Macrolide Antibiotics’ Effects on Sweating in Foals Examined

Researcher found that three commonly used macrolides suppressed foals’ normal sweat responses to varying degrees.
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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).

The team determined that after foals received any of the three macrolides, their sweat was significantly suppressed compared to the baseline measurements. Specifically, the team found that sweating suppression by erythromycin was significantly greater that that caused by clarithromycin or azithromycin, and there was no difference between the sweat suppression caused by clarithromycin or azithromycin

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Erica Larson, former news editor for The Horse, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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