Macrolide-induced hyperthermia in foals: Role of impaired sweat responses.

Abstract

REASONS FOR PERFORMING STUDY: The mechanism of hyperthermia, a potentially fatal adverse effect of erythromycin treatment of foals, is unknown. OBJECTIVES: To determine the cause of erythromycin-associated hyperthermia. It was hypothesised that the normal sweat response of foals is impaired by treatment with erythromycin. STUDY DESIGN: Blinded, crossover study in 10 healthy pony foals. METHODS: Foals kept in stalls were given either erythromycin (25&#xa0;mg/kg bwt orally, 3 times daily) or control for 10&#xa0;days then turned out for a further 10&#xa0;days. Quantitative intradermal terbutaline sweat tests were performed on Days 1 (baseline), 3, 10 and 20. The effects on terbutaline-induced sweating of erythromycin, terbutaline concentration and treatment day were analysed by repeated-measures ANOVA with Bonferroni-corrected pairwise post hoc comparisons. Peak temperatures were compared by Wilcoxon's signed rank test and proportions by McNemar's related samples test. Significance was set at P<0.05. RESULTS: There were significant 2-factor interactions for treatment &#xd7; terbutaline after baseline, treatment &#xd7; day at every terbutaline concentration, and day &#xd7; terbutaline for erythromycin (P<0.001) but not control (P&#xa0;=&#xa0;0.9) treatment. Sweating was significantly reduced from baseline in erythromycin-treated foals at all subsequent days. Erythromycin-treated foals produced less sweat at all time-points than did control-treated foals (P<0.05). Peak rectal temperatures of erythromycin-treated foals were significantly higher (P&#xa0;=&#xa0;0.02) than those of controls. During the first 3&#xa0;days outside more erythromycin-treated than control-treated foals required treatment for hyperthermia (6 vs. 0; P&#xa0;=&#xa0;0.03). CONCLUSIONS: We believe drug-induced anhidrosis is the likely cause of hyperthermia in some foals treated with erythromycin.