Treatment with gamithromycin in foals with pneumonia: comparative efficacy and adverse effects of i.m. versus i.v. administration

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 (R), 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 (R)) 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 (R) 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 (R) i.v. no adverse effects were observed. From these results, it can be concluded that i.v. administration of Zactran (R) 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.