too far … you don’t want him to aspirate the peroxide.  
  • He should vomit quickly. If not, repeat that dose once within 15-20 minutes.
  • If nothing happens the second time … switch to dosing the activated charcoal. Don’t wait too long. 

2 After the peroxide, give a dose of activated charcoal (see Dosing Charcoal above).“

Don’t be alarmed if your dog vomits or has diarrhoea. He needs to get the toxins out of his body. When you get to the vet, they’ll make sure he’s hydrated and will give fluids if needed. 

Once You’re At The Vet

Let your vet take over and do what’s necessary to save your dog. This is an emergency and not the time to be quibbling about sticking with natural care. 

However, the toxins in the algae put stress on your dog’s liver and can cause organ damage. So, when you get a chance  ask your vet to make sure your dog gets liver support. 

Many vets will give Sam-E, N-Acetyl-Cysteine (NAC) or vitamin C. If they don’t, ask if you can bring one or more of these supplements for them to give at the clinic. 

Some dogs with healthy immune systems may not even get sick. But the dogs who are affected can die very quickly. So you need to be prepared. 

And again, don’t waste time waiting to get to your regular vet.  Find the closest clinic that’s open and go straight there. 


Where has blue-green algae been reported? Below is a link to