Staphylococcus aureus is a common bacterium and an important cause of disease in many species. Approximately 10% of healthy horses carry S. aureus in their noses. This occurrence is termed “colonization,” as the bacteria are present without causing any problems. A smaller number are colonized in the intestinal tract or on the skin. S. aureus is an “opportunistic pathogen” that can cause disease under certain conditions. One problematic trait of S. aureus is its tendency to become resistant to antibiotics. Of particular concern is methicillin-resistant S. aureus (MRSA), which is resistant to all beta-lactam antibiotics (penicillin and cephalosporin families) and often many other antibiotics. This makes MRSA infections more difficult to treat.

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.

Like methicillin-susceptible S. aureus, MRSA can colonize horses without causing any problems. Studies have reported carriage rates of 0-5% in horses in the general population, but on some farms the prevalence can exceed 50%. Colonized horses may never have any problems with MRSA, but they are more likely to develop an MRSA infection under certain conditions. 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. In hospitalized horses, surgical site infections predominate. Invasive device (i.e., intravenous catheter) site infections and bloodstream infections can also occur, as well infections at a variety of other sites.

Despite the obvious concerns about MRSA, it can be a treatable condition. In a multicenter study, over 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