"Studies have reported carriage rates of 0-5% in horses in the general population, but on some farms the prevalence can exceed 50%," said Dr J. Scott Weese of the Department of Clinical Studies at the University of Guelph in Ontario, Canada.
"Colonized horses may never have any problems with MRSA, but they are more likely to develop an MRSA infection under certain conditions," he said. "Colonized horses are also of concern because they can transmit MRSA to other horses and people.
"Clinical MRSA infections can occur as sporadic cases or outbreaks. A wide range of infections can develop. In horses in the general population, skin and soft tissue infections - including wound and surgical site infections - and joint infections are most common.
MRSA is a growing problem in human hospitals, and likewise with horses.
Despite concerns about MRSA, a multi-centre study showed more than 80% of horses with MRSA infections survived, although they tended to have prolonged hospital stays and often required additional surgeries.
"While MRSA strains are resistant to many drugs, acceptable antibiotic options usually exist. The key to proper and successful management is early diagnosis of MRSA so that appropriate therapy can be instituted."
Dr Weese said staphylococcus aureus is a common bacterium and an important cause of disease in many species.
About 10% of healthy horses carry the bacteria in their noses, this so-called "colonization," causing no problems. A smaller number are colonized in the intestine or on the skin.
"S. aureus is an 'opportunistic pathogen' that can cause disease under certain conditions," he said.
However, its tendency to become resistant to antibiotics is of particular concern, with MRSA resistant to all beta-lactam antibiotics (penicillin and cephalosporin families) and often many other antibiotics.
In humans, MRSA infections are associated with increased illness and death compared to methicillin-susceptible S. aureus infections.
"MRSA is a tremendous problem in human hospitals and is now causing disease in people in the general population.
"But MRSA has also emerged as an important cause of disease in many animal species, including horses."
Dr Weese said most reports of MRSA in horses have involved one family (clone) of MRSA.
"This MRSA strain (or closely related strains) are recognized as a human strain, yet the strain is uncommon in people. Its predominance in horses suggests that it is somehow better adapted to horses than other strains. This strain has been reported in both North America and Europe and is likely widely disseminated internationally.
"One MRSA aspect of concern," he said, "is the potential for transmission between humans and horses, in both directions. People who work with horses appear to be at particularly high risk for MRSA colonisation.
"Studies of equine veterinarians have reported colonisation rates of 10-14%. The MRSA clone that predominates in horses has been the most common strain in equine personnel, providing further support for the notion that horses can infect humans."
Dr Weese said an outbreak of MRSA skin infections occurred in a teaching hospital in people working with a colonised foal.
"Precautions need to be undertaken to reduce the risk of infection of human contacts and to prevent transmission of MRSA on farms or in clinics."
Practices would vary depending upon the situation, but may include isolation of infected or colonized horses, the use of gloves and gowns when handling infected or colonized horses, improvement in general hygiene (especially hand hygiene among farm workers and veterinarians), screening of horses for colonization, limiting contact of different groups of horses, and other related infection control measures.
"No current evidence exists that antibiotics are useful for eradication of colonization, but MRSA can be eradicated without the use of antibiotics from farms with infection control practices if adequate time and energy are committed.
"All aspects of the equine industry need to be aware of this veterinary and zoonotic pathogen, because MRSA is likely to be an increasing concern in equine medicine."