BackgroundGamithromycin is active in vitro against the bacterial agents most commonly associated with bronchopneumonia in older foals. However, the clinical efficacy and safety of this drug have not been investigated.HypothesisGamithromycin is effective for the treatment of bronchopneumonia in foals.AnimalsOne hundred and twenty‐one foals on a farm endemic for infections caused by Rhodococcus equi.MethodsIn 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). Physical examination and thoracic ultrasonography were performed by individuals unaware of treatment group assignment. Foals that worsened were removed from the study.ResultsThe proportion of foals that recovered without the need to be removed from the study was significantly higher for foals treated with gamithromycin (38 of 40) or azithromycin with rifampin (39 of 40) than for controls (32 of 41). Treatment with gamithromycin or with azithromycin‐rifampin resulted in a significantly faster decrease in the clinical score and abscess score compared to the controls. Adverse reactions characterized by colic (n = 18) and hind limb lameness (n = 14) were observed only in foals treated with gamithromycin.Conclusion and Clinical ImportanceGamithromycin was noninferior to azithromycin with rifampin for the treatment of bronchopneumonia in the study population but had a higher frequency of adverse reactions.

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Summary: The aim of this study was to compare the frequency of adverse reactions and efficacy of i.v. administration of gamithromycin compared to i.m. gamithromycin for the treatment of foal bronchopneumonia on a farm endemic for infections caused by Rhodococcus equi. 60 foals with bronchopneumonia and treated with gamithromycin were retrospectively divided into two treatment groups. In the foals of group 1 (n = 40) gamithromycin (6 mg/kg body weight, Zactran ® , Merial) was injected once weekly for six weeks in the semitendinosus or the semimembranosus muscle. In the foals of group 2 (n = 20) gamithromycin (6 mg/kg body weight, Zactran ® ) was injected intravenously once weekly for six weeks in the jugular veins. The animals in group 2 were also treated with rifampin (10 mg/kg, q 24 h, p.o.). Local and systemic adverse effects were evaluated daily in all foals. Foals whose pneumonia worsened were removed from the study. In the group of foals treated with gamithromycin i.m. 38 of 40 (95 %) recovered without the need to be removed from the study compared to 16 of the 20 (80 %) foals treated with gamithromycin i.v.. The frequency of adverse drug reactions differed significantly between administration routes. In the group of foals treated with Zactran ® i.m. 23 of 40 foals (58 %) showed adverse effects. 18 foals (45 %) showed signs of colic that required treatment with analgesics, and 14 (35 %) developed a marked lameness on the side of the i.m. injection. In the group of the foals who got the Zactran ® i.v. no adverse effects were observed. From these results, it can be concluded that i.v. administration of Zactran ® induces significantly fewer adverse effects than i.m. administration. However, additional studies will be necessary to determine the optimal dose and dosing interval for i.v. gamithromycin that will result in similar efficacy as that achieved after i.m. administration.