The Canine Ledger

Blue-Green Algae: Why a Swim Can Kill

Toxic algal blooms kill dogs every year, often within hours. What the US case data show, how to spot risky water, and what to do if your dog swims in it.

Written by thecanineledger

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If your dog has been in or near water that might have a bloom and is vomiting, drooling, stumbling, trembling, weak or having a seizure, go to a vet now. Don’t wait to see if it passes. → Emergency Signs in Dogs: When to Go Now, Not Tomorrow

Of 260 dogs described in US news, agency and scientific reports of blue-green algae poisoning between the late 1920s and August 2012, “215 (83%) of the dogs died and 45 (17%) became ill and then recovered.” That’s from a review led by a scientist at the US Centers for Disease Control and Prevention (CDC), published in Toxins in 2013 (Backer et al., 2013).

While that figure comes from reports that may favour the worst cases, the other numbers in the same paper aren’t reassuring either. A swim in the wrong lake can kill a dog within hours. This page sets out what the data show, how to judge the water, and what to do if your dog has already been in.

1. What blue-green algae are

“Blue-green algae” is the common name for cyanobacteria. The CDC describes them as “microscopic organisms that can be found naturally in all types of water.” Some make toxins. The CDC says that when they bloom, they “can change the color of the water or look like foam, scum, mats, or paint on the surface of water” (CDC, Preventing Pet and Livestock Illnesses Caused by Harmful Algal Blooms).

Dogs are especially exposed. They swim in the water, drink it, and lick it off their coats afterward. Backer’s review notes that animals have died “after drinking contaminated water or grooming cyanobacterial scum from their fur or feathers after swimming.”

2. What the case data show

Backer’s team pulled together three US sources. Each tells a slightly different story:

SourceWhat it found
CDC surveillance, 2007–2011, 13 states67 suspected or confirmed dog cases. “Thirty-eight (58%) of the canine intoxications were fatal.”
News, agency and scientific reports, 1920s–2012260 dogs; 83% died.
A California veterinary teaching hospital, 1984–2012Testing to confirm the toxin was “rarely performed (4% of cases)”

Across all three, the team counted 368 dog cases. They warn the true number is far higher: the cases “likely represent a small fraction of cases that occur throughout the U.S. each year.”

The reasons are practical. Testing is expensive and hard to get. Some deaths can be wrongly put down to other causes, such as mushroom poisoning. The authors conclude that “the vast majority of cyanoHAB-associated dog deaths remain un-reported and often un-recognized by pet owners and veterinarians.”

Our note: The 83% figure comes from news and published reports, which are more likely to cover deaths than recoveries. It isn’t a death rate for any dog that swims in a bloom.

3. How fast it happens

Speed is the reason this page starts with an emergency box. The CDC says “Animals can get sick and die within hours after swallowing toxins made by harmful algae.” Its guidance for vets adds that signs “appear minutes to hours after an animal is exposed” (CDC, For Veterinarians).

Backer’s review describes dogs at the California hospital that “died peracutely (within 20–30 minutes of onset of illness)” after exposure in a backyard pond.

There’s no cure to wait for. The CDC tells vets: “Quickly providing supportive medical care can save animals’ lives. There are no known antidotes to these toxins.”

4. Signs to act on

The CDC tells owners to call a vet if their animal shows any of these after contact with water:

  • Loss of energy or appetite
  • Vomiting or diarrhoea
  • Stumbling or falling
  • Foaming at the mouth, or excessive drooling
  • Tremors, seizures or convulsions
  • “Any unexplained sickness that occurs within a day or so after being in contact with water.”

That last line matters most. You don’t need to be sure there was a bloom.

Our note: This advice is deliberately cautious. If your dog swam in a known bloom, call your vet even if it seems well.

5. Judging the water

You can’t tell by looking. The CDC is blunt: “You cannot tell for sure if water has a harmful algal bloom by looking at it.” Some blooms “grow on or near the bottom of water bodies and release toxins into the water”, so clear-looking water isn’t proof of safety (CDC, How to Recognize a Harmful Algal Bloom).

No warning sign doesn’t mean safe. Nobody tests every lake. The CDC warns “there will not always be an advisory posted near the water or online to warn you that a body of water has a harmful algal bloom.”

What to do instead:

  • Check local advisories before you go.
  • Keep your dog out of water that’s discoloured, scummy, foamy or smells bad.
  • Keep your dog away from algae washed up on the shore. The CDC says it “can still contain toxins.”
  • Follow the CDC’s rule: “When in doubt, keep out!”

Our note: Every source here is American. UK and European agencies run their own warnings and reporting.

6. If your dog has already been in

The CDC’s advice: “immediately wash them off with clean water to keep them from licking cyanobacteria off their fur.” Then watch closely for the signs in §4, and call a vet if any appear.

Our note: The CDC pages we read don’t say whether to protect your own skin when washing a dog off, or how long to watch afterward.

7. What we could not verify

Commonly saidWhat we found
“If the water looks clear, it’s safe”No. Some blooms grow underwater, the CDC says.
“Most dogs die”Reported cases are mostly fatal, but reports favour severe cases. We found no true death rate.
“There’s an antidote”No known antidote, the CDC says. Fast supportive care is the treatment.
“This is rare”Under-counted, not rare. The known US cases are likely “a small fraction.”

Sources

Always confirm with your vet before acting on anything on this page.