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Equine veterinary journal2026; doi: 10.1002/evj.70312

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

Abstract: 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.
© 2026 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2026-08-20 PubMed ID: 42625340DOI: 10.1002/evj.70312Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This research article reviews the difficulties in interpreting antimicrobial susceptibility test results for Rhodococcus equi infections in foals, particularly due to the lack of established clinical breakpoints and the intracellular nature of the pathogen.
  • It aims to provide guidance on integrating laboratory data with clinical pharmacology to improve treatment decisions for rhodococcal pneumonia.

Background on Rhodococcus equi and Treatment Challenges

  • Pathogen characteristics: Rhodococcus equi is a facultative intracellular bacterium, meaning it can live both inside and outside host cells.
  • Disease impact: It is a primary cause of pneumonia affecting foals globally, leading to significant veterinary and economic concerns.
  • Current therapy: The standard treatment involves a combination of macrolide antibiotics and rifampicin.
  • Resistance issues: Extensive use of this drug combination has promoted the emergence of multidrug-resistant (MDR) strains, complicating effective treatment.

Importance and Limitations of Antimicrobial Susceptibility Testing (AST)

  • Role of AST: Testing helps guide the choice of antimicrobials by determining the susceptibility of R. equi strains in vitro.
  • Lack of clinical breakpoints: There are no standardized breakpoints for interpreting macrolides or other agents against R. equi in foals, which means laboratory results cannot be reliably translated into clinical decision-making.
  • Consequences of missing breakpoints: Makes it difficult to decide whether an antimicrobial is likely effective or if resistance is present.

Challenges in Interpreting AST Results

  • Intracellular infection complexity: Because R. equi resides inside cells, drug activity measured in extracellular culture media may not reflect true clinical efficacy.
  • Extrapolation issues: Using breakpoints established for other bacteria or animal species is unreliable due to differences in pharmacodynamics and bacterial behavior.
  • Variability in testing methods: Different laboratories may use variable protocols, leading to inconsistent results.

Recommended Approach to Improve Interpretation

  • Minimum inhibitory concentration (MIC) integration: Consider actual MIC values rather than categorical susceptible/resistant labels.
  • Intracellular antimicrobial activity: Assess how well antibiotics penetrate cells and kill intracellular bacteria, as this affects clinical outcomes.
  • Pharmacokinetics and pharmacodynamics (PK/PD): Incorporate understanding of drug absorption, distribution, metabolism, elimination, and action relative to bacterial killing.
  • Evidence-based guidelines proposal: In the absence of formal standards, the review suggests practical guidelines to assist veterinarians and diagnostic labs in interpreting results and selecting therapies.

Implications for Veterinary Practice and Research

  • Improved diagnosis and treatment decisions may reduce morbidity and mortality in foals with rhodococcal pneumonia.
  • Highlighting the gaps in knowledge encourages further research to establish validated clinical breakpoints specific to R. equi and foals.
  • Enhances antimicrobial stewardship by guiding the use of effective drugs and reducing the risk of resistance development.

Cite This Article

APA
Seeger MG, Gressler LT, Huber L, Alvarez-Narvaez S, Cargnelutti JF, Bordin AI, Lawhon SD, Cohen ND. (2026). Challenges for interpreting results of antimicrobial susceptibility testing for treating infections of foals with Rhodococcus equi. Equine Vet J. https://doi.org/10.1002/evj.70312

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English

Researcher Affiliations

Seeger, Marlane Geribone
  • Graduate Program in Veterinary Medicine, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil.
  • Equine Infectious Disease Laboratory, Department of Large Animal Clinical Sciences, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, Texas, USA.
Gressler, Letícia Trevisan
  • Department Animal Health, Farroupilha Federal Institute - Frederico Westphalen Campus, Frederico Westphalen, Rio Grande do Sul, Brazil.
Huber, Laura
  • Department of Pathobiology, College of Veterinary Medicine, Auburn University, Auburn, Alabama, USA.
Alvarez-Narvaez, Sonsiray
  • Department of Infectious Diseases, College of Veterinary Medicine, University of Georgia, Athens, Georgia, USA.
Cargnelutti, Juliana Felipetto
  • Department of Preventive Veterinary Medicine, Center of Rural Sciences, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil.
Bordin, Angela Ilha
  • Equine Infectious Disease Laboratory, Department of Large Animal Clinical Sciences, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, Texas, USA.
Lawhon, Sara D
  • Department of Veterinary Pathobiology, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, Texas, USA.
Cohen, Noah D
  • Equine Infectious Disease Laboratory, Department of Large Animal Clinical Sciences, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, Texas, USA.

Grant Funding

  • Glenn Blodgett Chair in Equine Studies, Texas A&M University
  • Finance Code 001 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

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