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The Effect of Antibiotics on a Foal's Ability to Sweat.

A group of researchers from the University of Florida looked at the effect of treating foals with a specific antibiotic on their ability to sweat. Erythromycin, an antibiotic that belongs to a group of antibiotics called macrolides, is often used in conjunction with another antibiotic in the treatment of pneumonia caused by a bacteria called Rhodococcus equi.

Ten pony foals were kept in stalls and given either erythromycin or a placebo for ten days, then turned out for ten days. Rectal temperatures were also taken and recorded. Terbutaline tests (the intradermal skin test performed to assess the ability to sweat) were done on the foals on days 1 (baseline), 3, 10, and 20 and compared. Results showed that “sweating was significantly reduced from the baseline levels in the erythromycin-treated foals at all subsequent days. Erythromycin-treated foals produced less sweat at all time points compared to control-treated foals” (placebo group). Peak rectal temperatures of the treated foals were significantly higher than those of the untreated (placebo) group. The researchers concluded that “drug-induced anhidrosis is the likely cause of hyperthermia in some foals treated with erythromycin”.

Friesian foals are already at risk for issues of heat intolerance by the very nature of their color and hair coat. It is important to keep in mind this potential side effect of treating the Friesian foal with erythromycin in the face of Rhodococcus pneumonia. Use of this antibiotic may be unavoidable, but extra care to keep the foal cool and comfortable during treatment can help avoid hyperthermia during our hot summer months. It might also be worth exploring the use of a different antibiotic that can treat the foal successfully without this concerning side effect.

Scientific Article: Stieler, A.L. et al “Macrolide-induced hyperthermia in foals: Role of impaired sweat responses”. EVJ accepted and abstract published on line, doi:10.1111/evj. 12481.

Companion article: Larson, E., “Macrolide Antibiotics’ Effect on Sweating in Foals Examined”,, July 10, 2015.


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