The objectives of this study were to establish the prevalence of R. equi isolates resistant to macrolides or rifampin and to determine the outcome of foals infected with resistant isolates. Thirty-four isolates classified as resistant to erythromycin, clarithromycin, azithromycin, or rifampin, were obtained from 6 laboratories between 1997 and 2007. For each isolate, minimum inhibitory concentration of the 4 antimicrobial agents was determined using both the E test and broth macrodilution. Each isolate confirmed to be resistant to at least one antimicrobial agent was also evaluated for presence of the virulence plasmid using PCR amplification of the vapA gene.
Only 19 of the 34 isolates (55.9%) submitted as resistant were R. equi isolates resistant to at least one drug. Two isolates were resistant to rifampin only whereas 17 isolates were resistant to all 3 macrolides and rifampin. Two of the 19 resistant isolates were avirulent. The prevalence of resistant isolates at the University of Florida was not significantly different from that of Texas A&M University. The overall prevalence of R. equi isolates resistant to macrolides or rifampin was 3.7%. The survival rate of foals infected with resistant isolates (25%) was significantly lower (P < 0.004) than that of foals from which susceptible isolates were cultured (69.6%).
In conclusion, approximately half the R. equi isolates identified as resistant to a macrolide or rifampin by diagnostic laboratories are either not R. equi, not resistant, or avirulent. Foals infected with resistant isolates are less likely to survive than foals infected with susceptible isolates.