Clinical Report: Gamithromycin Treatment for Rhodococcus equi Pneumonia in Foals

Introduction
Rhodococcus equi (R. equi) is a common cause of pneumonia in foals between 3 weeks and 5 months of age. Macrolides in combination with rifampicin have been used to treat this disease over the past 20 years. Several different macrolides have been evalu- ated in foals for pharmacokinetic properties and/or efficacy, including erythromycin, azithromycin, clarithromycin, tilmicosin, tulathromycin, tulathromycin and gamithromy- cin. the combination of erythromycin, azithromycin, or clarithromycin with rifampicin is currently the recommended treatment of choice. Gamithromycin, a relatively new azal- ide developed for the treatment and prevention of bovine respiratory disease caused byMannheimia haemolytica, Pasteurella multocida, Histophilus somni and Mycoplasma bovis, has not been studied in foals in vivo previously.

Materials and methods
In this clinical report we share clinical results (see tables 1 and 2) achieved with gamith- romycin in foals with R. equi over the last three years. the foals were either admitted to Wolvega Equine Hospital or treated as outdoor-patients by this clinic. the diagnostic cri- teria on which the diagnosis R. equi pneumonia was made were clinical signs of lower respiratory tract disease and ultrasonographic evidence of pulmonary abscesses. for practical reasons pCr swabs and cultures were only taken from three foals. foals were between 2 weeks and 2.5 months old and received from 1 to 6 injections of gamithro- mycin (Zactran® 6 mg/kg iM every week) with or without rifampicin (rifadin® 5 mg/kg BiD po for 4-6 weeks). all foals made a full recovery. intramuscular injections were administered in the m. semimembranosus/semitendinosus, alternating left and right side. Stiffness and a painful injection site for one or two days were the only adverse reactions noticed and no permanent injection site lesions were recorded.

Results and conclusions
Although the numbers are small, there was no difference in outcome and time to clini- cal recovery between the foals that were on co-medication with rifampin and those that were not. further work needs to be done to evaluate if the efficacy of gamithromycin alone is the same as with co-medication with rifampicin. advantages of gamithromycin treatment include that it allows for a once a week administration only, compared, for example, to twice daily oral medication with clarithromycin, and there is no risk of acci- dental enterocolitis in the accompanying mare. Based on this clinical report it seems that gamithromycin is not only effective in vitro, but also shows clinical efficacy in vivo. [...]