• Open Access

Efficacy of Mass Antimicrobial Treatment of Foals with Subclinical Pulmonary Abscesses Associated with Rhodococcus equi


Corresponding author: Steeve Giguère, DVM, PhD, DACVIM, Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, 501 DW Brooks Drive, Athens, GA 30602; e-mail: gigueres@uga.edu.



Mass antimicrobial treatment of foals with small subclinical ultrasonographic pulmonary lesions is empirical practice on many farms with endemic disease caused by Rhodococcus equi.


Mass antimicrobial treatment of foals with subclinical ultrasonographic pulmonary lesions is unnecessary.


One hundred and eight foals on a farm endemic for infections caused by R. equi.


Controlled, randomized, and double-blinded prospective study. Foals with ultrasonographic evidence of pulmonary abscesses 5.0–10 cm in diameter (n = 108) were randomly allocated in 5 treatment groups: (1) tulathromycin IM; (2) doxycycline monotherapy PO; (3) doxycycline with rifampin PO; (4) azithromycin with rifampin PO, and (5) saline IM as a placebo. Physical examination and thoracic ultrasonography were performed by individuals unaware of treatment group assignment. Foals with evidence of disease progression were removed from the study and treated with azithromycin-rifampin.


Overall, 22/25 (88%) foals in the placebo group recovered without the need for treatment. The proportion of foals that had evidence of disease progression did not differ significantly between the treatment groups (> .05). Although the median duration of treatment was significantly (< .05) shorter in foals treated with azithromycin-rifampin (46 days) compared with foals treated with the placebo (73 days), the time frame of ultrasonographic lesion resolution did not differ significantly between the treatment groups.

Conclusions and Clinical Importance

The majority of foals with subclinical pulmonary abscesses <10 cm in diameter recover without antimicrobial treatment and treatment of affected foals does not provide a clear benefit over administration of a placebo.