FREDERICTON -- Health officials are trying to determine what killed three dogs who suffered convulsions and vomiting almost immediately after playing in the Saint John River.

New Brunswick's provincial veterinarian, Jim Goltz, said Thursday he is awaiting results from tests on two of the dogs to see if their sudden deaths were caused by exposure to toxicity from a possible blue-green algal bloom.

He said both dogs had seizures and other signs of neurological disease before they passed away within a half hour after playing in the river and likely ingesting the water.

Officials are examining two popular recreational sites on the river to look for possible algal blooms.

"We don't yet know what killed the dogs, but we do suspect the possibility of blue-green algal toxicity and this is being investigated as one of our primary considerations," he said.

"In the interim, we're advising people not to let pets or children in water that has a green scum on it ... and we're also advising people to stay clear of the sites on the Saint John River where the deaths of the dogs occurred."

Goltz said his laboratory received two of the dogs, but there were no immediate signs of what may have caused their deaths. Samples are being examined and tested for toxins at a specialized lab in Halifax, he said.

Two small dogs were together at Carleton Park near Fredericton on Sunday when they began experiencing difficulty breathing and became disoriented moments after coming out of the water. Their owner, Jeff Wilhem, said on his Facebook page that his pets -- Sookie and Peekaboo -- passed away a short time after playing in the water at the Carleton beach.

"Sookie died in my arms within minutes, and Peekaboo died about an hour later. There was nothing the vet could do," he wrote, adding that an autopsy was being done. "Please keep your children and pets away from the beach."

Another dog -- an eight-month-old Australian shepherd named Nike -- died suddenly Friday after playing in the river at a Hartt Island campground, about 10 kilometres away from Carleton Park.

Owner Shawn McFadden said Thursday that the puppy was happily playing in the river for about 20 minutes, got out and went back to the family's trailer when she started convulsing and vomiting before he rushed her to the veterinarian.

"I tried to bring her to the animal hospital and she died on the way, within a half hour," he said. "It was your classic seizure, like convulsing, eyes rolling and she went limp and it was quite something."

McFadden said they are awaiting test results to try to identify the cause, but that could take a couple of weeks. He added that he didn't notice anything unusual on the water's surface and the family has been going there for years with their dogs without incident.

"It's never been an issue and there was no reason to suspect anything," he said. "For us, it's just a waiting game to find out what happened."

Goltz stressed that they have not yet confirmed the deaths are linked to algal toxicity, but he is advising people to keep their pets and children out of water near the two sites and anywhere where there is a green scum on the water's surface.

According to the province's website, blue-green algae are photosynthetic bacterial organisms that occur naturally in lakes, rivers and wetlands. Depending on conditions, they can quickly form a bloom and produce toxins.

Goltz said exposure to a toxin can cause a host of symptoms in dogs since there are so many different species of the bacteria that produces the toxins. They can include excessive salivation, weakness, staggering, difficulty breathing, seizures and death. They can all occur within minutes of exposure, he said. In people, symptoms can include vomiting, diarrhea, numbness and abdominal cramps, amongst others.

Goltz said he's only seen one dog fatality in the province linked to algal bloom toxins. In 2010, a five-and-a-half-month-old Labrador retriever suffered hyper-salivation, red and rolling eyes, staggering and vomiting before laying down and dying, all within a half hour of being in the water.

Paul Bradley, a spokesman for the Health Department, said crews planned to visit and assess the two sites to determine if they need to take water samples for testing.

He said there are 14 sites with confirmed algal blooms, mostly in provincial lakes.

- By Alison Auld in Halifax