Emergence of Resistance to Macrolides and Rifampin in Clinical Isolates of Rhodococcus equi from Foals in Central Kentucky, 1995 to 2017

Abstract

The objective of this study was to determine the prevalence of <i>Rhodococcus equi</i> strains resistant to macrolides and rifampin over time in clinical samples from foals submitted to diagnostic laboratories in central Kentucky. We performed a retrospective observational study of all clinical samples from foals that were submitted to veterinary diagnostic laboratories in Kentucky between January 1995 and December 2017. Samples were included if the <i>R. equi</i> bacterium was cultured and tested for <i>in vitro</i> susceptibility to erythromycin or rifampin. <i>In vitro</i> susceptibility testing to erythromycin was available for 2,169 isolates of <i>R. equi</i>, while susceptibility testing to both erythromycin and rifampin was available for 1,681 isolates. Rifampin resistance was first detected in 2000, and erythromycin resistance was first detected in 2004. Between 1995 and 2006, the proportion of resistant isolates of <i>R. equi</i> was 0.7% for erythromycin and 2.3% for rifampin. There was a significant (<i>P</i> < 0.001) increase in the proportion of resistant <i>R. equi</i> between 2007 and 2017, with 13.6% of isolates being resistant to erythromycin and 16.1% being resistant to rifampin. Between 2007 and 2017, isolates of <i>R. equi</i> resistant to erythromycin or rifampin were significantly less likely to be isolated from feces than from the respiratory tract, other soft tissues, or musculoskeletal infections. The considerable increase in the prevalence of isolates of <i>R. equi</i> resistant to macrolides and rifampin since 2007 is of concern for both human and animal health.