Drug-resistant bacteria best known for causing hospital outbreaks may also be circulating more quietly between pets and people, according to new international research. The findings suggest that companion animals could play a larger role in the spread of antimicrobial resistance than previously recognized, highlighting another challenge in the global effort to combat hard-to-treat infections.
The study, published in Applied and Environmental Microbiology, focused on Acinetobacter baumannii, a bacterium the World Health Organization considers one of its highest-priority antibiotic-resistant pathogens. Although long associated with hospital-acquired infections, researchers say growing evidence indicates that the bacterium can also move between animals and humans.
Investigating the Spread of a Dangerous Bacterium
The research began in 2022 after dead chicken and duck embryos were submitted to a veterinary laboratory in China for diagnostic testing.
Scientists detected A. baumannii in the embryos’ brain tissue, but these poultry strains showed relatively low levels of resistance to antibiotics.
To place those findings into a broader context, the researchers assembled a global dataset containing 509 animal-derived A. baumannii isolates collected from 17 countries and representing 33 different animal species.
The samples included livestock, poultry, horses, cats, dogs, and wildlife such as white storks from Poland.
The team then compared these animal strains with thousands of A. baumannii samples collected from nearby hospitals, creating a detailed genetic map to examine how bacterial strains are shared across species.
Pets Shared the Closest Relationship With Hospital Strains
The analysis revealed two major groups of animal-associated A. baumannii.
One group, found primarily in horses, cats, and dogs, exhibited high levels of antimicrobial resistance.
The second group, largely associated with livestock, poultry, and wild animals, showed substantially lower levels of resistance.
Using an international classification system that tracks major outbreak lineages, the researchers identified several well-known hospital-associated clones among companion animals.
One lineage, known as ST25, appeared repeatedly in pets and horses and was detected across nine different animal species.
Genetic comparisons showed that strains isolated from cats, dogs, and horses were more than 99% genetically similar to strains causing infections in nearby hospital patients. In some cases, the similarity exceeded 99.5%.
By contrast, bacterial strains found in livestock, poultry, and wildlife typically shared only 97% to 98% genetic similarity with hospital isolates, suggesting considerably less overlap.
Companion Animals Carried Concerning Resistance Genes
The researchers also identified resistance genes in companion animals that are particularly concerning from a public health perspective.
Some bacterial samples collected from cats and dogs carried genes that provide resistance to carbapenems, a class of antibiotics often reserved as a last line of treatment for severe infections when other drugs no longer work.
These resistance genes were not detected in any of the livestock, poultry, or wildlife samples included in the study.
Because carbapenems are prohibited for livestock use in many countries, the researchers suggest that companion animals may represent a distinct pathway through which these resistance genes circulate.
The authors note that the close daily contact between people and their pets creates frequent opportunities for bacteria or resistance genes to move in both directions through shared living spaces, surfaces, and routine physical contact.
No Direct Proof of Transmission
Despite the findings, the researchers emphasize that the study does not demonstrate that any individual cat or dog directly transmitted A. baumannii to a human patient.
Instead, the results indicate that closely related bacterial populations are present in both hospital patients and companion animals, creating repeated opportunities for cross-species transmission.
The findings are consistent with the broader “One Health” approach, which recognizes that human, animal, and environmental health are closely interconnected.
Previous microbiology research has similarly called for greater surveillance of antibiotic-resistant bacteria in companion animals as part of efforts to slow the spread of antimicrobial resistance.
Looking Ahead
The researchers recommend future studies that simultaneously monitor both humans and animals living within the same households and communities.
Tracking bacterial genomes over time could help determine how frequently A. baumannii moves between species and identify the most effective strategies for preventing transmission.
The findings also reinforce the importance of responsible antibiotic use in both human and veterinary medicine. Reducing unnecessary antibiotic prescriptions remains one of the most effective ways to slow the emergence and spread of drug-resistant bacteria, regardless of whether they circulate in hospitals, homes, or animal populations.
