Antimicrobials, Lower Airway Disease, and Equine Athletes
Respiratory function—particularly lower airway disease—can limit performance in equine athletes. These horses typically face more lung-health challenges than pleasure horses, including different environments and climates at competitions vs. at home, traveling long distances regularly, and an increased risk of disease exposure that comes with commingling with horses from afar.

Veterinarians are tasked with treating lower airway diseases that arise to keep horses performing at their best. One tool they have is antimicrobials, which can be effective but also come with their own sets of challenges.

“We’re all aware that our professional duty is to reduce antimicrobial use where we can,” said Natasha Jocelyn, MA, VetMB, MVetMed, Dipl. ECEIM, MRCVS, during a presentation at the 2021 British Equine Veterinary Association Congress, held Sept. 5-7 in Birmingham, U.K. “We’re all aware of the risk of antimicrobial resistance.” Infections make using antimicrobials an easier choice.

For bacterial pneumonia or pleuropneumonia, for instance, it’s well understood that broad-spectrum antimicrobials (such as penicillin, gentamicin, and metronidazole) should be on board sooner rather than later, said Jocelyn, a medicine specialist at Sussex Equine Hospital in West Sussex, U.K.

And secondary bacterial infections can develop in horses with parasitic pneumonitis (specifically infection with Parascaris equorum or Dictyocaulus arnfieldi). While the parasites should be treated with (dewormers), concurrent antimicrobial treatment is key if veterinarians identify bacteria during diagnostics, she said.

The choice to use antimicrobials is less clear in other cases.

For equine multinodular pulmonary fibrosis—a type of pneumonia that’s been linked to equine herpesvirus (EHV)-2 and -5 but not to a bacterial component—doxycycline’s anti-inflammatory properties can help some horses battle this serious condition. She encouraged practitioners to consider antimicrobial-resistance risk and use it on a case-by-case basis.

Equine asthma is particularly challenging to treat because not all affected horses benefit from antimicrobials. The medications are helpful in some cases, however. For instance, researchers have identified an association between the presence of mucus and/or neutrophilia (an abnormal increase in circulating white blood cells called neutrophils), mild to moderate equine asthma, and positive bacterial cultures in young racing Thoroughbreds, Jocelyn said. They haven’t identified that association in other equine populations, so whether to reach for antimicrobials might depend on the type of horse you’re treating, she said.

Further, two recent studies found conflicting evidence on whether the antibiotic ceftiofur improved clinical scores in horses with both severe equine asthma and a positive tracheal wash bacterial culture, Jocelyn said. In the study, which supported ceftiofur use and showed a significant clinical improvement with treatment, the population of client-owned horses also received the usual treatments for severe equine asthma: clenbuterol, dexamethasone, and a modified environment. In the conflicting study’s population of research horses with induced severe asthma, ceftiofur had no impact on clinical score.

So what do we do?

“The decision needs to be based on getting a good diagnosis,” Jocelyn said.

A clinical exam is key, ultrasonography can be extremely useful, and lower respiratory tract sampling is crucial, she said. Culture can help identify the most effective drugs against the bacteria you’re battling, and cytology (examining cells under a microscope) can guide early antimicrobial choices before culture is completed.

“Undeniably, the athletic horse faces challenges which may compromise respiratory defenses and lead to greater risk of infection, so it’s more likely to have a bacterial component compared to other classes of horses,” Jocelyn said. “Antimicrobial use should be undertaken once you’ve done appropriate diagnostic testing. Weigh the evidence in light of the clinical picture and try as best as you can to use responsible choices.”