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Use Caution Around Lakes and Ponds This Time of Year
BOSTON, August 13, 2026 – Before pets dash into the water to cool off this summer, Angell Animal Medical Center is warning owners about the potentially life-threatening risks of cyanobacteria, also known as blue-green algae.
“Typically, this time of year, concerns about cyanobacteria begin to increase,” said Dr. Courtney Peck, Chief Medical Officer at the MSPCA-Angell West in Waltham. “Late in the summer, warmer temperatures, increased sun exposure, and stagnant water create the perfect environment for blue-green algae to rapidly grow.”
According to Dr. Peck, blue-green algae aren’t actually algae at all. Instead, they’re microscopic bacteria that use sunlight to make their own food, and can be found in fresh, brackish, and salt waters worldwide.
“Humans and animals can both be affected, but while we’re often cautious of swimming in discolored water, dogs usually have no problem jumping right in!”
“Even if you don’t see blue-green algae on the surface, that doesn’t necessarily mean the water is safe,” Dr. Peck noted. “Sometimes there’s a ‘spilled paint look’ when water shows bright green, blue-green, brown or red streaks, but in some cases, toxic cyanobacteria can be present without causing a change in the appearance of the water.”
Dr. Peck went on to say, “Cyanobacteria cannot be picked up in strands with a stick and will slip right through or look like a smear, unlike harmless filamentous green algae which forms clumps or stringy threads. Not all cyanobacteria are harmful. The only way to know this for certain is to test the water, so it’s best not to risk it.”
Stay Vigilant
In addition to avoiding letting pets swim in lakes and ponds during late summer and early fall, Dr. Peck recommends keeping a close eye on where pets stop to drink.
“It’s best to have fresh water on hand for your pet while out on walks,” Dr. Peck said. “You shouldn’t let them drink from still bodies of water, especially if there’s a foul odor or you’re able to see cyanobacteria or foam on the shore which can be a sign of a harmful algal bloom.”
“Pets don’t have to drink contaminated water directly to be exposed,” added Dr. Peck. “They could ingest the cyanobacteria by grooming themselves after they’ve gone for a swim.”
Dr. Peck encourages owners to bathe pets immediately after swimming to prevent this secondary risk.
“Make sure to thoroughly wash your own hands after bathing your pet. Humans are more commonly affected through skin contact.”
“Before heading to a lake or a pond, check for updates from local and state agencies and look for posted signs warning of cyanobacteria once there.”
Know the Signs and Act Quickly
Toxic cyanobacteria can affect the nervous system, liver, or skin.
“Unfortunately, once symptoms appear, they can progress rapidly,” stated Dr. Peck. “That’s why prevention and awareness are so important—they can go a long way in keeping pets safe.”
Signs a pet may have been exposed to toxic cyanobacteria include:
If pets develop these signs at home after spending time in or around water, Dr. Peck urges owners to seek veterinary care immediately.
Angell Animal Medical Center has 24/7 emergency care in Boston and Waltham. Urgent care is also available in Waltham—to determine if a concern is an emergency or if it can be addressed at an urgent care visit, visit angell.org/urgent.
