CORNELIUS, N.C. — The weather conditions are perfect for blue-green algae to pop-up in freshwater lakes and ponds in the Carolinas, and that has county officials testing the waterways and putting warning signs out for at least one pond that's already tested positive. 

"I was shocked," said Janice Burk as she reacted to the signs posted around a pond at Robbins Park in Cornelius. 

The signs urged people and pets to stay out of the water after county officials tested the pond and results came back positive for blue-green algae. 

The sometimes toxic bacteria has already claimed the lives of three North Carolina dogs in Wilmington. Their owners said the dogs had a great time swimming in a local pond, but hours later, died from the algae. 

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"I had no idea that the water would be dangerous," Burk said as she walked her 15-week-old Labrador, Kona. 

Nearby neighbors said dogs can often be seen walking around the pond with their owners, and sometimes get in the water to cool off. 

"Yeah, I had no idea," Cheryl Rotman said. 

Rotman has two dogs and walks them frequently around the pond that has blue-green algae. 

"I would have never known, I just thought -- this one would go in the water all the time to get a drink and you just don't realize it was that dangerous," she said about her Golden Retriever, Ellie. 

Some blue-green algae puts out toxins that could kill dogs and even young children, according to Rusty Rozzelle, Mecklenburg County's Water Quality Program Manager.

"In most ponds, you do have an algae to bloom this time of year," he said. "We find it in ponds every year, most ponds do have it. A lot of ponds probably have blue-green algae in it and people just don't know."

Charlotte-Mecklenburg Storm Water Services told NBC Charlotte its crews are out testing area ponds for the bacteria and they expect them to keep blooming as the heat continues after rainstorms. 

The blue-green algae blooms are created from bacteria, such as fertilizer and others, that run-off into freshwater ponds and lakes after a rain event. Mix that with the heat and low winds, it's a perfect recipe for the bacteria to bloom, according to Rozzelle.

The algae that are at times toxic can produce symptoms in minutes, including include vomiting, difficulty breathing, and seizures. For adults, the bacteria can irritate your skin, according to Rozzelle. 

Officials with the Town of Cornelius said they are treating the south pond at Robbins Park, but still advise people and pets to stay out. 

"[I'm] Definitely keeping her away from the water," Burk said. "Maybe a little sprinkler action at our house."

A deadly bacteria that has popped up in state and local waterways now brings a reminder: when in doubt, keep pets out.