Dogs and cats can carry antibiotic-resistant bacteria that are genetically similar to strains found in people worldwide.
Antibiotic resistance is often discussed as a hospital problem, where bacteria can survive medicines that once stopped them.
Resistant bacteria also occur outside human healthcare, and pets may carry strains that matter for both animal and human health.
One bacterium drawing attention is Klebsiella pneumoniae.
It can live in the body without causing illness, but some strains can cause serious infections. Treatment can become harder when the bacteria carry genes that help them resist antibiotics.
A new global analysis examined hundreds of K. pneumoniae genomes, the complete sets of genetic information inside the bacteria, from dogs and cats.
The researchers wanted to identify which bacterial groups were present and how resistant they were to antibiotics. They also compared them with strains found in people.
Antibiotic resistance in pets
The team studied 712 K. pneumoniae genomes from dogs and cats in 25 countries. This included 567 samples from dogs and 145 from cats.
They checked each genome, confirmed the bacterial species, and looked for antibiotic resistance genes. They also grouped the bacteria into sequence types, or STs, which are genetic groups of closely related bacteria.
The researchers found 263 sequence types. Several high-risk types appeared often, including ST307, ST11, ST15, and ST147.
The ten most common types made up 45.8% of all samples. All ten were found in both dogs and cats.
The team compared the pet collection with 38,106 human-associated K. pneumoniae samples collected from 2019 onward.
Those human records represented 1,661 sequence-type profiles from 88 countries and territories.
There was a strong overlap between the bacteria found in pets and people.
Of the 712 animal samples, 621, or 87.2%, belonged to bacterial types that had also been found in humans.
All ten of the most common bacterial types found in pets were also found in people. This overlap was greater than the researchers would expect by chance.
That does not show that a pet passed the bacterium to a person, or the reverse. Sequence types identify broad genetic groups, but they cannot prove a direct transmission event.
Cats carried more resistance
Among 706 animal samples with country information, 303, or 42.9%, carried extended-spectrum beta-lactamase genes, known as ESBL genes.
These genes can help bacteria resist important antibiotics, including many drugs in the cephalosporin family.
Another 98 samples, or 13.9%, carried carbapenemase genes. These genes can help bacteria resist carbapenems, antibiotics often used for hard-to-treat infections.
Overall, 345 samples, or 48.9%, carried at least one of these two major types of resistance genes.
Cats carried more multidrug-resistant bacteria than dogs. About 80% of cat samples carried resistance genes against at least three types of antibiotics, compared with 56.3% of dog samples.
Part of this difference was linked to the bacterial types found in cats.
Cats more often carried high-risk types such as ST147, although the difference did not disappear completely after the researchers considered other factors.
The researchers looked more closely at ST147 because it is a high-risk bacterial type linked to important antibiotic resistance genes.
They compared small differences in the DNA to see how closely the samples were related.
They found a closely related group containing bacteria from cats, dogs, and people. Among 25 dog and cat samples, the genomes differed by zero to 34 DNA changes, with a median of six.
The 17 cat samples from New York were even more similar, with a median of three DNA differences.
Some cat samples and both New York dog samples showed no detected differences from one reference cat genome, while the closest human samples differed by only two to nine DNA changes.
These similarities suggest that the bacteria shared a recent common ancestor.
However, the study cannot show whether the bacteria passed directly between pets and people, who passed them to whom, or where the bacteria originally came from.
Resistance appeared beyond infections
Multidrug-resistant K. pneumoniae was found in samples from both sick and healthy animals.
It appeared in 62.1% of colonization or screening samples and 52.4% of infection-related samples. The difference was not statistically clear.
This means resistant bacteria can be present even when they are not causing an active infection.
Dr. Stephen Fordham, Lecturer in Biosciences at Bournemouth University, led the work. The team also included researchers from University Hospitals Dorset NHS Foundation Trust.
“This study does not tell us that our pets are making us ill, but it does show that very closely related bacterial strains can be found across animals and humans,” Dr. Fordham said.
“There is no need for owners to be concerned but this does show the importance of including our pets in wider efforts to understand and tackle antimicrobial resistance.”
What the findings mean
Dogs and cats can carry the same K. pneumoniae lineages that worry doctors on the human side of medicine.
A few of the ST147 bacteria from animals and people sat almost on top of each other genetically.
None of that makes pets a common source of resistant infections in people. These genomes came out of public databases, where odd or clinically interesting cases tend to pile up more than ordinary ones do.
And there’s a lot the researchers simply didn’t know about each animal. That included its antibiotic history, veterinary care, illnesses, and who it lived with.
Nobody swabbed a pet and its owner on the same day, so there’s no way to say which one picked up a strain first, or whether it moved between them at all.
Pets should be included when tracking antibiotic resistance. This can help show where high-risk bacteria are spreading.
The study is published in the journal Transboundary and Emerging Diseases.
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