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In A Nutshell
- A global genetic study found that 87.2% of drug-resistant bacteria in pet dogs and cats belong to strain types also circulating in humans.
- Cats showed multidrug resistance in 80% of samples, compared to 56.3% in dogs, partly explained by one high-risk strain called ST147.
- Nearly half of all pet samples carried genes tied to resistance against key antibiotic classes, including some that fend off carbapenems, a last-resort treatment.
- The findings don’t prove pets are infecting their owners, but researchers say companion animals should be included in tracking drug-resistant bacteria worldwide.
A global genetic study found that 87.2% of Klebsiella pneumoniae samples from pet dogs and cats belonged to strain types that have also turned up in people. That means the family dog curling up on the bed or the cat napping on the couch could be carrying a bacterial lineage closely related to the kind doctors battle in sick patients, not some separate animal-only version of the bug.
Scientists examined 712 samples of Klebsiella pneumoniae, a bacterium that normally lives quietly in the gut but can cause serious pneumonia, urinary tract infections, and bloodstream infections when it strikes at the wrong time. Published in the journal Transboundary and Emerging Diseases, the study drew on samples from dogs and cats in 25 countries and compared them against a database of more than 38,000 human-linked samples of the same bug. The overlap in strain types was hard to ignore.
Cats fared worse than dogs in the analysis, with 80% of cat samples showing resistance to multiple classes of antibiotics compared to about 56% in dogs. Nearly half of all pet samples worldwide carried genes tied to resistance against key antibiotic drug classes, including some genes that fend off carbapenems, the medications doctors reach for when nothing else works.
Researchers Compared 712 Pet Samples Against Global Human Data
To build this picture, the research team gathered publicly available genetic records of the bacteria collected from dogs and cats worldwide, drawing on international databases maintained by health and research agencies. After filtering out low-quality or mislabeled samples, they were left with 712 confirmed genomes from pets, spanning 25 countries.
Each sample’s genetic code was analyzed to identify its strain type, essentially a genetic fingerprint that groups related bacteria together, and checked for genes known to make bacteria resistant to antibiotics. Two categories of genes drew particular attention: one that helps bacteria break down common antibiotics, and a tougher category tied to carbapenem resistance. Bacteria carrying resistance genes covering three or more different antibiotic classes were labeled multidrug-resistant.
To see how the pet bacteria compared with what is circulating in people, the researchers built a comparison set of 38,106 human-linked samples of the same germ, collected from 2019 onward across 88 countries. Then they measured the overlap between pet and human strains.
Four High-Risk Strains Turned Up Again and Again in Pet Samples
Among the 712 pet samples, researchers identified 263 distinct strain types, showing real genetic variety. Despite that variety, a handful of strains kept reappearing. Four of those strains in particular are already recognized internationally as high-risk because of their frequent ties to antibiotic resistance in human medicine, and all four turned out to be common in the pet samples too. One of them, known to scientists by the label ST147, would turn out to matter most.
Resistance genes were everywhere in the data. Among 706 pet samples with location information, 303 (42.9%) carried genes that break down common antibiotics, and 98 (13.9%) carried the harder-to-treat carbapenem-resistance genes.
That cat-dog split matched the overall pattern already noted, and part of the explanation traced back to ST147, the strain that showed up more often in cats and carries a heavy load of resistance genes. Just as important, resistant bacteria were not limited to animals that looked sick. Samples taken from healthy-seeming pets, through routine screening or stool testing rather than during an active infection, also frequently carried multidrug-resistant bacteria. That detail matters because some pets may carry these bacteria without outward signs, potentially contributing to a reservoir of resistant strains shared across animals and humans.
A Cat-Enriched Strain Cluster Links Genomes From Cats, Dogs, and People
Because ST147 stood out as both common in cats and heavily loaded with resistance genes, the research team took a closer look at its family tree. Using a method that strips out genetic noise caused by bacteria swapping DNA with each other, they mapped how ST147 samples from cats, dogs, and humans related to one another.
That map revealed a tightly related cluster of multidrug-resistant ST147 bacteria containing genomes from cats, dogs, and people, all closely linked genetically. Some pet and human samples in this cluster differed by only a handful of genetic changes, the kind of close relationship that points to a recent shared ancestor rather than bacteria that evolved separately in each species. The researchers were careful to note that this genetic closeness shows the bacteria belong to the same circulating population; it isn’t proof that a specific pet infected a specific person, or the other way around.
Pets Share a Bacterial Pool With People, Not a Separate One
None of this means owning a cat or dog is dangerous, and the study doesn’t show pets are directly spreading infections to their owners in any documented way. What it does show is that the drug-resistant bacteria showing up in veterinary clinics and human hospitals are drawing from the same genetic pool, not two disconnected ones. Treating antibiotic resistance as purely a human medical problem, or purely a farm-animal problem, leaves out a population that lives closer to people than almost any other animal on Earth. If health agencies want to get ahead of the next drug-resistant outbreak, the household pet may need a seat at the surveillance table alongside hospitals and farms.
Disclaimer: This article is based on a peer-reviewed genetic study and is intended for general informational purposes. It does not establish that pets transmit drug-resistant bacteria to their owners or that pet ownership carries a documented infection risk.
Paper Notes
Limitations
Public genetic records rather than newly collected samples formed the basis of the study, which means the findings reflect whatever data happened to be submitted to international databases. The authors note that public genome collections carry sampling biases tied to geography, which countries have strong sequencing programs, whether a sample came from an outbreak investigation, and a tendency for labs to submit resistant or unusual isolates more often than routine ones. The researchers also emphasize that shared strain types and close genetic relationships between animal and human samples show cocirculation of the same bacterial population rather than proof of direct transmission between a specific pet and a specific person. Detailed information such as an individual animal’s antibiotic treatment history, hospitalization records, or disease history was not available for analysis, limiting how precisely the causes behind cat versus dog differences in resistance could be pinned down.
Funding and Disclosures
According to the paper, no funds, grants, or other support were received for conducting the study or preparing the manuscript. The authors reported no conflicts of interest. Ethical approval was not required because the study used publicly available bacterial genome sequences and metadata rather than newly sampled animals or people.
Publication Details
Titled ‘Companion Animals Harbour Globally Circulating Human-Associated Klebsiella pneumoniae Lineages and High-Risk Antimicrobial Resistance Clones,’ the paper was published in Transboundary and Emerging Diseases (2026). The authors are Stephen Mark Edward Fordham, Elizabeth Sheridan, and Francis Drobniewski, affiliated with Bournemouth University, University Hospitals Dorset NHS Foundation Trust, and Imperial College London. DOI: 10.1155/tbed/5383720.
