Failure of antimicrobial therapy to accelerate spontaneous healing of subclinical pulmonary abscesses on a farm with endemic infections caused by Rhodococcus equi.

Mass antimicrobial treatment of foals with small ultrasonographic pulmonary lesions is common on farms with endemic disease caused by Rhodococcus equi. The objectives of this study were to compare the relative efficacy of three antimicrobial protocols for the treatment of pulmonary abscesses on a farm with endemic infections caused by R. equi and to determine the frequency of spontaneous resolution of subclinical pulmonary lesions. Foals with ultrasonographic evidence of pulmonary abscesses ≥ 1.0 cm in diameter (n=128) were randomly allocated to one of four equal treatment groups. Animals with respiratory distress or severe pulmonary lesions were excluded from the study. Treatment groups consisted of: (1) tulathromycin intramuscularly; (2) azithromycin monotherapy orally; (3) azithromycin in combination with rifampin orally; and (4) glucose orally as a placebo. Physical examination, thoracic ultrasonography and white blood cell (WBC) counts were performed weekly by individuals unaware of treatment group assignment. Foals that worsened were removed from the study and treated with azithromycin and rifampin. Overall, 14/32 (43.8%) foals in the placebo group recovered without the need for therapy. The proportion of foals that responded to the initial therapy, the duration of therapy, the kinetics of ultrasonographic lesion resolution, the proportion of foals that had to be removed from the study and the number of days to removal from the study did not differ significantly between treatment groups. The study showed that many foals with pulmonary abscesses recover without antimicrobial therapy. Moreover, treatment of sub-clinically affected foals with antimicrobial agents did not significantly hasten recovery.