Retrospective comparison of azithromycin, clarithromycin, and erythromycin for the treatment of foals with Rhodococcus equi pneumonia.

The objective of this retrospective study was to compare the efficacy of azithromycin-rifampin, clarithromycin-rifampin, and erythromycin-rifampin for the treatment of pneumonia caused by Rhodococcus equi in foals. Eighty-one foals with naturally acquired pneumonia caused by R. equi were included in the study. Information on age, sex, breed, physical examination findings, laboratory testing, and thoracic radiography was abstracted from each medical record. Foals were divided in 3 groups based on the antimicrobial agent selected for therapy. Short-term (discharge from the hospital) and long-term (apparently healthy as a yearling) success rates, days of hospitalization, days with fever, days with tachypnea, and percentage of radiographic improvement were compared among groups. Foals treated with clarithromycin-rifampin had significantly (P = .02) higher odds of overall short-term (odds ratio [OR] = 12.2) and long-term (OR = 20.6) treatment success and significantly fewer days with fever than foals treated with erythromycin-rifampin. Foals treated with clarithromycin-rifampin had a significantly (P = .03) higher percentage of radiographic improvement and a tendency (P = .06) toward higher odds of overall short-term (OR = 8.1) and long-term (OR = 11.8) treatment success compared to foals treated with azithromycin-rifampin. Among foals with severe radiographic lesions, the success rates of foals treated with clarithromycin-rifampin both short-term (88%) and long-term (83%) were significantly (P = .02) higher than that of foals treated with azithromycin-rifampin (0%). For each treatment group, the only reported adverse effect was diarrhea that was mild and self-limiting in most cases. The combination clarithromycin-rifampin is superior to azithromycin-rifampin or erythromycin-rifampin for the treatment of pneumonia caused by R. equi in foals in a referral population.