While folklore has it that humans and their pets tend to look alike, scientists have been finding deeper and even somewhat unsettling resemblances. Of particular concern are epidemiological and other data suggesting that humans and companion animals readily exchange and share a population of MRSA bacteria. While MRSA infection in cats and dogs is rare, and while humans generally have little risk of getting ill from their pets, companion animals may still act as a reservoir for human MRSA infection and vice versa.

This finding, recently confirmed by high-resolution genomic testing, lends support to the “one health” view of infectious diseases—the notion that the health of both human and animal populations are intrinsically linked. Here the link is a shared pathogen pool. Curiously, scientists plumbing the pool’s depths have found evidence that antibiotic usage in animal medicine is shaping the population of a major human pathogen.

Led by Mark Holmes, Ph.D., Vet.MB., senior lecturer in preventive veterinary medicine at the University of Cambridge, a team of scientists sequenced the genomes of 46 MRSA samples from cats and dogs, collected from two large veterinary hospitals and several smaller veterinary practices throughout the United Kingdom. These genomes were then compared to an extensive population framework of human isolates from the same lineage.

This work, described May 13 in mBio, in an article entitled “A Shared Population of Epidemic Methicillin-Resistant Staphylococcus aureus 15 Circulates in Humans and Companion Animals,” progressed to phylogenomic analyses. According to the authors, the analyses “showed that all companion animal isolates were interspersed throughout the epidemic MRSA-15 (EMRSA-15) pandemic clade and clustered with human isolates from the United Kingdom, with human isolates basal to those from companion animals, suggesting a human source for isolates infecting companion animals.”

Researchers also observed that samples from the same veterinary hospitals clustered together genetically, suggesting that as in human hospitals, MRSA can be readily transmitted in veterinary hospital settings.

“It’s a reminder that constant vigilance and high levels of hygiene are just as important when treating cats and dogs as with humans,” Dr. Holmes said.

Analysis of the genomes showed very little genetic discrimination between bacteria samples from humans and animals, indicating that the MRSA from cats and dogs had not undergone extensive adaptation to the companion animals, suggesting this type of MRSA has a broad host range. But the animal MRSA were significantly less likely than those from humans to have resistance to the antibiotic erythromycin, used rarely in English veterinary practices. Instead, these MRSA from animals were more likely to contain mutations making them resistant to the antibiotic clindamycin, used widely in veterinary medicine in the United Kingdom.

In a release that cited the publication of the mBio article, the researchers downplayed the risks of humans and pets infecting each other. Indeed, in their article, the researchers made a larger point of how MRSA processes—the development of drug resistance and the spread of infection—in human and animal populations appear to parallel each other. After highlighting the transmissibility of a hospital-acquired MRSA clone in a veterinary hospital setting, the researchers ventured that that “MRSA prevention practices used in human medicine, such as search and destroy or blanket decolonization, may also be appropriate in veterinary practice.”
Nonetheless, the researchers clearly demonstrated that cross-species transmission of MRSA is a serious concern. As they concluded in their article, “Further studies using whole-genome sequencing to investigate temporally paired human and companion isolates from veterinary hospitals and the human community are warranted to further understand the exact transmission dynamics between humans and companion animals.”
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