In the heat of summer, it’s tempting to take your dog swimming in lakes or rivers to help them cool down. But before you let your pup dive in, be aware that not all water is safe for dogs. That’s because toxic blue-green algae—also called cyanobacteria—is present in some bodies of water and can be toxic to dogs.

What is cyanobacteria?

These bacteria multiply in warm, stagnant, nutrient-rich fresh water and often appear blue or green but can be other colors. They can even hide on the bottom or shore of lakes and ponds. They’ve been identified in all 50 states and have occurred as far north as Finland and Alberta and Manitoba in Canada. With the higher temperatures and droughts that accompany global warming, as well as increased nutrient runoff from lawns and farms, they appear to be on the rise. They can be dangerous—even deadly—to dogs.

Microscopic view of algae
Toxic cyanobacteria can multiply in warm, stagnant, nutrient-rich water.
Elif Bayraktar

How do I know whether water is safe for my dog?

Dogs are particularly susceptible because they often drink water while swimming—or, after getting out of a pond or lake, they might lick water off their wet coats. Experts say the best way to protect dogs is to keep them away from outdoor bodies of water (ponds, lakes, sluggish rivers, even bird baths) especially in the summer. That means keeping them leashed when adventuring outdoors. Do not let them drink from stagnant water, and if they do go swimming, be sure to bathe or rinse them afterward.

Although some municipalities may post signs when there’s a known algal bloom, that’s not always the case. Avoid water with obvious blue-green algae blooms, which can look like shimmery paint or slime sitting on the water’s surface. It’s not always obvious when a lake or pond has cyanobacteria, so err on the side of caution during the warmer months.

What are the signs of cyanobacteria poisoning?

Be on the lookout for:

  • Vomiting
  • Diarrhea
  • Excessive drooling
  • Stumbling
  • Stiffness
  • Tremors
  • Weakness
  • Difficulty breathing
  • Convulsions

What do I do if my dog drinks water with algae?

If your pet has swallowed blue-green algae—and especially if they develop any symptoms of problems after exposure to water, mud or debris along the shore—rinse their coat off as soon as possible, call a vet to alert them to the emergency and rush your dog for medical treatment.

Don’t delay: The fastest-acting toxins attack the nervous system and can kill dogs within 24 hours. Slower-acting toxins attack the liver and can kill within several days. Dogs need immediate intervention to have a chance at surviving.

If you can, collect a water sample. If tests confirm toxic cyanobacteria, officials can place a warning sign near the body of water.