A day at a pond turned tragic for pet owners when their three dogs died after coming in contact with water full of toxic blue-green algae.

Melissa Martin and Denise Mintz took their three dogs to a pond in Wilmington, North Carolina, last Thursday to throw a ball around and play. Martin said their two West Highland terriers Abby and Izzy stayed on the water's edge but their Doodle mix Harpo saw another dog splashing around and got in.

The water appeared clear and there weren't any signs indicating the water could be dangerous, Martin said.

When they got home, though, things took a turn.

Martin said she took Harpo upstairs for a bath when Mintz yelled for her that something was wrong with Abby.

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"So I look at her and she's seizing," Martin told USA TODAY. Martin said put the dog in the car and rushed her to the veterinarian, but things got worse. "She's foaming at the mouth. She's hot to touch."

Martin said the vet asked if Abby had been near water. When she was told that her other dogs needed to come in, too, Martin called Mintz only to find out that Izzy was experiencing the same symptoms.

Then, when Harpo arrived at the vet, he also began showing symptoms. The dogs were brought to the ICU, but none improved, Martin said. All three had died by Friday.

"We sat there in shock," Martin said of their night in the ICU. "Like 'there's no way all three of our dogs are going to die. There's no way this can happen.'"

Martin said the veterinarian told them that blue-green algae sickened the dogs, causing their livers to shut down, which would lead to internal bleeding.

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Blue-green algae is a type of bacteria that blooms in warm, nutrient-rich ponds and lakes and can harm pets and humans, according to the Minnesota Pollution Control Agency.

The cyanobacteria produce cyanotoxins that can be harmful when swallowed, breathed in or in contact with skin, the pollution agency wrote on its website.

In some cases, like with the pond that sickened Martin and Mintz's dogs, the harmful bacteria can be hard to spot.

"That water was completely clear," Martin said. "You can't even see it unless you know exactly what you're looking for."

Martin said there should be signs at the pond indicating the harmful bacteria is present. She said she's been back since and saw a father and son fishing. Now, she's sharing her story to spread awareness around the issue.

"Please be aware. Don't think that this can't happen to you," she said. "If we had any indication that this was even a thing, there's no way our dogs would have been there."

Contributing: Jane Mo. Follow USA TODAY's Ryan Miller on Twitter @RyanW_Miller