Deadly Blue-Green Algae is On The Rise – How to Protect Your Dog

If you’re out walking this summer with your pup and you encounter a filmy, greenish scum on the surface of water that might make the water appear cloudy, you’ll want to get your dog away, and quick. There’s a chance it could be blue-green algae, which contains dangerous toxins that could be harmful and even fatal to pets, even if ingested in even small quantities. 

According to Vets Now customer data shared with Kinship, cases for blue-green algae poisoning are up 19% year on year in the UK.

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“Blue-green algae is something dog owners should take seriously,” Rachel Garbutt, veterinary nurse warns. 

What is blue-green algae?

“Despite the name, it’s not actually algae but a group of bacteria known as cyanobacteria,” Garbutt says. “When these bacteria clump together, they can look like algae and form blooms in still or slow-moving bodies of fresh water, especially during warm weather and dry spells.”

There are several causes of blue-green algae – long-term pollution from agriculture, wastewater discharge and atmospheric deposition (where substances in the air are deposited on the Earth’s surface) could all be to blame. 

While some blue-green algae contact can be inconvenient for people, causing skin rashes, eye irritations, vomiting, diarrhoea, fever and muscle pains, it can be even more dangerous for pets, including your dog. 

The UK Centre for Ecology & Hydrology has warned it has caused the deaths of dogs (and plenty of other animals). 

Why is blue-green algae dangerous for dogs?

Even though your dog may love the idea of jumping in that refreshing looking water while you’re out on a walk, if the water has blue-green algae in it, your pup will be at risk if they drink or swim in it.

Exposure to toxins produced by the group of bacteria is often fatal for dogs, and can cause long-term health problems in dogs that survive. 

“These toxins can cause liver failure and, in the most severe cases, can be fatal within an hour of exposure,” Garbutt says. “Dogs are at risk if they drink contaminated water or even just swim in it – the algae can stick to their fur and be ingested later when they groom themselves.”

Not all blooms are harmful, but the two most serious types of cyanotoxins (called microcystin and anatoxin) can cause shock, liver failure, respiratory arrest and even death can occur if in contact with your dog.

 There is no antidote for blue-green algae poisoning. 

What does blue-green algae look like?

Blue-green algae may look like green or greenish-brown scum, like a think mat of colour or pea soup, on the surface of water. But they could also appear as green, brown, red, pink, and blue and be quite foamy.

A rapid increase is called a bloom, and when these blooms occur, the number of cyanobacteria in the algae increases as well. 

They can also grow underneath the water, making some harmful algal blooms hard to see. 

The agal blooms might give off a foul odour too – sometimes described as swampy or fishy. 

You may notice dead fish in bodies of water with a high concentration of toxic blue-green algae.

“It’s impossible to tell whether a bloom is toxic just by looking,” Garbutt says. “And not all affected water looks suspicious – toxins can still be present even if the surface looks clear. That’s why the best approach is to avoid letting your dog swim in or drink from still water during high-risk periods, and always pay attention to any local warning signs.”

It might be helpful to look at these visuals from the UK Centre for Ecology & Hydrology to give you an idea of what to look out for. 

Where is blue-green algae found?

Blue-green algae is often found in fresh and salt water lakes, ponds and rivers, particularly in slow-moving or stagnant water. It can become abundant in warm, shallow, undisturbed, and nutrient-rich surface waters that get lots of sunlight. 

There is an app – Bloomin’ Algae – for reporting the presence of blue-green algae, and checking in to see if and where there have been reports of it made by others. 

What time of year is blue-green algae most common?

As algal blooms thrive in warm water, they are more common during the summer months, warm weather spells or after periods with a noticeable lack of rain.

“While it’s more common in summer, blue-green algae can appear at other times of year, too,” says Garbutt.

Increases in water temperature as a result of climate change are expected to increase the magnitude and duration of cyanobacteria in the future. 

Symptoms of blue-green algae poisoning in dogs

If after a walk or swim that you suspect your dog could have been exposed to cyanobacteria, you notice your dog has diarrhoea, vomiting, drooling, weakness, tremors, disorientation, collapse, unconsciousness, blood in stools, seizures or breathing difficulties, you should immediately get your dog seen by a vet. 

Symptoms could arise anywhere from 15 minutes to several days after exposure. 

What to do if your dog ingests blue-green algae

If you’ve been out and you’re sure your dog has been exposed to blue-green algae, you should get to an emergency vet right away. 

There is no antidote, so early intervention is critical to help give a chance of recovery. The vet might be able to flush the toxins out of your dog’s system before they become severely ill due to fatal liver failure, and treat any presenting symptoms.  

“If a dog has been in or near water and starts showing symptoms such as vomiting, diarrhoea, drooling, disorientation, breathing difficulties, seizures, or sudden weakness, it’s critical to get them to a vet immediately and mention the possibility of blue-green algae exposure,” Garbutt warns. “There’s no antidote for the toxins, but if caught early, vets may be able to treat the effects before they become life-threatening.”

Treatments for blue-green algae poisoning in dogs

There is no antidote for cyanobacteria toxicosis, but if you suspect or witness that your dog has come into contact with it, decontamination is the first step: the pet parent can immediately wash their dog off in fresh water so the dog doesn’t lick the bacteria from their coat. 

Then get your dog to the vet right way. 

A vet can induce vomiting with activated charcoal or cholestyramine resin. 

As hypoglycaemia (low blood sugar) is a risk, dextrose can be added to fluids to maintain blood glucose. 

Dogs can be placed on IV fluids and any seizures controlled with anticonvulsants. 

If vomiting and diarrhoea develop, medication designed to prevent and treat the symptoms can be administered. 

Whole blood transfusions might be considered if the heart isn’t able to pump enough blood to the body. 

Can a dog survive blue-green algae poisoning?

Cyanobacterial toxicity can be devastating for pet parents and their dogs. Many dogs exposed to it won’t survive before arriving a vet, and will require intensive care if they do. 

If they do survive, they could suffer from chronic liver disease, among other complications. 

How to prevent your dog from ingesting blue-green algae

The best way to keep your dog safe from blue-algae is to keep it away from water with visible algal blooms, scum, or foam on the surface of the water that has an odour, and keep them on a lead if you’re unsure. 

To stay safe, avoid lakes and ponds where blue-green algae could be present, especially during warm, dry weather,” Garbutt advises. “Don’t let dogs drink from or paddle in water that looks greenish, foamy, or scummy, particularly around the edges, where toxin levels can be higher. When in doubt, steer clear.”

You should check local advisories for warnings before visiting a particular body of water. If there are bodies of water where green-algae is suspected, keep your dog on a lead. 

Whenever your dog has been swimming outside, even if you think the water was safe, give them a good clean with fresh water afterwards. 

To help your dog cool down in the summer (without the threat of blue-green algae), fill a paddling pool and make sure there is lots of shade available. 

“The health risks are too serious to take chances,” Garbutt says. “A cautious walk and a bowl of fresh water are always better than a dip that could end in a vet emergency."

Bottom line: blue-green algae and dogs

Blue-green algae is a term to describe a group of bacteria that can contain highly poisonous toxins which can cause serious damage, and lead to death in dogs. 

Blue-green algae: frequently asked questions

Can my cat be affected by blue-green algae?

Yes, blue-green algae can be toxic to cats, although dogs are at a higher risk. All animals are susceptible to the toxins produced by cyanobacteria.

Is blue-green algae a health risk to humans?

Cyanobacteria blooms can produce toxins that cause rashes, stomach cramps, nausea, diarrhoea and vomiting in humans, and high exposure of the toxins can affect the liver and nervous system. People who are very sensitive to smells could get respiratory irritation. 

Is blue-green algae a bacteria or an algae?

Blue-green algae is not actually an algae, but a type of bacteria called cyanobacteria, which can produce toxic chemicals that are harmful to the health of people and animals. 

Is the algae in a dog's water bowl toxic?

If you’ve noticed green slime forming in your dog’s water bowl, don’t worry, this isn’t the same as the dangerous blue-green algae (cyanobacteria) found in lakes and ponds.The residue in bowls is usually a harmless biofilm made up of water-loving bacteria, algae, fungi and other microorganisms. While not toxic like blue-green algae, if left uncleaned, it can become a breeding ground for harmful bacteria such as E. coli or Salmonella, potentially causing stomach upsets or infections in dogs.

On the other hand, blue-green algae are a type of bacteria that can release potent toxins which affect the liver and nervous system, and can be deadly if ingested or licked from contaminated fur. They typically bloom in warm, stagnant freshwater during dry, hot spells. Regular cleaning of your dog’s bowl with hot, soapy water can prevent biofilm buildup and help keep your dog’s drinking water fresh and safe.

Resources

“Blue-Green Algae Poisoning: Cyanobacteria Toxicosis | Cornell University College of Veterinary Medicine.”

CDC. “Harmful Algal Blooms and Your Health.” Harmful Algal Bloom (HAB)-Associated Illness, 9 Jan. 2024.

“Ask a UW Veterinarian: Blue-Green Algae.” School of Veterinary Medicine, 15 Apr. 2024, www.vetmed.wisc.edu/ask-a-veterinarian-blue-green-algae/. Accessed 19 Aug. 2025.

BBC News. “What Is Blue-Green Algae and Why Is It Such a Problem?” Bbc.com, BBC News, 5 Aug. 2024.

“Bloomin’ Algae | UK Centre for Ecology & Hydrology.”

US EPA. “Climate Change and Freshwater Harmful Algal Blooms.” Www.epa.gov, 5 Sept. 2013.

Renee Schmid, DVM, DABT, DABVT for Pet Poison Helpline, et al. “Algae Poisoning.” Vca_corporate, 2024.

“Blue-Green Algae and Other Water Toxins & Treatments.” ASPCApro, 29 June 2016. Accessed 19 Aug. 2025.