Antibiotic Resistance in Horses: Is There a Problem?

While parasite resistance is currently a hot topic in the equine community, another form of resistance is taking its toll on horses worldwide: antimicrobial resistance. According to Imogen Johns, BVSc, MRCVS, Dipl. ACVIM, lecturer in equine medicine at the United Kingdom's Royal Veterinary College in Hertfordshire, the amount of bacteria that has developed resistance to antibiotics since the drugs become commonplace in the 1930s and '40s is "sobering." She presented on the topic at the American College of Veterinary Internal Medicine Forum, held June 15-18 in Denver, Colo.

At the time of their inception into society, antibiotics were considered miracle drugs, contributing to an increased human life expectancy by eight years. But Johns pointed out that antimicrobial resistance (AMR) began to occur within a relatively short time of widespread antibiotic use. This wasn't originally a problem since 29 new drug classes were developed between 1929 and 1969; however, since then, lack of discovery of new drugs poses a sobering contrast to the rapid and rising resistance of bacteria.

Bacteria can pass resistance genes on to future generations, as well as acquire resistance to multiple drugs--even those to which the bacteria have not been directly exposed. The result of this is the development of "super bugs." Acquired antimicrobial resistance is now a serious public health concern that needs to be addressed, Johns noted.

The link between AMR in humans and animals tends to focus on food-producing animals medicated for therapeutic reasons or for growth promotion. These animals can potentially transfer antibiotic-resistant bacteria via the food chain (i.e., the humans that consume the animals could be consuming resistant bacteria). If authorities implement antibiotic restrictions for all veterinarians (which is a future possibility) and not just those ministering to food-producing animals, the implications could mean changes in how we manage equine health in the future.

In addition, Johns discussed the role of antibiotic resistance development in commensal bacteria (those that live in harmony and synergy throughout the body), as antibiotics don't just target pathogenic (disease-causing) strains. Commensal bacteria also have the ability to form a multitude of resistant genes, and once resistant genes have been formed, the commensal bacteria can transfer the genetic information to other commensal forms as well as pathogens, ultimately adding to the antibiotic resistance problem.

For example, some forms of the bacterium E. coli aren't pathogenic and are actually considered commensals. Johns discussed several studies in which horses with low proportions of resistant E. coli commensal organisms at the time of equine hospital admission had developed more resistant bacteria only six days later. As the duration of a hospital stay persists, there is a significantly increased risk for these organisms to develop AMR. This risk is amplified if the horse receives antimicrobial treatment, particularly if given multiple antibiotics.

Another study Johns described reviewed the persistence of multiple drug resistance over time after horses were discharged from a hospital. By the end of two months the E. coli organisms returned to their pre-treatment levels of antibiotic susceptibility.

Johns stressed to veterinarians and horse owners that antimicrobial drugs should be used with caution and only in cases with an appropriate need for treatment. She added that bacterial cultures and sensitivity testing can help veterinarians choose very specific antibiotics that target the problem, which will likely achieve the best clinical outcome. Applying these strategies, she noted, along with continued AMR surveillance is critical to the future health of both human and animal species.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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