Algae in MacLure's Pond. (Maggie Brown/CBC)

A bloom of blue-green algae has closed MacLure's Pond near Murray River to swimming and fishing.

The province has posted signs warning people of the problem. The Cyanobacteria algae can make humans and animals sick.

Swimmers should be wary of the algae even in areas not posted, say health officials. (Maggie Brown/CBC)

"When it gets to heavy enough growth, there is a toxin that's released from the blue-green algae," said deputy chief health officer Dr. Lamont Sweet.

"This toxin is irritating to the eyes if a person is in the water. And if they swallow the water, it is irritating to the stomach. There can be nausea, vomiting, there can be stomach cramps and diarrhea at times, if a person ingests enough of the water."

The algae has been thriving in this summer's warm temperatures. Sweet said it's the third time blue-green algae has caused problems in P.E.I. ponds and lakes. The last time was in 2007.

Sweet said people affected would likely not end up in hospital, but the illness can be unpleasant, and not just for humans.

"Dogs particularly like to paddle in the water and they will drink a fair amount of water at times," he said.

"People need to be concerned about their pets, particularly dogs and not take them swimming in areas that have been posted."

Sweet said conservation officers are always on the look out for the algae in fresh water ponds and lakes, and added that people should also take care of themselves even if an area isn't posted. If you see thick, greenish scum in the water, he said, don't take any chances.