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

The objective of this study was to determine the prevalence of Rhodococcus equi 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 R. equi bacterium was cultured and tested for in vitro susceptibility to erythromycin or rifampin. In vitro susceptibility testing to erythromycin was available for 2,169 isolates of R. equi, 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 R. equi was 0.7% for erythromycin and 2.3% for rifampin. There was a significant (P < 0.001) increase in the proportion of resistant R. equi 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 R. equi 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 R. equi resistant to macrolides and rifampin since 2007 is of concern for both human and animal health.