It’s like the paleo diet, only for pets.

Grain-free, all-meat, and raw-food diets are hugely popular with pet owners who like the idea of feeding their cats and dogs a diet that’s closer to what their ancestor ate in the wilds.

The problem is, there’s no hard, scientific evidence that raw meat–based diets (RMBDs) are any healthier than traditional dry or canned pet foods.

According to clinical veterinary nutritionists at Tufts University, grain-free foods are one of the fastest-growing segments of the pet food market. They’re often marketed as being more natural (read: healthier) for pets than grain-based diets. But the experts at Tufts say there’s no reliable evidence that suggests feeding grain to dogs and cats is harmful. And while sometimes disparaged as “fillers,” whole grains can be a source of valuable nutrients to pets’ diets, including vitamins, minerals, essential fatty acids, and fiber.

Now, new research published this month in Vet Record shows that RMBDs could actually pose a health threat to both pets and humans.

Researchers at Utrecht University in The Netherlands tested 35 frozen pet-food products from eight different brands. All the products contained some combination of raw meat, bones, and animal byproducts from beef, duck, chicken, lamb, and horse, plus additional ingredients. The researchers were looking for any trace of zoonotic bacterial and parasitic pathogens—specifically, infectious bacteria and pathogens that could sicken not only pets, but potentially people, too, through contact with contaminated food or feces.

What they found is enough to make you sick. Literally.

They found potentially deadly E. coli bacteria in 28 products, or 80% of all the pet foods tested. More worrisome, they found E. coli 0157:H7 in 8 products, or 23% of the total. 0157:H7 is a particularly virulent strain of E. coli responsible for an infectious outbreak that’s killed two people and hospitalized more than 50 in the United States and Canada over the past seven weeks. In those cases, contaminated romaine lettuce rather than meat is believed to be the culprit.

There’s more.

Listeria monocytogenes was present in 19 products, or 54%, and other listeria species were found in 15, or 43%. Researchers wrote that the results of the study clearly demonstrate the presence of potential zoonotic pathogens in frozen RMBDs that may be a source of bacterial infections in pet animals and, if transmitted, pose a risk for human beings.

Of course, it’s not just handling raw pet food that can make people sick—people should thoroughly wash their hands after handling any kind of raw meat, including the meat they buy to feed their families.

The researchers added, “If nonfrozen meat is fed [to pets], parasitic infections are also possible.”

Lead researcher Paul A.M. Overgaauw, DVM, PhD, wrote in an email, “The presence of the hazard has been demonstrated in our study and studies from several other countries, so the question is how to minimize the exposure.”

Overgaauw suggests a two-pronged answer: “Better regulation for the origin of the raw materials and processing, and better informing owners about the risks, the need of personal hygiene, and proper handling of these products.”

This will require the efforts of both pet-care professionals and pet food manufacturers, says Overgaauw. “This education can be given by vets, pet stores, and by mentioning warnings and handling instructions on product labels and packages of these products.”

Martha G. Cline, DVM, ACVN, a veterinary clinical nutrition expert on the staff of AAHA-accredited Red Bank Veterinary Hospitals in New Jersey, is on board with Ovegaauw’s suggestion about educating pet owners about the risks.

“I think my job is to tell them that there are risks associated with (RMBDS), just like if we are recommending a procedure or recommending a medication,” Cline said. “If there are things that could be harmful about that, we should tell our clients and let them make an informed decision about whether they want to do that or not.”

Cline says the study backs up what veterinarians already knew. “We’ve known that this risk exits. [The study] reinforces the recommendations that we as veterinarians have been giving to our clients.”

More than anything else, Cline was struck by the limited sample size. “The infection rate is so high in a small number of samples,” she said. “What if they’d sampled a thousand?”

Meaning that the problem may be even worse than it appears.

Read AAHA’s position statement on raw protein diets here.

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