Antimicrobial susceptibility patterns of Rhodococcus equi from necropsied foals with rhodococcosis

Mainstay therapy for rhodococcosis in foals is the combination of rifampicin and a macrolide. While emergence of resistance to rifampicin and macrolides has been reported, studies demonstrating the development of resistance to such drugs is limited in necropsied foals with rhodococcosis. In this study, the foal necropsy records between 01/01/2011 and 08/30/2019 were reviewed for culture-positive R. equi with MICs and, whether or not the affected foals received any mainstay dual therapy before their deaths. Resistance to antimicrobials in the R. equi isolates from necropsied foals were then compared between treated foals with dual therapy and untreated foals to determine the association between the administration of antimicrobials and development of the drug resistance. In a total of 256 R. equi isolates from each of the 256 necropsied foals with rhodococcosis, rifampicin, azithromycin, clarithromycin and erythromycin showed high rates of resistance, 22.65 %, 16.01 %, 14.84 % and 15.23 %, respectively. The most active antimicrobials exhibiting MIC_50/90 values were imipenem, doxycycline, amikacin and gentamicin including in the rifampicin- and macrolides-resistant R. equi isolates. Based on the treatment histories available for the 114 necropsied foals with rhodococcosis, R. equi isolates resistant to rifampicin, and macrolides were significantly more isolated from treated foals with mainstay dual therapy compared to untreated foals. Despite dual therapy, development of resistance against rifampicin and macrolides warrants evaluation of new treatment protocols in foals.