A lot of dogs love to swim. And for some the dirtier the water, the better. But beware: Don’t let your dog dive into a lake or a pond that has a noticeable layer of algae on top. Veterinary toxicologist Dr. Deon van der Merwe says that’s because most algae produces toxins that are harmful to both pets and their owners. And the problem is particularly bad in the summer, when hot weather and sunshine create ideal conditions for explosive algae growth.

So how can you tell if your “swimmer dog” has been exposed to algae toxins? The first signs are vomiting and diarrhea, and if your dog has those symptoms after a swim, take them to the vet fast. Because left untreated, algae poisoning can cause liver failure and can even kill a dog.

And if you think your dog’s been exposed to algae, it’s also a good idea to avoid their “wet dog shake." Wear gloves when handling them and scrub down their crate with bleach and water once you bring them home. It may take a large dose of algae toxin to cause dangerous liver problems in people. But it only takes a small amount to cause skin rashes and asthma-like breathing.