Blue-green algae can kill a dog in hours and there's no antidote. Where it shows up near Toronto, what it looks like, and what to do if your dog swam in it.
Onslow's Team · Published July 14, 2026 · Updated August 3, 2026 · 5 min read
Do this first
Don't wait to see if it passes. Call ahead and say you suspect blue-green algae.
Blue-green algae isn't algae. It's cyanobacteria: photosynthetic bacteria that live in fresh water year-round in small numbers and multiply fast when the water is warm, slow-moving, and full of nutrients. Late summer and early fall is peak season in Ontario.
Some blooms produce toxins. Some don't. You cannot tell which by looking, which is the entire problem.
Dogs are more exposed than people for three reasons.
Why dogs are more exposed than people
01
They drink lake water on purpose
02
They lick their coat afterward
03
Nobody puts up a sign at the pond behind the soccer field
Attack the nervous system
Works fastAttack the liver
Hours to a dayThere is no antidote, and poisoning can turn fatal quickly.
Cornell's Riney Canine Health Center is blunt about it. Health Canada notes animal deaths are usually from rapid respiratory failure.
A documented case · Moose Lake, Alberta
A nine-year-old springer spaniel was in the water for under thirty seconds. She was visibly green when she came out. Her family rinsed her immediately.
<30 sec
in the water
~2 hrs
to vomiting and coughing
Same night
died that night
Health Canada and Ontario's environment ministry describe the same four signs.
The paint comparison is the useful one. Green algae looks like plant material. A cyanobacteria bloom looks like something was poured into the water.
The water can be clear at one end of a beach and thick at the other. Look upwind and downwind.
Toxins release when the cells break down, so a fading bloom can be more dangerous than one at its peak.
If the water looks off, smells off, or has anything floating on it you can't identify, keep the dog out.
Not “keep an eye on them.” Out.
Lower risk
The opposite of what cyanobacteria want.
Higher risk
Warm, shallow, nutrient-loaded from lawn and road runoff, no current.
Tap anything you're seeing. The read-out is under the lists.
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Neurological signs can start within minutes. Liver signs can take several hours to a day.
Skin contact alone can cause rashes and eye irritation. And a dog that seems fine four hours after a swim isn't necessarily fine.
Most of the dose comes from grooming afterward, not from what they swallowed while swimming.
A bottle, a tap, anything that isn’t the lake. Wear gloves if you have them.
Not tomorrow, not if symptoms appear. Treatment is supportive and it works better the earlier it starts.
It changes what they prepare for.
It helps the vet and it helps the next person.
Investigates suspected blooms and arranges testing. Open 24 hours.
1-800-268-6060Before any lake swim
Rinse your dog with fresh water afterward regardless of how the water looked. It costs thirty seconds.
Generally yes at Toronto’s monitored beaches, where conditions don’t favour cyanobacteria. Sheltered bays, inland ponds, and slow river sections are the higher risk. Dogs are banned from Toronto’s supervised swimming beaches April 1 to October 31. Cherry Beach’s off-leash area is the exception.
Neurological symptoms can begin within minutes. Liver symptoms may take several hours to a day. Both are emergencies.
No. Both Health Canada and Ontario’s environment ministry advise treating every bloom as harmful.
No. Rinsing reduces further exposure but doesn’t undo what’s been ingested or absorbed. Go to a vet.
Late summer through early fall, roughly August to October, with peak risk during hot stretches with little rain.
Sources: Cornell University College of Veterinary Medicine, Riney Canine Health Center · Health Canada, Recreational Water and Health · Ontario Ministry of the Environment, Conservation and Parks · City of Toronto beach water quality monitoring.
Related: emergency vets · summer heat safety · dog-friendly beaches
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