Effects of clarithromycin, azithromycin and rifampicin on terbutaline-induced sweating in foals.

Background: Erythromycin (ERY) induces anhidrosis in foals. Azithromycin (AZI) and clarithromycin (CLA), often combined with rifampicin (RIF), are commonly used to treat Rhodococcus equi infections, but effects on sweating have not been investigated.

Objective: To determine the effects of AZI, CLA and RIF on sweat responses in normal foals.

Study Design: Each experiment was a blinded, duplicated, six foal × three period counterbalanced within subjects design (12 foals/experiment).

Methods: Antimicrobials were given orally for 5 days. In Experiment 1, ERY, AZI and CLA were given. In Experiment 2, ERY, RIF and ERY/RIF combination were used. Quantitative intradermal terbutaline sweat tests were performed daily for 3 days before and 1, 2, 5, 9, 24, and 39 days after treatment. Data were analysed by repeated measures analysis of variance procedures. Significance was P≤0.05.

Results: In Experiment 1, all macrolides suppressed sweating although CLA and AZI were less potent than ERY. In Experiment 2, significant sweat suppression occurred in foals given ERY with or without RIF, but there was no effect of RIF alone. Rifampicin reduced sweat suppression by ERY on Day 1 of treatment but not thereafter.

Main Limitations: Because ERY blood concentrations were not measured, effects of RIF on ERY-induced anhidrosis could not definitively be ascribed to altered ERY bioavailability.

Conclusions: All macrolides commonly used to treat R. equi pneumonia, i.e. ERY, AZI and CLA, induce anhidrosis in foals. The potent anti-sudorific effect of ERY is delayed, but not substantially affected by concurrent RIF administration.