Sixteen brands of dog food may be associated with a heightened risk of heart failure in dogs, according to the Food and Drug Administration. The FDA isn't suggesting that pet owners stop feeding their dogs the particular brands yet, but some vets are already advising against "grain free" foods.

The FDA is currently investigating more than 500 reports that appear to link dog foods that are marketed as "grain free" to canine dilated cardiomyopathy. The FDA has been warning about the foods based on peas, lentils or potatoes since July 2018, but the statement released late last week is the first time the agency has identified the 16 brand names.

The brands are ordered by the number of cases linked to them, which ranged from a high of 67 to 10:

Acana

Zignature

Taste of the Wild

4Health

Earthborn Holistic

Blue Buffalo

Nature’s Domain

Fromm

Merrick

California Natural

Natural Balance

Orijen

Nature’s Variety

NutriSource

Nutro

Rachael Ray Nutrish

Most of the reports were associated with dry dog food formulations, but raw food, semi-moist food and wet foods were included. The FDA has not suggested owners change their pets’ diets.

“We’re not saying don’t use these brands, we’re just telling pet owners to work directly with their veterinarians because we’re still investigating,” Lindsay Haake, a spokesperson for the FDA, said.

While the vast majority of cases have been in dogs, there have also been some in cats.

Veterinary cardiologists told NBC News that they aren’t waiting for the FDA investigation to conclude before advising owners to stop feeding the suspect pet foods.

“When a dog comes to us and we learn during the history that it’s on a grain-free diet, we advise switching to a non-grain-free diet,” said Dr. Anna Gelzer, a veterinary cardiologist and an associate professor at the University of Pennsylvania School of Veterinary Medicine. “There’s no scientifically proven benefit to grain-free foods, so why take a chance?”

In a statement, the Pet Food Institute, whose members make 98 percent of U.S. pet foods and treats, said it and its members “have convened nutritionists, veterinarians and product safety specialists for more than a year to better understand whether there is a relationship between dilated cardiomyopathy (DCM) and diet. PFI agrees with the FDA's statements that this is a complex issue with many factors requiring scientific evaluation.”

If you think about wolves, they may ingest the contents of ruminant animals they preyed on, so they are certainly capable of eating grain. There’s no scientific reason for going without grain.

Heart failure is a known issue for larger breeds of dogs, such as Great Danes and German Shepherds, Gelzer said. The breeds most frequently reported to the FDA for the heart disease were golden retrievers, mixed and Labrador retrievers.

But recently the condition started showing up in smaller breeds, which is what caught the attention of veterinarians and eventually the FDA.

“For us at Penn, we started to see cases in late 2017 that stuck out as unusual because they were in smaller breeds such as springer spaniels and beagles that you don’t typically see with canine dilated cardiomyopathy,” Gelzer said.

Knowing there had been studies showing that diet could play a role in the development of heart disease in dogs, “we started making investigations into what each owner was feeding.”

Ultimately Gelzer and others found there was a common denominator: grain-free dog foods, which had replaced grains with substitutes such as lentils, peas and chickpeas.

Gelzer doesn’t know exactly how the grain-free pet food trend started, but suspects it had to do with consumer demand for what seemed like a healthier alternative.

“It didn’t come from the science side,” Gelzer said. “If you think about wolves, they may ingest the contents of ruminant animals they preyed on, so they are certainly capable of eating grain. There’s no scientific reason for going without grain.”

Generally when dogs develop a food allergy, it’s to a protein, Gelzer said. So, veterinarians will often switch to a food with a different protein source to treat the problem.

One of the big problems with DCM is that dogs don’t show symptoms of the disease — lethargy, exercise intolerance, shortness of breath — until they’re very sick, Gelzer said.

She points to the example of clients who came in with a very sick dog that was diagnosed with DCM. The family had a second dog that seemed healthy, but because both were eating a grain-free dog food, Gelzer suggested bringing in the second dog to be checked.

What we don’t know is if [the foods] used in these diets in place of grains are causing the problem. It’s also possible that could be some kind of toxin.

“When we evaluated that dog, it also had decreased heart function, but it was still subclinical,” she said. “The dog didn’t look abnormal because it was at an early stage.”

Gelzer isn’t sure what the results will be if those early stage dogs are switched to a different food. “Some get better when the diet is changed,” she said. “Some stay the same and keep the status quo with medications and some die despite changing the diet and getting all the heart medications we can give.”

While it’s clear that pet owners are feeding these brands because they “are trying to do what they perceive as the right thing for their dogs, unless the dog has a documented sensitivity to grains, it’s probably not worth the risk at this point to feed these products,” said Dr. Bruce Kornreich, a veterinary cardiologist in the department of clinical sciences at the Veterinary College of Cornell University and associate director of the Cornell Feline Health Center.

“What we don’t know is if [the foods] used in these diets in place of grains are causing the problem,” Kornreich said. “It’s also possible that could be some kind of toxin.”

Kornreich suggests pet owners switch to a brand “produced by a company with a long-standing history.”

If a pet dog is showing any symptoms of the heart condition, including decreased energy, cough or difficulty breathing, the FDA urges owners to contact a vet as soon as possible.