Infectious Diseases

Study Reveals Gaps in Equine Antibiotic Practices

By Paul Basilio

Antimicrobial resistance is a growing global health concern in both human and veterinary medicine. While antimicrobials are an irreplaceable tool in clinical practice, every prescription carries a risk.

There is no central entity responsible for regulating antimicrobial use, so the brunt of responsibility falls to individual practitioners through a multipronged approach of biosecurity, infection prevention, and more mindful prescription of antimicrobials.

The Role of Veterinarians
Recent studies have shown that some equine practitioners—both in ambulatory practices and in equine hospitals—are overprescribing some antimicrobial drugs, said Pamela S. Velarde, DVM, a resident in the department of Clinical Sciences at Cornell University, during a presentation at the 70th AAEP Annual Convention in Orlando. In some cases, antimicrobials that the World Health Organization has classified as High Priority and Critically Important Antimicrobials (HPCIA and CIAs) have been overused when other antimicrobials that are less important in human medicine can be effective.

“We can help to decrease this problem by decreasing our overall prescription of antimicrobials and, if we need to prescribe something, decrease the use of those that are considered the highest priority antimicrobials,” she said.

Studying Cornell
To highlight potential areas of improvement in antimicrobial stewardship, Dr. Velarde and her mentors Gillian Perkins, DVM, DACVIM and Kevin Cummings, DVM, PhD, conducted a retrospective study of systemic antimicrobial prescribing practices at the Cornell University Equine Hospital for the 2021 calendar year. The researchers used electronic health records (EHRs) to identify the most prescribed systemic (ie, oral, IM, and IV) antimicrobials, stratified by hospital service.

Overall, about 1600 equine visits were recorded, and approximately one-third of horses were given antimicrobials.

The most commonly prescribed antimicrobials were penicillins. Aminoglycosides, sulfonamides, tetracyclines, and nitroimidazoles rounded out the top 5. Fluoroquinolones, third-generation cephalosporins, and polymyxins—classified as HPCIAs in the 2024 WHO List of Medically Important Antimicrobials—were less commonly used (<15%).

The highest percentage of antimicrobials were prescribed for patients experiencing musculoskeletal issues, followed by the upper airway, the reproductive system, and the GI system. Dr. Velarde again noted that these numbers are a breakdown of the types of patients seen at the hospital, and not an indication that a disorder in one body system led clinicians to prescribe more antimicrobials over another body system.

Of all of the horses who received antimicrobials, most (up to 90%) received 3 or fewer. Some horses were outpatients and did not receive antimicrobials while hospitalized, but received an antimicrobial prescription on discharge.

Evidence for Indications
To look more closely at which horses were receiving antimicrobials and for what reason, Dr. Velarde and her team categorized the horses as having a confirmed infection, having a suspected infection, or having no evidence of infection.

“Suspected cases were those where we had clinical evidence,” she explained. “They either had a fever, purulent discharge, or open wounds with full-thickness lacerations, joint involvement, or bone exposure. They also included horses with laboratory evidence of infection, with findings such as a high cell count in body fluids and inflammatory white blood cell counts with toxic changes, left shifts, etc.”

Of the horses who were given antimicrobials, about 50% had a confirmed infection or a suspected infection—leaving the other nearly 50% with no evidence of infection.

For horses undergoing a surgical procedure, the team divided antimicrobial prescription by classification of the operative wound. They found that antimicrobials were often prescribed in operations classified as ‘clean’ or ‘clean-contaminated.’

A further look into the hospital’s practices showed that very few of the horses (<20%) that received antimicrobials during 2021 had culture and sensitivity testing performed.

“I believe that sometimes we don’t think it’s important to get a sample and submit it for culture later on, and then we don’t understand why our treatment isn’t working” Dr. Velarde said. “But if you do have the sample and you submit it, you may get results in a few days that assist with your treatment plan, especially if the patient is not improving as you expect.”

Future Research
Dr. Velarde noted that the antimicrobial prescription rate was higher in horses with confirmed or suspected infection, as well as in those that died or were euthanized in the hospital due to severe disease, but more work can be done to protect high-priority antimicrobials from overuse.

The infrequent use of culture and sensitivity testing and the use of post- or intraoperative antimicrobials in horses undergoing clean procedures offer focal points for improvement.

“We want to do better on those cases that were prescribed antimicrobials with no evidence of infection. If an antimicrobial is prescribed and it isn’t needed, that could mean multiple treatments per day over many days. This is an area where we could have a big impact,” she said. “We should also try to understand whether clean surgical procedures can be performed with no use of antimicrobials, or just when necessary.”

The hospital plans to establish an antimicrobial stewardship policy that the clinicians will use to guide their prescribing practices, especially as it relates to the use of high priority critically important antimicrobials. MeV