It can look like nothing more than a bit of scum on a pond. For a dog, it can be fatal within hours. Here is what every UK owner should know before summer walks near water.
Most dogs love water. A pond, a lake, a slow river on a warm day, and plenty will be straight in without a second thought. For the vast majority of walks, that is completely fine. But during the warmer months, still and slow-moving fresh water across the UK can carry a hidden danger that every owner should be able to recognise.
Blue-green algae is one of the few seasonal risks that can turn a normal walk into an emergency very quickly. It is not common, but it is serious, and because it peaks in exactly the warm weather when dogs most want to swim, it is worth understanding before you need to.
Despite the name, blue-green algae is not really algae at all. It is a group of bacteria, called cyanobacteria, that live in water and multiply in warm, calm conditions. When they multiply quickly they form what is known as a bloom, which can gather at the surface and around the edges of lakes, ponds, reservoirs, canals and slow rivers.
The important part is this: some of these bacteria release toxins that can seriously harm a dog, and you cannot tell which blooms are toxic just by looking. A harmless-looking patch of water can be dangerous, so the only safe approach is to treat any suspected bloom as a risk.
Dogs are especially exposed because of how they behave around water. They drink from it, they swim in it, and then they lick the water off their coat afterwards. Any of those routes can be enough. Some dogs are even drawn to eating the dried crusts of algae that gather at the water's edge, which can be more concentrated still.
The toxins act fast. Depending on the type, they can affect the liver or the nervous system, and signs of illness can appear anywhere from a few minutes to a few hours after contact. In serious cases it can be fatal, and there is no antidote. Treatment is about supporting the dog and managing the effects, which is why getting to a vet quickly makes such a difference.
There is no antidote for blue-green algae poisoning. The thing that most often saves a dog is speed.
Blue-green algae does not always look dramatic, and its appearance varies. These are the things to watch for, and where and when the risk is highest.
What it looks like. Green, blue-green or brownish scum, foam or a film on the surface. It is often described as looking like pea soup, spilt paint, or grass clippings floating on the water. It can also collect as a scummy line around the edge.
Where it appears. Still or slow-moving fresh water. Lakes, ponds, reservoirs, canals and sluggish rivers are the usual places, and blooms tend to gather where the water is calmest, often at the downwind edge.
When to be most careful. The warmer months, roughly late spring through to autumn, with the highest risk in the heat of summer. Warm, still, sunny spells are when blooms are most likely.
If you think your dog has been in contact with affected water, do not wait for symptoms. Treat it as an emergency and call your vet straight away.
If your dog becomes unwell after being in or near water, especially in warm weather, blue-green algae is worth mentioning to your vet immediately. Signs can come on fast, sometimes within minutes.
Get to a vet urgently if you see any of these after contact with water:
If you think your dog has been exposed, act quickly and do not wait to see whether they become ill. Time genuinely matters here.
The good news is that avoiding blue-green algae is mostly about awareness. A few simple habits during the warmer months keep the risk very low.
None of this needs to stop you and your dog enjoying the water this summer. The vast majority of walks near lakes and rivers are perfectly safe. It simply means knowing what a bloom looks like, keeping clear when something looks off, and acting fast on the rare occasion it matters. Awareness costs nothing, and with blue-green algae it is the single most useful thing an owner can have.
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This article is general guidance and is not a substitute for veterinary advice. Blue-green algae poisoning is a medical emergency. If you think your dog has been exposed, or they become unwell after contact with water, contact your vet immediately.
