The 101st Swim: Blue-Green Algae and Dogs

A dog can visit the same lake for years without a problem. One spell of hot weather can change the water.

At the height of summer, a familiar lake can feel like a known quantity. Your dog has paddled there for years, the usual walkers are gathered around the bank and nothing appears noticeably different. It is easy to assume that another swim will be much like all the others.

The change can happen surprisingly fast. Cyanobacteria already present in the water can increase sharply during a run of hot, dry days. A breeze may then push the bloom across the surface until it gathers in a quiet bay or forms a line around the bank.

For dogs, this can be extremely serious. They do not have to stop and drink from the lake to be exposed; water is easily gulped during a swim and anything left on the coat may be swallowed later during grooming. If you think your dog has been in affected water, there is no time to watch for a while and see what happens.

The sensible habit is to look at the water each time you arrive. It does not matter how often your dog has swum there before.

A misleading name

The name is not especially helpful. Blue-green algae is really a collection of bacteria known as cyanobacteria, found naturally in many types of water.

Most of the time, there may be too little to notice. That can change when several favourable factors arrive together: bright sunshine, warm water, plenty of nutrients and little flow to break up the surface. The bacteria increase in number and a visible bloom begins to develop. This happens most often during summer, but there is no fixed blue-green algae season. Blooms have been recorded during other parts of the year too.

Ponds, lakes, reservoirs and canals are among the places most commonly affected, although cyanobacteria also occur in rivers, streams, estuaries and coastal water. Blooms often drift towards the edge, which is precisely where a dog is likely to paddle or drink.

Some cyanobacteria produce toxins and others do not. The problem for anyone standing on the bank is that appearance alone cannot reveal whether a particular bloom is toxic. Suspected blue-green algae should therefore be treated as potentially harmful.

Checking the water

There is no single appearance to look for. A severe bloom may turn a large area of water bright green, but many are less obvious. Sometimes the only visible change is a patch of cloudy water or a thin line of material collected along the bank.

The colour varies too. Although blue-green is in the name, a bloom might be green, turquoise, brown, red or almost black. It can resemble pea soup, spilled paint or green sawdust floating on the surface. Some blooms form streaks and wisps; others appear as flakes, small clumps, foam or a thicker scum. The water may also smell earthy, musty or grassy.

Duckweed and harmless types of filamentous algae can look similar, so not every green pond presents the same danger. This is not something a dog owner needs to settle at the water’s edge. If the water has changed colour, looks unusually cloudy, carries surface scum or simply does not look right, keep your dog out.

Warning signs placed by a council or landowner should always be followed. Equally, the lack of a warning sign should not be read as confirmation that the water is clear. The bloom may be new, unreported or visible only in one section.

How poisoning affects dogs

A dog does not have to drink deliberately from an affected lake to be exposed. Water swallowed during swimming may be enough, as may licking residue from the coat afterwards. Material gathered along the shoreline can be especially concentrated.

The illness will not look identical in every dog because cyanobacteria produce more than one kind of toxin. Those that attack the liver can lead to vomiting, diarrhoea, weakness and collapse. Toxins acting on the nervous system may cause disorientation, breathing problems, twitching, tremors or seizures. Heavy drooling is another possible sign, and vomit or diarrhoea may contain blood.

Symptoms often begin within an hour and can appear within minutes, although delayed illness is also possible. A dog who seems normal immediately after leaving the water is not necessarily in the clear. Cases can progress rapidly and may be fatal.

There is no single drug a vet can give to cancel out the toxins. Instead, treatment is directed at what the toxins are doing to the dog’s body. The sooner the veterinary team knows what may have happened, the sooner that treatment can begin.

If your dog has been exposed

Move your dog away from the water and phone a vet straight away. Tell the practice that you suspect blue-green algae exposure and, if someone else is with you, ask them to call while you begin the journey. Do not wait for vomiting, weakness or another symptom before seeking advice.

Try to stop your dog licking their coat. If clean tap water is immediately available, rinse the coat thoroughly while wearing gloves, but do not allow this to delay the journey to the vet. Avoid getting the contaminated water on your own skin and do not attempt to make your dog vomit unless a veterinary professional instructs you to do so.

Tell the vet when the exposure happened, whether your dog drank from the water and what you noticed on its surface. A photograph taken safely from the bank may help identify the possible source, but there is no need to collect a sample and no photograph is worth delaying treatment for.

Making summer swims safer

Keeping a lead on while approaching unfamiliar water gives you a chance to inspect it before your dog dives in. Look along the shore as well as across the open water, paying particular attention to sheltered edges and anywhere surface material has collected.

Put a bottle of water and a portable bowl in your walking bag or car. Offering your dog a drink before reaching the bank makes a muddy puddle or lake edge less appealing. After a period of hot weather, check notices from the landowner, local council or environmental authority as part of planning the walk, then still look at the water when you get there.

The free Bloomin’ Algae app, created by the UK Centre for Ecology & Hydrology, shows submitted records and allows users to report a suspected bloom with a photograph and location. Reports can help local authorities, landowners and environmental agencies investigate and issue warnings. The app includes maps for England, Scotland, Wales, Northern Ireland and Ireland.

In England, suspected blooms and other pollution incidents can also be reported to the Environment Agency’s 24-hour hotline on 0800 80 70 60. In other parts of the UK, reports can be made through the relevant environmental authority or local council.

Swimming can be a wonderful part of a dog’s summer, and blue-green algae does not make every pond or lake unsafe. It does mean that no stretch of water should be taken for granted. Look before your dog goes in, check again on every visit and walk away if anything about the water gives you reason to hesitate.