Humans and Animals Share MRSA Strain

A new study offers evidence that methicillin-resistant Staphylococcus aureus (MRSA) can be passed from human to animal and back again without undergoing any adaptation.

Previous studies have inferred that the bacteria are transmissible from human to animal and vice versa.

According to the AVMA, a 2008 report in the New England Journal of Medicine documented a MRSA strain that was the same in a chronically infected woman, her family members, and one of the family’s pet cats. After the cat was "de-colonized," the human patient's infection went away. The AVMA also notes a 2005 case report in the Journal of Clinical Microbiology, which documented colonization of family members and a pet dog during the investigation of chronic, recurrent MRSA infections in a human patient. The study concluded that either a family member or the pet dog served as a reservoir for reinfection of the patient.

But this latest study, led by researchers at the University of Cambridge in the United Kingdom, showed that 46 different animal samples of MRSA from a strain known as sequence type 22 (ST22) were essentially genetically identical to ST22 samples from human hosts. This suggests that the bacteria are easily passed back and forth between humans and pets without having to adapt.

"[Companion animals] pass it backwards and forwards - they are a reservoir of infection," Mark Holmes, VetMB, PhD, one of the study’s authors, said in a BBC news article.

The researchers also compared the MRSA isolates from animals and people to see if there was any mutation when the bacteria were passed between species. "The analysis showed very little genetic discrimination between isolates from different hosts, suggesting that the ST22 isolates had not undergone extensive adaptation to companion animals," the study says.

The scientists conclude that their research contributes to the "one health" view of infectious diseases. The fact that animals and humans can be infected by the same strains of MRSA shows that antibiotic usage in both human and veterinary medical settings can affect bacteria populations, the authors say.

The study, "A Shared Population of Epidemic Methicillin-Resistant Staphylococcus aureus 15 Circulates in Humans and Companion Animals," was published in the American Society for Microbiology’s journal, mBio.

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