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Blue-green algae in dogs: how do you recognise it and what should you do?

Blue-green algae in dogs: how do you recognise it and what should you do?

Written by Mara | Dierenarts | May 13, 2026

On warm days, many dogs seek cooling relief in the water. Lovely of course, but unfortunately stagnant water can sometimes hide a serious danger: blue-green algae. Especially during summer, warnings about blue-green algae appear regularly. For dogs, contact with contaminated water can be dangerous and sometimes even life-threatening.

Cocker Spaniel running through the water

What is blue-green algae?

Blue-green algae is not actually a true algae, but a bacterium from the cyanobacteria group. These bacteria grow particularly well in:

warm water
stagnant water
nutrient-rich water

In warm weather, these bacteria can multiply quickly and produce toxic substances. This is known as a blue-green algae bloom.

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What does blue-green algae look like?

To prevent your dog from becoming ill from blue-green algae, it is important to know how to recognise it. Blue-green algae can appear as:

a blue-green or greenish film on the water
floating streaks or clumps
a layer that resembles pea soup
foam along the water’s edge
water with a musty or unpleasant smell

However, blue-green algae is not always clearly visible. Even water that appears reasonably clean can still be contaminated.

Why is blue-green algae dangerous for dogs?

Dogs are at extra risk because they:

swallow water while swimming
lick their wet coat
sometimes drink directly from natural water sources

As a result, they can quickly ingest a dangerous amount of toxins.

Symptoms of blue-green algae poisoning in dogs

Symptoms of blue-green algae poisoning can develop rapidly. The first signs may appear within 30 minutes to several hours after contact with contaminated water.

Possible symptoms of blue-green algae poisoning in dogs include:

vomiting
diarrhoea
lethargy
weakness
excessive drooling
muscle tremors
breathing difficulties
disorientation
seizures

In severe cases, the toxins can damage the liver, nervous system and other organs.

How quickly do symptoms appear after exposure to blue-green algae?

How quickly symptoms appear depends on several factors:

the amount of toxins ingested
the type of blue-green algae
the size and health of the dog

Sometimes symptoms can appear within 30 minutes. In other dogs, it may take several hours before symptoms become visible. That is why it is important to monitor your dog closely after swimming.

What should you do if you suspect blue-green algae poisoning?

Do you think your dog has come into contact with blue-green algae? Then act quickly.

1 Rinse your dog immediately

Rinse the coat thoroughly with clean tap water. This helps prevent your dog from ingesting even more toxins by licking itself dry.

2 Do not let your dog lick itself

Try to prevent your dog from licking its coat after swimming.

3 Contact a veterinarian

Contact a veterinarian immediately. Blue-green algae poisoning is an emergency. Do not wait if you notice symptoms.

Is there a treatment for blue-green algae poisoning?

Unfortunately, there is no specific antidote for blue-green algae poisoning. Treatment is therefore supportive and focused on limiting the damage. This may include:

fluid therapy
medication against nausea or seizures
support for the liver and other organs

The faster a dog receives treatment, the greater the chance of recovery.

How can you prevent blue-green algae?

Prevention is extremely important when it comes to blue-green algae. Always check whether the water is safe before allowing your dog to swim. Via the website Zwemwater.nl you can check by province and municipality whether there are warnings for blue-green algae or other water quality issues.

In addition, avoid stagnant warm water, water with a greenish film and places where warning signs are displayed.

Also read our blog 7 tips for safe swimming with your dog.

Blue-green algae poses a serious risk to dogs during warm weather. Because symptoms can develop quickly and there is no antidote, it is important to stay alert for signs of possible poisoning.

Always check the water quality beforehand, do not let your dog swim in suspicious water and take symptoms after swimming seriously. When in doubt, always contact a veterinarian.

Frequently asked questions about blue-green algae in dogs

How do you recognise blue-green algae?

Blue-green algae is often recognised by a blue-green or greenish layer on the water. Sometimes you may see floating streaks, clumps or a kind of “pea soup” effect. The water may also smell musty or develop foam along the shore.

What symptoms does blue-green algae cause in a dog?

A dog with blue-green algae poisoning may develop symptoms such as vomiting, diarrhoea, lethargy, weakness, drooling, muscle tremors, breathing difficulties or seizures. In severe cases, blue-green algae can damage the liver and nervous system.

How quickly can symptoms of blue-green algae appear in a dog?

Symptoms can appear within 30 minutes to several hours after contact with contaminated water. That is why it is important to monitor a dog carefully after swimming.

Also read our other blogs

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About the author

Mara van Brussel-Broere, Veterinarian at Vetsend

Graduated as a companion animal veterinarian from Utrecht University, she uses her practical experience to advise customers and share knowledge at Vetsend. She finds it important to provide pet owners with clear information about health and wellbeing.