Antimicrobial Use in Horses: Double-Edged Sword

Veterinarians prescribe antibiotic drugs to horses faced with infections that cannot be effectively managed otherwise. Antibiotics come with a price, such as adverse side effects and long-term resistance.

“When antimicrobial drugs are administered to horses for any reason, they can potentially alter the intestinal microbiome, resulting in antibiotic-associated diarrhea. In turn, any horse with diarrhea, regardless of the inciting cause, is at risk for sepsis and laminitis,” explained Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research.

Because it may not be possible to avoid antibiotic administration altogether, steps should be taken to protect the microbiota. According to Crandell, these steps include maintaining the horse on a diet high in forage and providing a hindgut buffer to maintain a steady, healthy pH in the cecum.

“Make sure your horse has abundant good-quality forage available. A variety of roughage types, such as grass hay, alfalfa hay or pellets, beet pulp, or soy hulls, will encourage a healthy diversity of beneficial bacteria in the hindgut,” Crandell added.

Another major concern currently plaguing the medical industry concerns growing resistance to all classes of antimicrobial drugs. Overuse of antibiotics, especially prescribing unnecessary antibiotics, serves as a primary reason for this resistance. As recently highlighted in an article published in the British Journal of Nursing, “There is a dearth of new antibiotics and, if nothing is done to restrict the use of those that remain effective, there is a risk of returning to the pre-antibiotic era where simple infections could result in death.”^*

Multiple health agencies have a clear stance on antimicrobial stewardship for “promoting and monitoring judicious use of antimicrobials” to preserve effectiveness. Those organizations clearly state that stewardship involves prescribing the right antibiotic for the right patient at the right time with the right dose and the right route, causing the least harm to the patient and future patients.^*

Veterinarians from the New Bolton Center at the University of Pennsylvania recently relayed their concerns about overuse of antibiotics in a retrospective study.^** Patterns of antibiotic use in the equine industry were evaluated by reviewing medical records from nearly 40,000 patient visits between 2013 and 2018. Of those, 8.5% of visits ended in antibiotic prescription. Antibiotics were given most commonly for ocular, skin, and respiratory conditions, with 56%, 41%, and 24% of patients prescribed antibiotics, respectively.

For nonemergency veterinary examinations, 8% of cases were prescribed antibiotics. This number, however, skyrocketed to 30% for horses examined on an emergency basis. When an antimicrobial was dispensed, a culture was submitted in only 3% of the visits.

Other findings of the retrospective study included:

• Aminoglycosides (e.g., amikacin) were most frequently prescribed, followed by sulfonamides (e.g., trimethoprim sulfamethoxazole or TMZ) and tetracyclines;
• Antimicrobials classified as “high priority critically important antimicrobials,” such as systemic ceftiofur and topical polymyxin B, made up 26% of the total antibiotics dispensed; and
• In most cases (90%), only one antibiotic was prescribed, but up to five were given on rare occasions.

These data improve understanding of antibiotic use and prescribing patterns in the equine industry. Additional study in this field will help promote antimicrobial stewardship, which is essential for preventing further increases in antibiotic resistance, such as methicillin-resistant Staphylococcus aureus (MRSA) and others.

^*Edwards-Jones, V. 2020. Antimicrobial stewardship in wound care. British Journal of Nursing. 29(15):S10-S16.

^**Rule, E.K., A.G. Boyle, and L.E. Redding. 2021. Antimicrobial prescribing patterns in equine ambulatory practice. Preventative Veterinary Medicine 193:105411.