Drug-Resistant Bugs

Methicillin-resistant Staphylococcus aureus (MRSA) is a multidrug-resistant bacterium that has received ample attention in the press lately. Much concern has been expressed because of the tremendous increase in human infections, including people in the general population that would typically be considered low-risk. In people, MRSA can cause a wide range of infections, from skin infections and abscesses to severe pneumonia and necrotizing fasciitis ("flesh-eating disease").

MRSA is an emerging problem in horses. There are increasing reports of MRSA infection and colonization (the presence of MRSA at a body site, usually the nose, without any signs of infection) in horses internationally. The prevalence of colonization in horses is rather variable and probably ranges from 0-5% in most populations. Colonization rates of greater than 50% have been identified on some farms. Colonized horses might not develop MRSA infections, but they are at increased risk of developing MRSA infections under certain conditions and can be sources of infection for other horses or people. In horses, as in people, a wide range of infections can develop. Skin, soft tissue (including wound and surgical site infections), and joint infections are most common in horses in the general population. Surgical site infections predominate in hospitalized horses.

While the mention of MRSA often raises fears, it can be a treatable infection if addressed promptly and appropriately. And even though it's potentially difficult and expensive to treat, one multi-center study reported a survival rate greater than 80%. MRSA strains are multidrug resistant, but there are usually some options. The key for proper and successful management is early diagnosis so that appropriate therapy can be instituted. This highlights the need for bacterial culture before treatment.

Most reports of MRSA in horses and the people that work with horses have involved one particular strain. It is likely this strain originated in people and eventually spread to horses. For some reason, it must be more adept at surviving in horses than other MRSA strains, and it has since become widespread. This strain has been reported in North America and Europe.

Another concerning aspect of MRSA is the potential for transmission between humans and horses. Recent studies reported that people who work with horses, particularly equine veterinarians, have greater rates of MRSA colonization (10-14%) than the general population. Horses have also been implicated as causes of clinical MRSA infection in people, including an outbreak of skin infections in people working with a colonized foal in a neonatal intensive care unit.

Infection control precautions need to be considered to reduce the risk of infection in horses and people. The intensity of required practices is variable and depends on the situation. It might include isolation of infected or colonized horses, the use of barriers (gloves, gowns) when handing infected or colonized horses, improvement in general hygiene (especially hand hygiene among farm workers and veterinarians), screening of horses for colonization, limiting contact between different groups of horses, and related infection control measures. MRSA can be eradicated from farms with infection control practices if adequate time and energy are committed. This can be done without the use of antibiotics, and there is currently no evidence that antibiotics are useful for eradication of colonization. Judicious use of antibiotics is probably very important.

While MRSA is a concerning pathogen, it is important not to overreact. MRSA will probably never be eradicated from the equine population, and the risk of infection (for both horses and people) will continue to exist. However, basic infection control and farm management practices can reduce the risks.

About the Author

J. Scott Weese, DVM, DVSc, Dipl. ACVIM

J. Scott Weese, DVM, DVSc, Dipl. ACVIM, is an associate professor in the Department of Pathobiology at the University of Guelph's Ontario Veterinary College in Canada.

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