Linda Simon
Dr Linda Simon (MVB MRCVS, University College Dublin)


Blue-green algae is emerging as a real threat to our canine population, though many owners remain unaware of the risk it poses. While most dogs will be exposed when out and about in parks and fields, some pet owners will have blue-green algae growing in their own back garden without even realising it.

The potential consequences of blue-green algae exposure can be devastating, with reports of animals dying being published every year. As treatment options are limited, the key to minimising the risk is to simply avoid exposure at all costs. It is essential that all dog owners educate themselves on what blue-green algae is and become aware of the damage that this seemingly innocuous substance can pose to our much-loved pets.

What Is Blue-Green Algae?

Blue-green algae colony

Blue-green algae colony

While one may assume that blue-green algae is indeed a form of algae, it is actually a cyanobacteria, which is a microscopic bacteria that will grow on the surface of stagnant water and mimic the appearance of algae. These cyanobacteria are so toxic that they pose a significant threat to many animals, including livestock, wildlife and our pet cats and dogs. In fact, even us humans can be affected by them when swimming or through ingestion of contaminated water.

They can produce an array of toxins, including potent Anatoxins. Knowing what the substance looks like is important in order to be able to avoid it. Water containing the cyanobacteria will often have a film of slime on the surface which will be a light green colour. Some people will refer to the characteristic hue produced as a ‘pea green’. Not all algal blooms will produce dangerous toxins, but it is impossible to know which will, so it is best to simply avoid all algae growth.

Where Is It Commonly Found?

Many bodies of water can contain blue-green algae and it is typically seen on top of ponds, lakes and streams. Worryingly, the growth tends to accumulate at the edge of the water, making it more accessible for thirsty animals. On top of this, we can commonly see blue green algae in our back gardens, such as in ornamental garden fountains and even within small bird baths.

The biggest blooms are seen in nutrient dense water, particularly in water surrounded by fields which have been fertilised, as the fertiliser can run off and basically ‘feed’ the growth of the toxins. Stagnant water should never be trusted and any water with a film or scum on the surface should not be used by an animal to drink from or bathe in.

What Time of Year Does It Typically Occur?

As the algae thrive in warmer weather, we see most growth in the summer and early autumn months. Many public health departments will post signs near affected water at certain times of the year to inform the public of the risk involved. Owners should be on the lookout for these signs during sunny weather.

Is It Harmful to Dogs?

There is no doubt that blue-green algae is very harmful to dogs, particularly those that spend more time outdoors and swimming, such as hunting dogs who are at an increased risk of exposure. Those that are lucky and receive immediate treatment may survive the exposure, but many will die within a matter of hours. Animals may be put at risk from drinking the water or simply from swimming in it.

Common Signs & Symptoms of Blue-Green Algae Poisoning

The symptoms caused by the blue-green algae are dependent on which type of toxin has been produced. There are two major toxins which we need to be concerned about: Anatoxins and Microcystins.


Microcystins tend to target the liver, an organ that is essential to life as it filters toxins and plays a major role in metabolism. Symptoms can initially include lethargy, vomiting, reduced appetite and diarrhoea. Within hours, animals may become jaundiced (their skin will take on a yellow appearance).

Blood tests carried out would reveal elevated liver enzymes and a reduction in the substances normally produced by the liver, such as proteins. The animal’s ability to clot its blood will be affected, and spontaneous bleeding may occur. Over the following 24 to 48 hours, affected animals become progressively unwell and may fall into a coma and ultimately pass away.


Anatoxins are the more potent and dangerous of the toxins and can cause severe symptoms in a matter of minutes. As the nervous system is targeted, signs can be particularly dramatic for owners to witness. Initially, dogs may salivate and shake, they may then find it difficult to walk and breathe and can go on to suffer from paralysis and seizures. The final effects are normally falling into a coma, followed by a quick death. Tragically, animals die rapidly after Anatoxin exposure, sometimes within as little as 30 minutes.

What Is the Treatment Plan?

Vet nurse looking after sick dog

The sad truth is that treatment for blue-green algae exposure is limited and there is no known cure or antidote. They key is for owners to recognise where there has been exposure and to bring their pet to the veterinary clinic as soon as possible. On the weekend, this often means attending an ‘out of hours’ emergency clinic. If animals are presented to the vet before symptoms develop (which is rarely achieved due to the fast-acting nature of the toxins), an attempt will be made to rid the animal of what they have ingested.

This can include inducing vomiting and providing the animal with activated charcoal meals. Treatment is supportive and aims at controlling the symptoms the animal is experiencing. This may include aggressive fluid therapy, liver support, anti-nausea medication, anti-seizure medication and oxygen provision. Intensive nursing may be required for several days.

What Is the Prognosis for Blue-Green Algae Poisoning?

Sadly, those who have been exposed to blue-green algae have a poor prognosis and the mortality rate remains high, even in those that are treated promptly. Even young, fit animals who have not been exposed to a large amount of the poison are at a high risk of death.

Is It a Common Problem Found in Veterinary Practices?

Certain places in the world will see more cases of blue-green algae poisoning than others, and areas with temperate climates are generally the worst affected. Rural areas where fertiliser is used more often will typically have the largest bloom growths.

Luckily, blue-green algae toxicity is not especially common when compared to other toxicities, perhaps due to the good work of the local authorities, who work hard to publicise the risk of the toxin and to discourage owners from allowing their animals nearby with posters and public announcements. The more the general public become aware of the risks, the less cases vets will be presented with, so education is critical.

Are Cats at Risk?

Thankfully, it is very rare for cats to be exposed to blue-green algae toxins and it is not something typically seen in practice. The reason for this is thought to be a combination of the cat’s natural reluctance to drink anything but the cleanest water, as well as their general dislike of swimming.

Are There Any Preventative Methods Available to Reduce Exposure to Blue-Green Algae?

Given the awful prognosis, prevention is by far the best way to combat this condition. People need to prevent any access to areas where algae is blooming, which may mean fencing off bodies of water and turning un-used water bowls or fountains upside down to avoid rain entering them.

Owners should bring their own bottled water along with them on walks and offer it frequently to their dogs to prevent them seeking water from inappropriate sources. Dogs should only be allowed to swim in bodies of water that do not contain any algae and it is safest to wash their coats off after a swim when possible, as even the ingestion of small amounts of the toxin can prove fatal.

How to Know Whether the Water Is Safe for Your Dog

Dog swimming in clear, cyanobacteria-free water

The take home message is that owners should always check water before allowing their dog near it. This is especially true during warmer weather. Any water with a film or ‘scum’ growing on it, must be avoided. Though it is true that not every algal bloom will produce toxins, without performing laboratory tests it is impossible to know which pose a risk and which do not.

On top of visually inspecting water, owners should be on the lookout for any signs or posters warning of dangers that have been put up by the local councils. If in doubt, avoid the water entirely and stick to bottled water for drinking and a paddling pool in the back yard to cool off!

References and Further Reading

Diagnostic and clinically important aspects of cyanobacterial (blue-green algae) toxicoses: