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Veterinarians typically fight Rhodococcus equi—one of the most common causes of pneumonia in foals—with two antibiotics, a drug called rifampin combined with a macrolide. While this combination is often effective, researchers from the University of Florida (UF) College of Veterinary Medicine previously showed that one macrolide (erythromycin) can cause anhidrosis—a reduced or absent ability to sweat—and subsequent hyperthermia (body temperature greater than 103.5$deg;F) in foals. They weren’t sure whether all macrolides had the same impact.

With the Grayson Jockey Club Research Foundation’s support, Amy Stieler, DVM, and colleagues from UF tested three macrolides’ impact on foals’ sweating abilities. Stieler, a resident in large animal medicine at UF, shared the results of two studies on the topic at the 2015 American College of Veterinary Internal Medicine Forum, held June 4-6 in Indianapolis, Indiana.

First, Stieler and colleagues employed 12 foals (eight colts and four fillies) and treated each with three macrolides—azithromycin, clarithromycin, and erythromycin—at the label dose for five days. For three days before macrolide administration began and on Days 1, 2, 5, 9, 25, and 39 after treatment, the team evaluated the foals’ perspiration using a quantitative intradermal terbutaline sweat test (which involves injecting terbutaline—a beta agonist similar to epinephrine known to cause horses to sweat—into the skin and seeing whether the horse produces sweat).