Horse-specific form of MRSA appears to dominate in France – study

A scanning electron micrograph of a human neutrophil ingesting MRSA. Image: National Institutes of Health, public domain,, via Wikimedia Commons
A scanning electron micrograph of a human neutrophil ingesting MRSA. Image: National Institutes of Health, public domain, via Wikimedia Commons

A form of horse-specific methicillin-resistant Staphylococcus aureus (MRSA) now appears to dominate in France, according to researchers, replacing another form of the bacterium in just a few years.

French researchers undertook molecular-based testing of 130 MRSA samples taken between 2010 and 2015 from infected cats, dogs and horses. They originated from cases across the country.

Most of the MRSA samples were isolated from skin and soft-tissue infections, while MRSA from reproductive tract infections were also prevalent in horses.

The scientists identified a dominant horse-specific form among equids, while cats and dogs were mainly infected by human-related MRSA – that is, clones (genetically identical forms) that were usually reported in human infections.

The MRSA clone dominating among the French horse samples used in the study is known as CC398-IV-t011, which is now massively dominant among horses around the world, the study team said.

Marisa Haenni and her colleagues, writing in the journal Frontiers in Microbiology, said their work showed that the MRSA clones detected in the horse samples were independent of the human MRSA forms.

Two major animal-associated clones were identified in the tested horse samples: CC8-IV USA500 and CC398-IV-t011.

CC8-IV USA500, which is the main clone infecting horses in North America, was detected in the French samples between 2011 and 2013, with a last occurrence in early 2014.

“This suggests that this clone, uncommon in the human population in France and worldwide, is prone to colonize and/or infect horses worldwide,” they reported.

Yet CC8-IV USA500 seemed to have been totally replaced by the horse-specific CC398-IV-t011 clone, confirming its international spread and high fitness/adaptation to horses.

Between 2013 and 2015, only 7 of the 39 horse samples tested did not belong to the CC398 clone.

The replacement of CC8-IV USA500 by the CC398-IV-t011 clone may be good news, they said, since CC398 showed resistance to fewer antibiotics. That said, differences in virulence would also have to be considered.

Given the possibility of human re-infection, the prevalence of MRSA in cats and dogs was undoubtedly a public health issue that deserved to be monitored, they said.

Conversely, horses are mostly infected by animal-associated isolates.

They recommended regular surveillance of MRSA in companion animals in coming years to assess whether the CC398 form would replace human-related MRSA clones in cats and dogs, and whether those clones would be able to re-adapt to the human host.

The study team comprised Haenni, Pierre Châtre, Céline Dupieux-Chabert, Véronique Métayer, Michèle Bes, Jean-Yves Madec and Frédéric Laurent, from a range of French institutions.

Haenni M, Châtre P, Dupieux-Chabert C, Métayer V, Bes M, Madec J-Y and Laurent F (2017) Molecular Epidemiology of Methicillin-Resistant Staphylococcus aureus in Horses, Cats, and Dogs Over a 5-Year Period in France. Front. Microbiol. 8:2493. doi: 10.3389/fmicb.2017.02493

The study, published under a Creative Commons License, can be read here

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