In April, a scruffy pug named Winston started coughing and wouldn’t touch his chow—a clear sign the pup wasn’t feeling well. Three of the four humans in his family in Chapel Hill, North Carolina had come down with COVID-19, and they were concerned he’d caught it too. Later testing showed the pug didn’t, in fact, have a SARS-CoV-2 infection. But a handful of other household pets have tested positive for the virus in the last few months, a cue for scientists to take a serious look at whether our canine and feline companions could transmit the virus to one another or to humans. There’s no evidence so far that they can.
However, SARS-CoV-2 is not the only infectious agent that scientists worry our furry friends might share with us. Researchers have also been studying whether multidrug-resistant organisms—and the genes they use to resist death by antimicrobial drugs—are transferred from pets to humans. There have been cases of infection of humans by pet food, at least: in 2019, for example, more than 150 people in 34 states were sickened with multidrug-resistant Salmonella after handling contaminated pig-ear pet treats. Other research has also shown pet food to be a source of antibiotic-resistant bacteria.
These findings led Ana Raquel Freitas and Luísa Peixe of the Research Unit on Applied Molecular Biosciences at the University of Porto to ask whether or not dog food, specifically raw dog food, played host to antibiotic-resistant Enterococcus faecium, a bacterium that has been causing a growing number of infections worldwide. “For many years, we have known that humans share some lineages, some antibiotic-resistant lineages of Enterococcus with dogs,” Freitas tells The Scientist. “We don’t know the origin of transmission, and we thought, ‘Why not test the food?’”
Between September and November last year, Freitas and Peixe gathered samples of dog food from eight supermarkets and a veterinary clinic in Porto. The researchers collected a total of 46 samples: 22 wet foods, 15 dry foods, and 9 raw-frozen foods, which are growing in popularity despite questions about their safety. Swabbing each sample on an agar plate, the team cultured any bacteria living in the food, then treated the plates with various antibiotics. All nine of the raw food samples tested positive for multidrug-resistant enterococci, while only one of the wet food and none of the dry food samples had lineages resistant to antibiotics.
“What we saw in the raw foods was surprising,” Freitas says. The team had expected those foods to have more bacteria because, unlike dry foods, they aren’t sterilized, “but the type and amount of bacteria in the samples was unexpected.” (The team had planned to present these early findings at the European Congress of Clinical Microbiology and Infectious Diseases in April, but the conference was canceled due to the COVID-19 pandemic.) Preliminary analysis of the data suggests that some of the bacterial strains identified in the raw foods have signatures of mobile genetic elements that promote antibiotic resistance, and some of those same signatures have been detected in people hospitalized for bacterial infections, Peixe notes, an important finding to follow up on.
Another recent observational study from an independent team in Portugal supports Freitas and Peixe’s results. In their study, Constança Pomba of the University of Lisbon and colleagues analyzed fecal samples of more than 100 humans and 84 of their companion animals. The great majority of the humans and animals tested positive for nonpathogenic strains of E. coli; most didn’t show signs of a bacterial infection. Digging deeper, the team identified evidence of genes resistant to colistin, a last-resort treatment to kill multidrug-resistant bacteria, in the fecal samples of two healthy humans and one dog that all lived in different households. Researchers should be aware of the risk that already multidrug-resistant bacteria could acquire a genetic defense against colistin if they pick up these genes while in humans or pets, the authors say.
Overall, however, evidence is weak that large quantities of antibiotic-resistant bacteria or resistance genes are actually being transferred to humans from pets, at least at levels great enough to make us sick. Some studies have found that if it happens at all, it’s likely very rare. For example, Carolin Hackmann of the Charité-University Hospital in Berlin and colleagues have so far taken nasal and rectal swabs from 1,500 hospital patients and from several dozen of those patients’ pets, and in only two cases were pet and owner colonized with the same strain of multidrug-resistant organism. “We were surprised that the rate of pet-owner pairs who shared the same [strain] was so small,” Hackmann writes in an email to The Scientist. She adds that “it is too early to draw any final conclusions,” but based on the data she does have, owning a pet doesn’t appear to significantly raise the risk of pets and humans swapping multidrug-resistant bacteria.
Clarification (July 31): This article has been updated to clarify that Ana Raquel Freitas and Luisa Peixe are researchers affiliated with the Research Unit on Applied Molecular Biosciences at the University of Porto.