Unusual cases of potentially deadly heart disease have been cropping up in dogs fed dog food containing legumes and potatoes that is often marketed as being "grain free," prompting the U.S. Food and Drug Administration to investigate.

Canine dilated cardiomyopathy affects a dog's heart muscle and often leads to congestive heart failure.

These reports are highly unusual as they are are occurring in breeds not typically genetically prone to the disease. - Dr. Martine Hargogensis, FDA

Affected dogs may seem tired, weak, lose weight and even collapse suddenly. They may also develop irregular heart rhythms that could predispose dogs to sudden death, according to the Cornell University College of Veterinary Medicine.

Certain large dog breeds are predisposed to the disease, including Great Danes, Newfoundlands, Irish wolfhounds, St. Bernards and Doberman pinschers.

But the FDA said it's concerned about reports from veterinarians of the disease in other kinds of dogs. They include golden and Labrador retrievers, a whippet, a shih tzu, a bulldog, miniature schnauzers and some mixed breeds.

"These reports are highly unusual as they are are occurring in breeds not typically genetically prone to the disease," said Dr. Martine Hartogensis, deputy director of the FDA's Center for Veterinary Medicine's Office of Surveillance and Compliance in a brief released earlier this month.

Legumes, potatoes

According to the veterinary cardiologists who made the reports, the affected dogs consistently ate foods containing peas, lentils, other legume seeds or potatoes as their main ingredients for months to years.

"That's why the FDA is conducting an investigation into this potential link," the briefing said.

It urged pet owners and veterinarians to report DCM cases in dogs not normally predisposed to the disease.

It added that it has been in contact with pet food manufacturers and the veterinary community to discuss the recent reports.

"In the meantime," it said, "the FDA continues to recommend that changes in diet, especially for dogs with DCM, should be made in consultation with a licensed veterinary professional."

Grain-free dog food has become popular in recent years, despite its premium price.

Dr. Kevin McIntosh, a veterinarian and owner of the Algonquin Animal Hospital in Ottawa, says he has noticed grain-free dog food becoming popular in the past five or six years because of the perception that it's healthier and more natural. (CBC)

Dr. Kevin McIntosh, a veterinarian and owner of the Algonquin Animal Hospital in Ottawa, said he's noticed its rise in the past five or six years.

"There's a perception that it's a healthier thing to feed, it's more natural, it's closer to what they would eat if they were wolves," he told CBC's Ottawa Morning Thursday.

"Certainly, the marketing that's out there really reinforces that."

He said the FDA investigation is still preliminary and not all dogs fed a grain-free diet end up with heart disease.

"But it's a good conversation to have with your veterinarian — is my dog one of these at-risk breeds? And maybe should I think about switching away until we know for sure."