Chemoprophylactic effects of azithromycin against Rhodococcus equi-induced pneumonia among foals at equine breeding farms with endemic infections.
OBJECTIVE
To determine the effect of azithromycin chemoprophylaxis on the cumulative incidence of pneumonia caused by Rhodococcus equi, age at onset of pneumonia, and minimum inhibitory concentration (MIC) of azithromycin for R equi isolates cultured from fecal and clinical samples.


METHODS
Controlled, randomized clinical trial.


METHODS
338 foals born and raised at 10 equine breeding farms; each farm had a history of endemic R equi infections.


METHODS
Group 1 foals were control foals, and group 2 foals were treated with azithromycin (10 mg/kg [4.5 mg/lb], PO, q 48 h) during the first 2 weeks after birth. Foals were monitored for development of pneumonia attributable to R equi infection and for adverse effects of azithromycin. Isolates of R equi were tested for susceptibility to azithromycin.


RESULTS
The proportion of R equi-affected foals was significantly higher for control foals (20.8%) than for azithromycin-treated foals (5.3%). Adverse effects of azithromycin treatment were not detected, and there were no significant differences between groups for the MICs of azithromycin for R equi isolates cultured from fecal or clinical samples.


CONCLUSIONS
Azithromycin chemoprophylaxis effectively reduced the cumulative incidence of pneumonia attributable to R equi among foals at breeding farms with endemic R equi infections. There was no evidence of resistance to azithromycin. Nonetheless, caution must be used because it is possible that resistance could develop with widespread use of azithromycin as a preventative treatment. Further investigation is needed before azithromycin chemoprophylaxis can be recommended for control of R equi infections.
Chaffin
M Keith
MK
Equine Infectious Disease Laboratory, Department of Large Animal Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843-4475, USA.
Cohen
Noah D
ND
Martens
Ronald J
RJ
D000193:Actinomycetales Infections / Q000453:epidemiology / Q000517:prevention & control / Q000662:veterinary*; D000818:Animals; D000831:Animals, Newborn; D000900:Anti-Bacterial Agents / Q000627:therapeutic use*; D019072:Antibiotic Prophylaxis / Q000662:veterinary*; D017963:Azithromycin / Q000627:therapeutic use*; D004305:Dose-Response Relationship, Drug; D024881:Drug Resistance, Bacterial; D005260:Female; D006734:Horse Diseases / Q000453:epidemiology / Q000517:prevention & control*; D006736:Horses; D008297:Male; D008826:Microbial Sensitivity Tests / Q000662:veterinary; D018410:Pneumonia, Bacterial / Q000453:epidemiology / Q000517:prevention & control / Q000662:veterinary; D016929:Rhodococcus equi / Q000187:drug effects / Q000472:pathogenicity*; D016896:Treatment Outcome
D000900:Anti-Bacterial Agents; D017963:Azithromycin