New research being presented at the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID), has found raw dog food sold in supermarkets and pet shops to be a major source of antibiotic-resistant bacteria, making it an international public health risk. Scientists at the UCIBIO, Faculty of Pharmacy, University of Porto found that more than half of the dog food samples they tested contained Enterococcus species of bacteria. More than a third of the enterococci strains demonstrated resistance to multiple antibiotics, and in excess of 20% were resistant to linezolid, a “last-resort” antibiotic that is used when all other treatment options have failed.

Some of the multidrug-resistant bacteria in raw dog food were identical to those found in hospital patients in several different European countries, and the researchers say the trend for feeding dogs raw food may be fuelling the spread of antibiotic resistant-bacteria.

Ana R Freitas, PhD, an author of the team’s published paper in the International Journal of Food Microbiology, stated, “The close contact of humans with dogs and the commercialization of the studied brands in different countries poses an international public health risk.” Freitas, together with co-authors Carla Novais, PhD, Luísa Peixe, PhD, and UCIBIO colleagues, published their data in a paper titled, “Industrial dog food is a vehicle of multidrug-resistant enterococci carrying virulence genes often linked to human infections.”

The number of pets worldwide has increased exponentially during recent decades, and figures cited by the researchers indicate that in 2019 around 85 million households in Europe owned at least one pet animal. This increase in the number of pets has, understandably, been followed by an exponential growth of the industrial pet food sector, and this, the team noted, “… has been accompanied by new food safety risks, namely antibiotic resistance.”

Drug-resistant infections kill an estimated 700,000 people a year globally and, with the figure projected to rise to 10 million by 2050 if no action is taken, the World Health Organization (WHO) classes antibiotic resistance as one of the greatest public health threats facing humanity. Yet among microbiological hazards, “… antibiotic-resistant bacteria (ARB) and genes with public health impact remain scarcely studied among pet food,” the scientists stated.

There are, of course, EU directives and regulations covering the entire manufacturing process, storage, and distribution of pet food in Europe, as well as third country imports into the EU. Nevertheless, Freitas and colleagues commented, “… microbiological hazards (e.g., pathogenic or antibiotic-resistant bacteria) may be introduced into the pet food along different food-chain phases (e.g., animal/vegetable primary production, formulation, packaging, transportation, storage).”

To find out whether pet food might represent a potential source for the spread of antibiotic-resistant bacteria, Freitas, Novais, Peixe, and colleagues from UCIBIO, Faculty of Pharmacy, University of Porto, Portugal analyzed samples of dog food from supermarkets and pet shops to look for species of Enterococcus bacteria. Enterococci are opportunistic bacteria that can live harmlessly in the guts of humans and animals, but can cause severe infections if they spread to other parts of the body. “Our aim was to characterize antibiotic resistance profiles and main clinically relevant features of Enterococcus species among processed (dry/wet/semi-moist) and non-processed (raw-frozen) foods belonging to the main brands commercialized for dogs in Portugal and abroad,” they stated. A total of 55 samples of dog food (22 wet, 8 dry, 4 semi-wet, 7 treats, and 14 raw-frozen) from 25 brands available nationally and internationally were included in the study. The raw-frozen foods included duck, salmon, turkey, chicken, lamb, goose, beef, and vegetables.

The results showed that thirty of the 55 dog food samples (54%) contained enterococci. These included 100% of the raw frozen food samples tested, 88% of the dry foods, 43% of the treats, and 27% of wet foods. The predominant species of enterococci identified in the samples were E. faecalis and E. faecium.

Forty-seven percent of the enterococci isolates were resistant to at least one of the antibiotics including erythromycin, tetracycline, quinupristin/dalfopristin, streptomycin, gentamicin, chloramphenicol, ampicillin, or ciprofloxacin. There was also resistance to vancomycin (2% of isolates) and teicoplanin (2%), while 23% of the enterococci strains across the samples were resistant to linezolid, a last-resort antibiotic that is considered a critically important treatment by the WHO. The major source of ARB was frozen raw dog food. “Multidrug-resistant isolates (31%), including vancomycin and linezolid, were obtained mostly from raw foods, although also detected in wet samples or treats,” the team noted.

All of the raw dog food samples contained multidrug-resistant enterococci, whereas in contrast, only three of the non-raw samples contained multidrug-resistant bacteria.

Genetic sequencing revealed that some of the multi-drug resistant bacteria in the raw dog food were identical to bacteria isolated from hospital patients in the U.K., Germany, and the Netherlands. Genetically identical bacteria have also been found in farm animals and wastewater in the U.K. In another experiment, the researchers transferred antibiotic resistance genes from the bacteria found in dog food to other, experimental, bacteria—suggesting this can also occur in nature.

The researchers concluded from their findings that dog food is a source of bacteria that are resistant to last-resort antibiotics and could potentially spread to humans. “ … these results unveil an unnoticed source of bacteria resistant to last resort antibiotics,” the investigators wrote. Dog food, they added, could be an overlooked driver of antibiotic resistance globally. “To the best of our knowledge, this is the first study analyzing a variety of dog food samples with respect to antibiotic resistance in enterococci … Our results demonstrate that dog food from different international brands is a potential vehicle of clinically-relevant antibiotic resistance and virulence genes, which may constitute a hazard to human health … Dog owners should always wash their hands with soap and water right after handling pet food and after picking up feces.”

Freitas added: “The close contact of humans with dogs and the commercialization of the studied brands in different countries poses an international public health risk … European authorities must raise awareness about the potential health risks when feeding raw diets to pets and the manufacture of dog food, including ingredient selection and hygiene practices, must be reviewed.

The investigators acknowledged that it still remains to be determined if resistant strains are more susceptible to heat, given that samples from foods undergoing thermal treatment mostly carried non-MDR isolates. Nevertheless, they wrote, “ … independently of the ABR profile, the high incidence of Enterococcus in dog food indicates that temperatures, hygiene practices, and selection of raw materials need urgently to be revised in the production of diverse brand dog food products.” Moreover, they stressed, “… it is also important to raise public awareness about the potential health risks when handling dog food if proper hand hygiene of pet owners is not conducted.”