Challenges for interpreting results of antimicrobial susceptibility testing for treating infections of foals with Rhodococcus equi

Rhodococcus equi is a facultative intracellular pathogen and a major cause of pneumonia in foals worldwide. The standard treatment for affected foals is a combination of a macrolide with rifampicin. The widespread use of this combination has led to the emergence of multidrug-resistant (MDR) strains of R. equi. Consequently, antimicrobial susceptibility testing is essential for guiding treatment of rhodococcal pneumonia. Unfortunately, clinical breakpoints for macrolides and other antimicrobials against R. equi for foals are lacking, thereby complicating clinical interpretation of in vitro results. This review summarises current knowledge of antimicrobial resistance in R. equi, the methods used for susceptibility testing and the challenges of interpreting susceptibility testing for guiding treatment of this intracellular infection. We discuss the limitations of extrapolating breakpoints from other species of bacteria and animals and emphasise the importance of integrating minimum inhibitory concentration data, knowledge of intracellular antimicrobial activity and principles of pharmacokinetics and pharmacodynamics to inform therapeutic decisions. In the absence of standardised interpretive criteria of results of susceptibility testing, we propose evidence-based guidelines to support veterinarians and diagnostic laboratories in managing R. equi infections of foals.