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Raw food has a following in the pet food market even as researchers uncover microbial hazards, including a greater chance of contamination by multidrug-resistant bacteria than cooked foods and treats.
Scientists have found drug-resistant superbugs harmful to humans in two raw dog-food brands, stirring fresh discussion on the safety of the increasingly popular diets.
The research, conducted in Portugal and presented this month at the European Congress of Clinical Microbiology & Infectious Diseases, raises pressure on governments to more tightly regulate the production of raw pet food.
At least one of the researchers suggests that veterinarians, too, consider warning clients about superbug risks if they haven't been doing so already.
Antimicrobial resistance, or AMR, occurs when pathogenic microbes evolve to evade drugs meant to kill them, a process that can be hastened by overuse of the drugs. Many scientists consider AMR to be one of the biggest threats to humankind, alongside climate change and nuclear war.
To combat drug resistance, regulators worldwide have been for years discouraging doctors from overprescribing antibiotics and other antimicrobial medications. More recently, the agricultural sector has been urged and compelled to limit the use of antimicrobials in animals raised for food. Less attention has been paid to tackling direct infection risks posed by pet food.
Raw food is of particular concern because many pathogens die when cooked at sufficiently high temperatures, although cooked food, too, can become contaminated.
Scientists at the University of Porto tested 55 samples of dog food, 14 of them frozen raw, for the bacteria Enterococcus, which can cause urinary tract infections, blood infections and meningitis, among other conditions. The samples were taken from various types of dog food — frozen raw, wet, dry, semi-wet and treats — collected from nine retail stores in northern Portugal. The samples encompassed 25 brands, which the researchers did not name. Twenty-one of the brands are sold in multiple countries, including both raw brands.
The researchers were alarmed by the results. Thirty of the 55 samples contained Enterococci, and 26 carried isolates resistant to at least one antibiotic. Enterococci found in all 14 of the raw-frozen samples were resistant to three or more different families of antimicrobial drugs — in other words, they were multidrug resistant. By comparison, multidrug-resistant isolates were found in three samples of cooked products: two wet foods and one treat.
"These bacteria can be transmitted to humans by direct contact with dogs, their food or their feces," said Ana Freitas, a University of Porto molecular geneticist who led the research.
The drug-resistant pathogens can sicken victims immediately or establish themselves more subtly in the digestive tracts of pets and their owners.
"People can have a gut reservoir of resistant bacteria that may amplify in opportunistic situations, such as hospitalizations with antibiotic treatments," Freitas said.
Resistance to last-resort drugs of particular concern
The Enterococci in the samples tolerated a variety of antibiotics, including erythromycin, tetracycline, ampicillin and ciprofloxacin, among others. Of particular concern to the researchers was the level of resistance they found to linezolid, which is categorized by the World Health Organization as critically important in human medicine because it is usually effective against drug-resistant bacteria. Of the 55 samples, seven were found to contain linezolid-resistant Enterococci, all of them raw.
"Those results were surprising because rates of linezolid resistance are still overall low, according to available data," Freitas said. "The hidden spread of those linezolid-resistant isolates could lead to their increase in the clinical setting, with a consequent increase in morbidity and mortality of enterococcal infections."
Another scientist said the results of the research, while providing an important reminder of AMR risks, aren't groundbreaking in a historical context. The finding "probably raises less concern than various [other] studies over the past 10 to 15 years," said Dr. Scott Weese, an infectious diseases expert at the University of Guelph's Ontario Veterinary College in Canada.
Weese, who in 2019 blogged about AMR in raw pet food, opines that gram-positive bacteria, such as Enterococci, are of less concern than gram-negative species. Gram-negative bacteria have a harder cell wall that can make them more difficult to kill. Their presence has been identified in earlier studies, including research published in 2008 by Weese and colleagues that found that dogs that ate raw meat were 15 times more likely to pass in their feces cephalosporin-resistant Escherichia coli. More recently, research conducted in the United Kingdom and the Netherlands also identified high levels of drug-resistant E. coli in pets on a raw diet compared with those eating cooked diets.
The researchers in Portugal acknowledged the earlier findings but said investigations of AMR in pet food remain relatively scarce, especially for gram-positive bacteria such as the gut colonizer Enterococcus. Linezolid-resistant bugs, they noted, have only very recently been observed in pets.
Drug-resistance aside, the findings in Portugal add to existing concerns about general infection and other safety risks posed by uncooked food. Marketed as being healthy for pets because it resembles animals' diets in the wild, raw meat has been involved in multiple recent pet-food recalls around the world.
In a recent incident, at least 10 dogs in the Australian state of Victoria died after consuming raw food produced at Maffra District Knackery. The slaughterhouse issued a recall on July 20, but the incident has nevertheless sparked requests for tougher regulation of the pet-food market in Australia, where recalls are voluntary. All of the dogs died from liver failure caused by the plant toxin indospicine, which investigators suspect was ingested by grazing animals before they were butchered. (Cooking may not degrade indospicine, however.)
Other recent incidents include a recall in 2019 of raw pet food sold by Natural Instinct in the U.K. suspected of causing an outbreak of feline tuberculosis that killed at least five cats. In a separate incident, Natural Instinct this month recalled raw duck food because it contained Salmonella. In the United States last November, Albright's recalled products containing raw chicken because they had "the potential" to be contaminated with Salmonella.
In a study published in 2014, researchers at the U.S. Food and Drug Administration and collaborators found that raw pet food is more apt to be contaminated with pathogens than dry pet food. The agency summarizes the findings under a web posting with an unambiguous heading that reads in part: "Raw pet food diets can be dangerous to you and your pet."
Are purported benefits of raw food worth the risk?
An increasing number of owners appear to be feeding raw meat to their pets, driven by perceived health benefits and a suspicion of industrially produced pet food, according to research published in 2019. Separate research published last year, based on responses from 3,673 pet owners in Australia, Canada, New Zealand, the U.K and the U.S., indicated that 9% of dog and 6% of cat owners fed their pets exclusively raw food; and 66% and 53%, respectively, a diet that included raw food. The researchers compared those results with nine peer-reviewed articles published over the previous 10 years and found that feeding unconventional diets — such as raw, homemade and vegetarian — was becoming more prevalent.
Little scientific research exists on the nutritional benefits of raw pet food. "The idea is that animals have shinier coats, cleaner teeth, find it more palatable and, because it's more digestible, have smaller poos," said Dr. Scott Campbell, a veterinarian who practices in Ipswich, Australia. "I think there's an element of truth in all of that."
At the same time, Campbell — who also is a clinical nutrition consultant for the Veterinary Information Network, an online community for the profession and parent of the VIN News Service — said the risks of raw dog food are well known. Apart from the threat of infectious diseases, he said, dogs might suffer from nutritional imbalances. Campbell also said he regularly sees dogs, typically puppies, presenting with dental fractures because they've been injured biting on bones. Some animals also develop gastrointestinal blockages caused by bones in raw food.
Moreover, Campbell notes that dogs' undomesticated predecessors tended to live for only five or six years in the wild and probably were killing much smaller prey and eating it fresh, rather than, say, cattle or sheep weeks after they'd been slaughtered.
Dr. Susan G. Wynn, senior director of scientific affairs at raw-food producer Instinct Pet Food, agreed that few, if any, studies have confirmed that uncooked food offers nutritional benefits, but said evidence is mounting that it does. For instance, Wynn cited ongoing work at the University of Helsinki called the DogRisk study, which has indicated that feeding pets raw food reduces the likelihood of atopic dermatitis and inflammatory bowel disease. She pointed to other studies that found that dogs fed raw food were less likely to be infected with roundworms or become obese.
Wynn, a veterinary nutritionist and an alternative-medicine consultant at VIN, personally suspects that raw food retains nutritional complexity because hundreds of compounds can be lost during cooking, a process that itself can contaminate food. "The fact that use of raw pet food has grown worldwide despite active opposition from government and professional officials suggests something good is happening for many pet owners," she said.
Not all raw-food companies are alike, of course. "Formulating a complete and balanced diet out of raw ingredients is easy to do," said Wynn, an advocate of minimal processing. "What's not easy is ensuring that the final product is complete and balanced, which almost all homemade raw feeders neglect to do, and many commercial raw pet food companies fail to do, as well."
For his part, Campbell tells clients to exercise caution. "I'm reluctant to say commercial food is better, but it's easier to get right, especially for people living busy lives with regular pets," he said. "When owners do want to do it, you can give them some direction around grinding bones and trying to achieve nutritional balance."
When it comes to drug resistance, Wynn said there are various ways that producers can mitigate risks, such as applying high-pressure processing or adding probiotics to suppress growth of bacterial pathogens in meat. The University of Porto's Freitas said regulators could play their part by at least mandating high standards of raw-material selection, food manufacturing, hygiene and disease-screening practices.
Consumers who choose raw foods can contribute, too. "Dog owners should always wash their hands with soap and water right after contact with animal feed or their feces," Freitas said. "This is only a drop in a big ocean contributing to the global antimicrobial resistance crisis, but it can help us contain it better."