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Rise of MRSA strain in European horses described

MRSA, magnified 20,000 times. Photo: Public Health Image Library

MRSA, magnified 20,000 times. Photo: Public Health Image Library

A form of Methicillin-Resistant Staphylococcus aureus (MRSA) first identified in 2005 has displaced other strains to become the leading cause of MRSA infection in horses in some areas of Europe, it has been reported.

The emergence of the strain is described by Dr Scott Weese, from the Ontario Veterinary College at the University of Guelph in Canada, in the latest issue of Equine Disease Quarterly.

Weese said while it appeared to be rare at this time in horses outside of northern Europe, highly mobile horses had the potential to assist with global spread of the pathogen.

In the Netherlands in 2005, a small number of unexpected MRSA infections in people with pig contact led to identification of the novel MRSA clone, sequence type (ST) 398.

This clone was quickly called “livestock-associated” MRSA, Weese said, although recent evidence suggested it originated in humans as a methicillin-susceptible strain that subsequently moved to pigs, became methicillin-resistant, and lost some human-adaptive traits.

“Nonetheless, whatever it is termed, ST398 MRSA is an important issue in people and animals in some regions,” he said.

The organism has since been identified in pigs worldwide, and to a lesser degree other livestock such as cattle and poultry, and is commonly found in people who have contact with livestock.

In some northern European countries, it is now the leading cause of MRSA infection in people.

Equine MRSA infections were first reported in the late 1990s, and it soon became apparent that MRSA was endemic in the horse population in many regions, being found in a small percentage (typically less than 3 percent) of healthy horses and causing sporadic infections and outbreaks.

“Abnormally high rates of MRSA colonization of horse owners and equine veterinarians have also been reported, along with smaller numbers of zoonotic infections,” he said.

“Until the late 2000s, the vast majority of MRSA isolates from horses were common human epidemic clones, suggesting that equine MRSA was ultimately human in origin.

“Yet recent years have ushered in a potential new concern, with identification of ST398 in horses.

“ST398 has now been reported in horses in many European countries as well as in a single Canadian horse.”

Cross-species infection from a horse has also been reported. Highly variable colonization rates (0.5-11 percent) have been described, and ST398 has displaced other strains to become the leading cause of MRSA infection in horses in some areas of Europe, he said.

As is typical for any MRSA strain, a wide range of clinical infections has been reported, including skin and soft tissue infections, surgical site infections, pneumonia, catheter site infections, mastitis, and metritis.

“Whether the emergence of ST398 in horses indicates a clinically relevant change for the horse population, a concern restricted to certain regions or farm types, or simply a shift in predominant strains with limited clinical relevance remains to be seen,” Weese said.

“While ST398 appears to be rare at this time in horses outside of northern Europe, the highly mobile nature of the horse population indicates a potential for horses to assist with global dissemination of this animal and human pathogen.”

Weese said horse owners and veterinarians should be aware of the risk of MRSA (ST398 and others), and use:

  • Basic infection control practices at all times.
  • Enhanced infection control practices for any animal with an infection.
  • Antimicrobials prudently.
  • Bacterial culture and susceptibility testing routinely on animals with opportunistic infections.

Weese said the emergence of this livestock-associated MRSA clone also highlighted the often-overlooked potential for pathogen exposure of horses that have direct or indirect contact with food animals.

Equine Disease Quarterly is funded by Lloyd’s, London, brokers and their Kentucky agents.

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