Study links erm(46) copy number to R. equi macrolide resistance

Version 2 — Full analysis

A new JAVMA study adds an important layer to the antimicrobial resistance story in Rhodococcus equi: the number of erm(46) copies appears to influence macrolide resistance, not just the gene’s presence or absence. The work also describes a multiplex real-time PCR assay designed to detect R. equi, the virulence marker vapA, and the macrolide-resistance determinants erm(46) and erm(51) in a single test, then applies that approach to 229 clinical isolates collected from 14 U.S. states between 2021 and 2023. (pmc.ncbi.nlm.nih.gov)

That finding lands in a field that has been tracking R. equi resistance for years. Macrolide-rifampin therapy has long been a mainstay for foal rhodococcosis, but broad use, including treatment of subclinical lesions on some endemic farms, has been linked to the emergence of multidrug-resistant strains. Earlier studies identified erm(46) as the core macrolide-resistance determinant in U.S. equine clinical isolates and showed it is carried on pRErm46, a conjugative plasmid associated with the expanding MDR-RE 2287 clone. (pubmed.ncbi.nlm.nih.gov)

The background matters because R. equi resistance has become more complicated than a simple susceptible-versus-resistant split. Research over the past several years has shown that erm(46) can move horizontally, that the transposon TnRErm46 can generate additional insertions, and that some resistant strains may retain macrolide resistance even after plasmid loss if transposed copies remain in the genome. In parallel, erm(51) has been identified mainly in environmental isolates from Kentucky, underscoring that the resistance reservoir extends beyond clinical cases. (pubmed.ncbi.nlm.nih.gov)

Against that backdrop, the new study’s copy-number finding is notable because it may help explain variation in phenotypic resistance among erm(46)-positive isolates. Based on the study abstract, the assay was paired with broth microdilution susceptibility testing and temporal surveillance of clinical isolates submitted to the Hagyard Equine Medical Institute laboratory. The implication is that molecular resistance testing may need to move beyond binary detection toward a more quantitative model, especially when clinicians are trying to reconcile PCR findings with MIC data. Because the full article text was not readily accessible in the search results, this interpretation is based on the abstract and related primary literature on erm(46) mobility and amplification potential. (beva.onlinelibrary.wiley.com)

Outside commentary in the recent literature supports the clinical relevance of that shift. A 2024 Equine Veterinary Journal update described macrolide-resistant R. equi as an evolving problem tied to U.S. farm management and antimicrobial use patterns, while another 2026 review highlighted the growing challenge of interpreting susceptibility results in foals as resistance mechanisms diversify. Recent reviews also emphasize that erm(46) and erm(51) are not just markers of resistance, but part of a broader ecology of transferable determinants in farm environments. (beva.onlinelibrary.wiley.com)

Why it matters: For veterinary professionals, especially equine practitioners, internists, and diagnostic laboratorians, the practical takeaway is that a more granular molecular test could sharpen both treatment and stewardship. If copy number predicts resistance magnitude, labs may be able to better flag isolates likely to fail standard macrolide-based therapy, and clinicians may gain a clearer basis for avoiding unnecessary exposure to drugs that further select for MDR clones. That’s especially relevant because R. equi is not only an equine pathogen, but also an opportunistic zoonotic pathogen, and pRErm46 has shown the ability to move among environmental actinobacteria. (pubmed.ncbi.nlm.nih.gov)

There’s also a farm-level implication. Antimicrobial stewardship guidance has increasingly cautioned against reflex treatment of every ultrasonographic lesion in foals, in part because selective pressure appears to sustain resistant R. equi populations in endemic environments. A diagnostic approach that distinguishes mere gene presence from higher-risk resistance profiles could help align treatment decisions more closely with likely clinical benefit. (amrvetcollective.com)

What to watch: The next step is whether this assay, or a similar quantitative PCR approach, is validated for routine clinical use on respiratory samples or cultured isolates, and whether future studies define actionable copy-number cutoffs tied to MICs, outcomes, and stewardship protocols on endemic breeding farms. (pubmed.ncbi.nlm.nih.gov)