In a controlled , randomized , and double blinded clinical trial , foals with ultrasonographic evidence of pulmonary abscesses ( abscess score 8.0-20 cm ) were randomly allocated in 3 treatment groups : ( 1 ) gamithromycin IM q7 days ( n = 40 ) ; ( 2 ) azithromycin with rifampin , PO q24h ( n = 40 ) ; or ( 3 ) no antimicrobial treatment ( controls ; n = 41 ) .