Blue-Green Algae: The Water That Can Kill in an Hour

A dog can be dead within the hour of a swim in the wrong pond. There is no antidote, so the only thing that helps is not waiting.

There is a documented UK case of a young retriever that went for a swim and died forty-five minutes later. That is the fact worth carrying into summer, because it explains why the advice on this one is so blunt.

Blue-green algae is not really algae. It is cyanobacteria, and it blooms in warm, still, nutrient-rich freshwater, usually in the middle and back end of summer. It produces two families of toxin: one that destroys the liver, one that attacks the nervous system and can stop a dog breathing.

There is no antidote for either. Treatment is supportive, so the only advantage available to you is speed.

What to do

1. Ring your vet or a poison line immediately. Do not wait for symptoms. There is nothing to be gained by watching.
2. If you can do it without losing time, rinse your dog with clean water so it cannot lick more toxin out of its coat. Do not let this delay the journey.

In the UK, the Animal PoisonLine. In the US, the Pet Poison Helpline.

Very small exposures can be fatal. A few mouthfuls taken while swimming, or a good lick of a wet coat afterwards, is enough. This is not a poison where you can reason about how much went in.

What a bloom looks like

• Water like pea soup
• A blue-green tint, as though paint has been stirred in
• Foam or scum collecting in mats on the surface

The important detail is where it collects. Wind pushes blooms into thick mats along the downwind shore, which is exactly the shallow edge where a dog wades in and where you would throw a ball from.

And the uncomfortable caveat: clear-looking water is not proof of safety. Toxins persist after a bloom disperses, and blooms are not always obvious. If a lake has posted warnings, believe them even if the water in front of you looks fine.

What you might see

• Vomiting and diarrhoea, sometimes with blood or black, tarry stools
• Weakness and pale gums
• Yellowing of the gums or eyes
• Muscle tremors and rigidity
• Seizures and disorientation
• Blue-tinged skin and gums, laboured breathing, collapse

That last group is the neurological form, and it can progress to respiratory paralysis within minutes to hours. If you see breathing difficulty after any contact with still water, that is the most urgent version of this there is.

Do not try to make your dog sick unless a vet has told you to.

The summer rule worth adopting

Hunting dogs and dogs that live to swim are the ones most at risk, simply because they are in the water most.

The workable rule is this: in hot weather, keep dogs out of still or slow-moving freshwater that looks discoloured, scummy or soupy, and out of anywhere with a warning sign, however unbothered the ducks look. Lakes, canals, reservoirs, ponds and sluggish rivers across the UK, Europe, North America, Australia and New Zealand all bloom.

Running water and the sea are a different proposition. It is the still, warm, green stuff that kills dogs.

What to tell them

Which water, roughly when, and whether your dog swallowed any or only swam in it. Also whether it has been licking its coat since, because that is a continuing exposure and it changes the picture.

A photograph of the water helps more than you would expect.

Checking before you throw the ball

Bloom warnings are usually posted by the local authority or the landowner at the access points to a lake, and many councils publish current warnings online. Worth a thirty-second check before driving somewhere for a swim in August.

If there is no sign and you are unsure, use the simple test: does the water look like pea soup, is there a green or blue-green scum along the shore, is there foam collecting where the wind pushes in? If any of those, walk the dog on the lead and find somewhere else.

Sources

This article was written using guidance from Pet Poison Helpline, a 24/7 animal poison control service for North America; Animal PoisonLine (VPIS), the UK's only 24/7 poison advice line for pet owners, run by the Veterinary Poisons Information Service; and RSPCA, the Royal Society for the Prevention of Cruelty to Animals, the UK's oldest animal welfare charity. This guidance is for general information and does not replace a consultation with your own veterinarian. In an emergency, always contact a vet. Read [how we write our health guidance](https://rochsociety.com/how-we-write-our-health-guidance/). Last reviewed July 2026.