Equine Rhodococcosis management: Comparative evaluation of antimicrobial combination regimens and treatment outcomes in Foals

Abstract:

While antimicrobial therapy constitutes the cornerstone of equine rhodococcosis management, the selection of optimal combination regimens remains empirical in many veterinary practices owing to the limited availability of comparative efficacy data from controlled clinical investigations. This research conducted a prospective comparative evaluation of six antimicrobial combination regimens for the treatment of Rhodococcus equi pneumonia in 144 thoroughbred foals at three breeding facilities in Buenos Aires Province, Argentina, between January 2020 and December 2023. Foals aged 4-12 weeks with confirmed R. equi pneumonia were randomly allocated to one of six combination regimens: Azithromycin-Rifampicin (AZM-RIF, n = 28), Erythromycin-Rifampicin (ERY-RIF, n = 24), Tulathromycin-Rifampicin (TUL-RIF, n = 22), Clarithromycin-Rifampicin (CLR-RIF, n = 25), Doxycycline-Rifampicin (DOX-RIF, n = 22), and Gentamicin-Rifampicin (GEN-RIF, n=23). Primary outcome measures included clinical cure rate at 60 days, bacteriological clearance, survival rate, and adverse effects. AZM-RIF demonstrated the highest clinical cure rate (88.3%) and survival rate (92.1%), while TUL-RIF showed the lowest adverse effect rate (12.4%). The heatmap analysis across five treatment parameters confirmed AZM-RIF as the overall best-performing regimen. Age-stratified analysis revealed that foals aged 4-8 weeks experienced the highest infection severity and longest treatment duration across all regimens. These findings provide evidence-based guidance for antimicrobial selection in equine rhodococcosis management and identify TUL-RIF as a promising alternative with comparable efficacy and improved tolerability.