In the early 2000s, we took a lot of bad publicity in Ontario (particularly at the Ontario Veterinary College) because of MRSA in horses. While MRSA had been found in horses before and there were anecdotal reports of MRSA infections and outbreaks in different areas, the fact that we intensively investigated the issue and published a lot of our findings made it look like we were the hotbed of MRSA internationally. We suspected at the time that MRSA was widespread in horses and that the limited reports were because few people were looking or publishing their observations. That suspicion has been supported by reports over the past few years of MRSA in horses in many countries, and it appears that MRSA is present in horses around the world.

A recent study from Australia (Axon et al, Australian Veterinary Journal 2011) provides more support for this conclusion. In the study, horses that were admitted to a veterinary hospital’s intensive care unit over a 30 day period in 2008 were tested for MRSA carriage by culturing swabs from their noses. MRSA was isolated from 3.7% of horses, which is similar to the prevalence here based on data we’ve gathered over the years.

For the second part of the study, the authors looked at medical records from horses at the hospital from 2004-2009 and collected data on MRSA infections. During that time, MRSA was isolated from 75 horses.

  • That number (75) surprises me a little, since it’s much higher than what we see here. Even though we see approximately 2% of horses carrying MRSA when they arrive at the hospital, we have a very low MRSA infection rate in our patient population. A few of those 75 horses probably didn’t really have MRSA infections, since nine horses only had positive nasal samples which is more likely to be from subclinical colonization rather than infection of the nasal passages. A few others had MRSA isolated from catheter sites, and it’s hard to say whether those are truly infection or just contamination of the skin. So, the number of true infections might be lower, but it’s still a significant issue. It would be interesting to know how many of those horses came in with MRSA infections versus how many picked up MRSA in hospital.

Wound infections were most common, accounting for 43 (57%) of the cases. Five horses were euthanized because of the MRSA infection, all of which had joint infections that did not respond to intensive treatment.

One farm accounted for 18 MRSA-positive results in the second part of the study, as well as two positive horses in the surveillance part of the study. This farm would seem to have a pretty big MRSA problem, which we’ve seen occasionally on a few biohazardous breeding farms that we’ve found over the years.  MRSA can be controlled on farms like that but it takes effort. We’ve had some farms address the issue properly and eliminate MRSA, while others essentially ignored the problem and continued to have widespread MRSA for years. 

Not surprisingly, most of the MRSA isolates in the Australian study belonged to sequence type 8 (ST8), the group of MRSA that we find in horses here in Ontario and internationally. This is a recognized human strain that seems to have become adapted to horses. It’s also found in a disproportionately high percentage of horse owners and horse vets, likely indicating movement of teh strain between horses and people.

Overall, the results of this study are not surprising, but are very useful in that they support the notion that MRSA is present in horses around the world, and the situation with MRSA in horses is probably quite similar in many different countries. 

More information about MRSA in horses can be found in on the Worms & Germs Resources – Horses  page.