As the dog days of summer continue officials are warning pet owners of blue-green algae blooms appearing in ponds and lakes across the region.

The latest warning comes from South Dakota, where many small, shallow ponds develop the deadly algae in the heat of summer and into autumn. But the problem can occur in almost any body of water.

“Blue-green algae blooms happen every year when summer really gets hot,” said Mark Emmer, a South Dakota Game, Fish and Parks regional fisheries manager Mark Ermer. “It’s nearly impossible to tell if algae in a pond or lake are poisonous or not, so we recommend not letting dogs swim in a body of water that has a visible layer of thick, floating algae on the surface. Even one drink of water that has a blue-green algae bloom can be fatal for dogs.”

Though most often a blue-green color, the algae can also be blue, green, reddish-purple or brown. Blue-green algae blooms are caused by cyanobacteria, which grow particularly well in slow-moving or stagnant water with high phosphorus or nitrogen content, which is often the case in agricultural areas. Some of these cyanobacteria may produce dangerous toxins which, if ingested, can lead to liver or nervous system damage in animals. These toxins cause serious damage quickly, so prompt medical care is critical following potential exposures.

Not all blue-green algae contain toxins, but it’s impossible to tell just by looking.

If you think you or your pet has come into contact with blue-green algae, contact your doctor or veterinarian immediately, said Mendel Miller, South Dakota Assistant State Veterinarian. Symptoms of blue-green algae poisoning include lethargy, an inability to walk, hyper-salivating, weakness, vomiting, diarrhea, pale gums, shock, seizures, loss of appetite, tremors and difficulty breathing. The symptoms are sometimes mistaken for heatstroke.