More On MRSA In Horses
Methicillin-resistant Staphylococcus aureus (MRSA) is an antibiotic-resistant bacterium and a major concern in both human and veterinary medicine. This bacterium can be carried by healthy horses and is an opportunistic pathogen, meaning it can cause infection in certain situations, particularly when the body's normal defensive barriers (like the skin) are breached (e.g. by surgery, wounds, compromised immune system).
We've done several surveillance studies across North America and we consistently find MRSA in a small but not insignificant percentage of horses. In most studies, we see it carried in the nasal passages of 1-5% of horses. A couple of recent studies provide more evidence that this bug has spread widely in horses internationally.
In a retrospective study of bacterial infections in horses at the University Equine Clinic of Bern, Switzerland (Panchaud et al, Schweiz Arch Tierheilkd 2010), MRSA was on the most important bacterium isolated from affected horses. Additionally, screening of horses at admission to the hospital revealed that 2.2% were carriers.
In a retrospective study of samples submitted for bacterial culture from horses in Ireland (Abbott et al, Vet Rec 2010), MRSA accounted for 5.2% (20/383) of isolated bacteria, which was higher than the percentage of MRSA in samples from dogs (1.1%) and cats (0.7%). In the prospective part of the study, MRSA was isolated from only 4.6% (3/65) of infections where clinicians suspected that MRSA was the cause. Its interesting that it was isolated less commonly in cases where people thought MRSA might be the problem than overall. In contrast, while MRSA was only found in 1.1% of culture samples from dogs, it was found in 8.1% of infections where clinicians suspected an MRSA infection. It's not clear why there was such a difference between horses and dogs. It doesn't mean the equine clinicians are clueless, it just supports the notion that there are few clear indicators of that an infection is being caused by MRSA. MRSA infections occur in horses without any obvious risk factors, which complicates diagnosis and management of the infection.
MRSA must be considered in any situation when a horses has an infection. There's evidence that it's more common in horses that have been recently treated with antibiotics or in a veterinary hospital, but the link is not particularly strong and MRSA infections do occur in horses with no identifiable risk factors. That's why culturing infected sites - rather than just trying different antibiotics and seeing what happens - is critical. Money and effort spent culturing infected sites is a good investment.
