A two-part study (Maddox et al. 2011) was recently published online in the Equine Veterinary Journal looking at antimicrobial resistant "superbugs" in horses in the UK. The first part of the study had the simple objective of estimating the prevalence of both MRSA nasal colonization and fecal shedding of antimicrobial resistant E. coli in the UK horse population. The reason they were looking at E. coli is because this bacterium is part of the normal intestinal flora of most animals (including horses), so it’s easy to find, and it is frequently exposed to antimicrobials whenever a horse is treated systemically (i.e. with either oral or injectable antibiotics), so researchers use it as an "indicator" for resistance that may develop in other bacteria as well. E. coli doesn’t typically cause gastrointestinal disease in horses like it can in people, nonetheless E. coli is a common cause of uterine, urinary and wound infections in horses, as well as septicemia in foals, and antimicrobial resistant infections in these situations can certainly be a big problem.
Out of 678 nasal swabs (taken from horses on over 500 premises), only 4 (0.6%) were positive for MRSA. That’s good to see, as it’s very easy for MRSA to spread in a horse population "under the radar," because colonized horses do not have any outward signs that they are carrying the pathogen. The overall low prevalence in the community setting is similar to previous community studies in the UK and other areas. Out of 650 fecal samples, 452 (72.2%) were positive for an E. coli that was resistant to at least one antimicrobial. That’s not too surprising. The bigger concern is that 233 (37.6% – over one third of all horses tested) samples contained multidrug-resistant E.coli (defined in this study as resistance to more than three antimicrobial classes) and 42 (6.3%) samples contained an E. coli that produced an extended-spectrum beta-lactamase (ESBL). ESBLs are particularly problematic because they can be relatively easily transferred between bacteria, they confer resistance to a large number of commonly used antimicrobials in the beta-lactam class (which includes penicillins and cephalosporins), and are frequently associated with genes that confer resistance to other antimicrobial classes as well. Bacteria that produce ESBLs are a significant problem in human medicine and have been reported to cause infection in horses as well.
The second part of the study used information collected from questionnaires filled out by the horse owners to try to determine risk factors that affected the odds of a particular horse shedding antimicrobial-resistant E. coli. They used some pretty heavy-duty statistical analysis because they were looking at so many factors and different outcomes, and any time that happens you have to take the final numbers with a grain of salt. Nonetheless, the results can help point the way for future studies to help determine which factors may have the biggest impact on the risk. The authors found recent hospitalization and veterinary treatment for various conditions over the last six months were associated with higher odds of shedding multidrug-resistant strains of E. coli, and the type of farm/facility also affected the odds. In addition, having a recently hospitalized horse on the premises (among other things) increased the odds of a horse shedding ESBL E.coli.
What does all this mean for the average horse owner? The results really aren’t new or startling. We already know that antimicrobial resistance is a growing problem in equine medicine, as it is in veterinary and human medicine in general, and this is one more set of studies that provides evidence to that effect. I have no doubt that if a similar study was performed in North America the same kinds of resistant bacteria would be detected, although the numbers may vary somewhat one way or another. The second part of the study also re-emphasize the role that antimicrobial use plays in promoting development of and selection for resistant bacteria, and the potential for the treatment of certain horses to affect the microbes being carried and shed by the animals around them. In the end, it comes down to being responsible about how we use antimicrobials in order to curb the development of resistance, so that these important drugs remain effective for treating serious infections in the future. This applies equally to their use in people and animals of all kinds.