Chemoprophylactic Effects of Azithromycin against Rhodococcus equi Pneumonia among Foals at Endemic Equine Breeding Farms

Azithromycin, when administered orally at 10 mg/kg, q 48 h, to foals for the first 2 wk of life, is effective at reducing the incidence of Rhodococcus equi pneumonia among foals at endemic equine breeding farms.

1. Introduction

Rhodococcus equi causes severe pneumonia in foals worldwide. Most affected foals likely become infected very early in life. A novel strategy for prevention of R. equi pneumonia is the administration of effective antimicrobials to foals during the first few days of life. The objective of this study was to determine the effect of azithromycin (AZ) chemoprophylaxis on the incidence of R. equi pneumonia among foals at R. equi-endemic equine breeding farms.

2. Materials and Methods

A controlled, randomized, clinical trial was performed at 10 equine breeding farms with history of endemic R. equi infections. Group 1 foals were untreated controls, and group 2 foals were treated with AZ [a] (10 mg/kg, PO, q 48 h) during the first 2 wk of life. Enrolled foals were monitored for development of R. equi pneumonia. Fecal and tracheobronchial isolates of R. equi were tested for susceptibility to AZ. Data were compared between treatment groups.

3. Results

There were 338 foals studied. The proportion of R. equi-affected foals was significantly higher for control foals (21%) than for AZ-treated foals (5%). The estimated protective efficacy of AZ chemoprophylaxis was >85%. Neither adverse effects of AZ therapy nor AZ-resistant isolates were observed.

4. Discussion

The results of this study showed that AZ chemoprophylaxis effectively reduces the prevalence of R. equi pneumonia among foals at endemic breeding farms. Adverse effects of AZ chemoprophylaxis were not detected. The investigators failed to identify AZ-resistant R. equi in this limited study. Nonetheless, antimicrobial resistance could develop with indiscriminant widespread use of AZ as a preventative therapy. Further study is needed to identify methods for control of R. equi infections.

This study was supported by the Morris Animal Foundation and the Link Equine Research Endowment, Texas A&M University. The authors thank Dr. Jeane Mounce for technical assistance and the participating veterinarians and equine breeding farms who contributed labor, effort, interest, and time toward completion of this study.

Footnote

[a] Zithromax, azithromycin for oral suspension, 1-g packets, Pfizer Laboratories, New York, NY 10017.