Two studies by Portuguese scientists presented at the European Congress of Clinical Microbiology and Infectious Diseases (ECCMID) highlight concerns about the potential for transmission of multidrug-resistant (MDR) bacteria between companion animals and humans.

In one study, researchers from the University of Porto found high levels of MDR enterococci in raw-frozen dog food sold in the European Union. In another study, a household survey and molecular screening by researchers from the University of Lisbon found the colistin-resistance gene MCR-1 in two healthy humans and one dog with a skin infection.

But in a third study, researchers in Germany reported that pet ownership does not appear to be a significant risk factor for colonization with MDR organisms (MDROs).

MDROs in raw-frozen dog food

The study on dog food looked at 46 samples of dry, wet, and raw-food products purchased from eight supermarkets and one veterinary clinic in Europe, with particular attention paid to raw-food–based products. These products have become increasingly popular among dog owners in recent years, but some evidence has emerged that they may be vehicles for transmitting antibiotic-resistant bacteria.

The raw-frozen products were a mixture of different meat or seafood types (including chicken, turkey, duck, and salmon), fruits, and vegetables.

The researchers focused on Enterococcus bacteria because previous research has shown that dogs are frequent carriers of ampicillin-resistant Enterococcus faecium and may play a role in community spread.

The researchers identified Enterococcus isolates in 19 of the 46 samples overall (8 of 15 dry-food samples, 2 of 22 wet-food samples, and 9 of 9 raw-frozen samples). In the 9 raw-frozen products, they recovered 20 MDR E faecium and 22 MDR Enterococcus faecalis isolates. Only one MDR E faecium was identified in the wet-food sample, and none in the dry food.

All MDR isolates in the 9 raw-frozen samples were resistant to ampicillin, ciprofloxacin, erythromycin, tetracycline, streptomycin, and chloramphenicol. In addition, 7 of the 9 samples contained enterococci resistant to the last-line antibiotic linezolid.

"Our study demonstrates that raw-frozen-foods for dogs carry MDR enterococci including to last-resource molecules for the treatment of human infections (linezolid)," the authors wrote in the abstract. "The close contact of pets with humans and the commercialization of the studied brands in different EU countries pose an international public health risk if transmission of such strains occurs between dogs and humans."

In the other study, University of Lisbon researchers collected fecal samples from 70 households occupied by healthy people (106), healthy companion animals (49), and companion animals with skin and soft-tissue infections (19) and urinary tract infections (16). Their aim was to investigate the presence of the plasmid-mediated colistin-resistance gene MCR-1 in Escherichia coli bacteria.

Since it was first identified in China in 2015, the MCR-1 gene has been detected in humans and animals around the world. The fear with the transmissible gene is that, when acquired by bacteria harboring other resistance genes, it can cause infections that are essentially untreatable.

Of the 161 E coli isolates identified in the fecal samples (89 from people, 45 from dogs, and 21 from cats), colistin resistance was found in 3.1% (5 of 161 isolates). Molecular analysis found the MCR-1 gene in three of the isolates—two from healthy people and one from a dog with a skin infection. None of the three isolates were from the same household, however.

Pets not a risk factor for MDROs

The two studies add to a growing body of research about the potential for transmission of antibiotic-resistant bacteria between human and companion animals. Bacterial species like enterocci and E coli are of particular concern because they live in the guts of cats and dogs and can be shed in feces, which provide a possible avenue of transmission to pet owners. Research has also been conducted on the role of pets in the transmission of methicillin-resistant Staphylococcus aureus (MRSA).

While transmission of pathogenic bacteria from pets to their owners, and vice-versa, has been observed, whether owning a pet is a risk factor for acquiring antibiotic-resistant pathogens is unclear. But early results from an ongoing study being conducted in Germany suggest it's not.

In the unmatched case-control study, researchers from University Hospital Berlin are collecting nasal and rectal swabs of hospital patients and their pets to test them for MDROs, including MRSA, vancomycin-resistant enterococci, third-generation cephalosporin-resistant Enterobacteriaceae, and carbapenem-resistant Enterobacteriaceae. Phenotypically matching MDROs from pets and their owners are then tested for genetic relatedness—an indication of transmission—using whole genome sequencing (WGS).

The researchers report that among the first 1,500 participants enrolled in the study (out of a planned 4,000 to 6,000), one-third (495) tested positive for an MDRO, and of the 296 people who owned at least one pet, 112 (38%) had an MDRO.

But a univariable analysis found no significant difference in the proportion of pet owners between the patients with MDROs and the control patients, and further analysis of the health status of pets, and the closeness of contact between pets and their owners, also found no significant difference between cases and controls.

But some transmission of MDROs between patients and their pets was found. Among the 77 dog and 71 cat samples from the 112 owners that have been analyzed, 16% (23 dogs and 1 cat) tested positive for an MDRO. In two of the cases, the MDROs found in a patient and their dog were phenotypically matching, and WGS confirmed that the isolates from the owner and their dog were genotypically identical.

"A transmission of MDROs between human and animal was confirmed in only 1.8% of 112 pet owners and their respective pets," the authors wrote. "So far, the preliminary data does not indicate pet care as a significant risk factor for MDRO colonisation in hospital patients."

Although ECCMID was cancelled this year because of the COVID-19 pandemic, the studies were published in a book of abstracts, along with others that were to be presented at the conference.