Organism of the Week
Abstract
Rhodococcus equi is an aerobic, Gram-positive, facultatively intracellular actinomycete associated particularly with impaired cellular immunity. Pulmonary infection is characteristic and may progress to cavitation, abscess formation or empyema; bloodstream, device-associated and other extrapulmonary infections also occur. Diagnostic delay can follow dismissal of the organism as a coryneform contaminant or confusion with mycobacteria. Reliable species identification, careful interpretation of minimum inhibitory concentrations and assessment for dissemination underpin management. Treatment evidence remains largely observational. Combination therapy is commonly used for severe disease and immunocompromised hosts, while duration depends on disease burden, source control, immune recovery and clinical and radiological response. This review integrates clinical and laboratory evidence while distinguishing measured findings from expert practice.
Keywords: Rhodococcus equi; cavitating pneumonia; opportunistic infection; actinomycetes; antimicrobial susceptibility; transplantation.
Introduction
Rhodococcus equi causes uncommon but potentially serious human infection. Its intracellular survival and association with impaired cellular immunity explain its prominence in advanced HIV infection and transplantation. A persistent pulmonary lesion and a compatible Gram-positive coccobacillus should prompt investigation rather than automatic dismissal as a “diphtheroid”. Conversely, isolation alone does not establish disease. [1–4]
Nomenclature, ecology and pathogenesis
LPSN lists R. equi as the correct name and recommends it for medical use. Alternative names encountered in publications and databases include Rhodococcus hoagii and Prescottella equi. LPSN also records assignment of subgenus Prescottella to Rhodococcus in 2025. These names matter when searching older evidence and laboratory records. [5–7]
The organism occurs in soil and environments contaminated by grazing animals; pneumonia in foals is its best-recognised veterinary manifestation. Inhalation and direct inoculation are plausible acquisition routes, but an animal source is seldom established in individual human cases. Absence of horse contact does not exclude infection. [1–3]
Virulence-associated protein genes occur on plasmids. The best-characterised groups are pVAPA, associated predominantly with horses; pVAPB, associated with pigs; and pVAPN, associated with ruminants. All have been detected in human isolates. Bryan and colleagues examined 65 human-derived isolates collected during 1984–2002: 37 carried a detected virulence plasmid, and 16 were vapN-positive, representing 25% of the complete collection and 43% of plasmid-positive isolates. These historical findings do not establish contemporary prevalence or identify the animal source of an individual infection.
Susceptible hosts
Human infection is reported predominantly in patients with advanced HIV, solid-organ or haematopoietic stem-cell transplantation, haematological malignancy and other immunosuppressive conditions. Apparently immunocompetent individuals can also develop disease. Invasive infection without an evident predisposition should prompt assessment for an underlying immune disorder, including consideration of HIV testing. [1–4]
Vergidis and colleagues studied 18 transplant recipients with Rhodococcus infection and 36 matched controls. Diabetes and recent opportunistic infection were independently associated with disease. Only nine isolates underwent species identification, eight of which were R. equi: the cohort must therefore not be described as exclusively species-confirmed R. equi.
A 2026 series of 14 patients with AIDS and R. equi infection reported a median CD4 count of 26.0 cells/µL. Findings from such selected cohorts should not be generalised to all host groups.
Clinical manifestations
Pulmonary infection
Pulmonary disease commonly evolves subacutely, with fever, cough, fatigue, weight loss and sometimes chest pain or haemoptysis. Imaging may demonstrate consolidation, nodules, masses, cavitation, lung abscess or pleural infection. Contiguous extension and other destructive complications have been described. [1–4]
Cavitation is characteristic but not obligatory, and its frequency varies by population. In the transplant cohort, only two of eleven patients with pneumonia had cavitation. This contrasts with higher frequencies in historical descriptions dominated by advanced HIV; a single percentage should not be applied across host groups.
Malakoplakia
Pulmonary malakoplakia is a reported manifestation and can resemble a neoplasm. Histology classically shows histiocytic inflammation with Michaelis–Gutmann bodies. A lung-transplant case documents malakoplakia secondary to R. equi mimicking lung cancer. The association can provide a diagnostic clue, but histology should be interpreted alongside culture and species identification rather than treated as species-specific proof. [2,10]
Bloodstream and extrapulmonary infection
| Syndrome | Reported manifestations or diagnostic considerations |
|---|---|
| Bloodstream infection | Pulmonary dissemination or an isolated bloodstream process; assess indwelling vascular devices |
| Central nervous system infection | Brain abscess or meningitis; neurological symptoms may signal dissemination |
| Skin and soft-tissue infection | Subcutaneous abscesses, wound infection and inoculation-associated infection |
| Bone and joint infection | Osteomyelitis or septic arthritis |
| Ocular infection | Endophthalmitis, corneal infection and trauma-associated disease |
| Other focal or device-associated infection | Lymphadenitis, intra-abdominal infection and peritoneal dialysis-associated infection |
These are documented presentations, not frequency estimates. Extrapulmonary infection can occur with or without pulmonary disease. Importantly, Gundelly and colleagues found isolated extrapulmonary involvement more common in immunocompetent than immunocompromised patients in their assembled case series and literature comparison. Immunocompetence should not be used to exclude focal extrapulmonary infection. [1,2,4]
In a study of seventeen cancer patients with Rhodococcus bacteraemia and central venous catheters, 94% had central line-associated bloodstream infection and 82% of isolates were R. equi. Experimental work demonstrated heavy catheter-surface biofilm formation. These selected findings support assessment of a vascular device as the source, but do not imply that most bacteraemia in all patients is catheter-associated.
Diagnosis and laboratory recognition
Sampling and identification
Culture from an involved site is the foundation of diagnosis. Obtain blood cultures in systemic illness and consider sputum, bronchoalveolar lavage, pleural fluid, tissue or abscess material according to the syndrome. Neurological symptoms require investigation for CNS involvement. Communicating suspicion to the laboratory helps prevent dismissal of a clinically significant isolate. [1–3]
The organism can grow on routine aerobic media. It is pleomorphic, with bacillary-to-coccoid changes, and may be weakly or partially acid-fast. Pale pink, coral or pale yellow colonies can develop, but colourless variants occur. Neither pigmentation nor acid-fastness establishes identification.
UK SMI ID 10 describes pale pink, slimy colonies after 2–4 days on specified enriched media at 35°C. Its current issue 2.3 is dated 17 July 2025, but this was an administrative update: the recorded last scientific and clinical review was January 2015. It remains useful for classical morphology, not as contemporary treatment guidance.
MALDI-TOF mass spectrometry—matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry—and sequencing can establish identification, subject to database coverage and discriminatory performance. Ranganath and colleagues used MALDI-TOF and/or partial 16S rRNA gene sequencing; 19% of their overall isolate collection remained identified only to genus level.
Clinical significance and differential diagnosis
A compatible isolate from a normally sterile site warrants careful assessment. Respiratory recovery requires correlation with host factors, imaging, specimen quality and competing diagnoses. In the 2024 centre series, ten of 32 locally evaluated cases were treated as clinically significant infection; most excluded isolates were non-equi species. This fraction is not the probability that an individual R. equi isolate is incidental.
The differential diagnosis of cavitating disease includes tuberculosis, nocardiosis, fungal infection and malignancy. Partial acid-fastness can confuse the distinction from mycobacteria. Microscopy, growth characteristics and reliable species identification should be considered together; radiological patterns alone are insufficient. [1,2]
Antimicrobial susceptibility and resistance
Testing and interpretation
Specify the testing method and interpretive framework. EUCAST provides dedicated guidance when species- or agent-specific breakpoints are unavailable and cautions against uncritical adoption of another organisation’s breakpoints. Published CLSI-based susceptibility categories should not be transferred automatically into EUCAST reports. In-vitro activity is not equivalent to demonstrated clinical efficacy. [1,13]
Ranganath and colleagues evaluated 322 Rhodococcus isolates overall; 199 underwent susceptibility testing, including 105 R. equi and 94 non-equi isolates. Their Methods section specifies Sensititre RAPMYCO plates and CLSI criteria for aerobic actinomycetes. Thus, an unqualified statement that no CLSI testing framework exists would be misleading. Availability of a testing framework does not establish that every drug–organism interpretation has outcome-validated human breakpoints.
In-vitro findings
The 2024 study found substantially poorer activity of amoxicillin–clavulanate and cephalosporins against R. equi than against non-equi isolates. The authors reported 99% imipenem susceptibility and greater than 95% susceptibility to clarithromycin, rifampicin and linezolid among R. equi; all isolates tested were susceptible to vancomycin under their framework. Aminoglycoside results varied. Moxifloxacin performed more favourably than ciprofloxacin in the overall collection; pooled percentages must not be labelled as species-specific results.
Co-trimoxazole activity also varies between reported populations. Gundelly and colleagues found a marked difference between historical European and US isolates; these small, geographically and temporally selected data support isolate-specific assessment rather than a universal susceptibility assumption.
One Health resistance
Equine antimicrobial use and resistant strain dissemination are relevant to human microbiology. Val-Calvo and colleagues reported Irish equine isolates belonging to the MDR-RE 2287 clone and carrying erm(46) and the rifampicin-resistance-associated rpoB S531F mutation. These findings support testing rather than assumed macrolide–rifampicin activity, but do not estimate resistance prevalence in UK human infection.
Treatment
Initial treatment and definitive therapy
No regimen has demonstrated universal superiority in comparative human trials. Severe infection and immunocompromised hosts are commonly managed with two or three agents, selected according to site, severity, MICs, toxicity, drug interactions and source control. Observational series and reviews support intravenous imipenem and vancomycin as potential initial backbone agents, with additional or subsequent agents chosen individually. [1–4]
Rifampicin, macrolides, fluoroquinolones and linezolid feature in published regimens. Inclusion of an agent with intracellular activity is biologically attractive, but this rationale does not establish the clinical superiority of a particular combination. CNS disease additionally requires consideration of drug penetration at the actual site; macrolide activity should not be assumed to provide adequate CNS treatment. [1,2]
Monotherapy has been used in selected immunocompetent patients and uncomplicated infections, but evidence remains limited. Decisions should account for source control and disease burden rather than immune status alone. [1,2,4]
Interactions and toxicity
Drug interactions are particularly consequential in transplantation. Clarithromycin and erythromycin can increase exposure to calcineurin and mTOR inhibitors, whereas rifampicin can markedly reduce exposure. Azithromycin has fewer relevant metabolic interactions, but medication review and monitoring remain necessary.
Long courses require agent-specific safety monitoring. Potential constraints include renal toxicity with intravenous regimens, cytopenias with prolonged linezolid, and additive QT effects with some oral combinations. These considerations may determine whether an apparently active regimen is sustainable. Doses and monitoring should follow current drug guidance, organ function and local specialist protocols rather than a fixed organism-specific dosing table. [1,2]
Duration and follow-up
Duration is uncertain and should reflect site, cavitation or mass formation, source control, ongoing immunosuppression and clinical and radiological response. Published practice frequently uses months of therapy for severe pulmonary or disseminated disease and immunocompromised hosts. [1–3]
The 2024 authors proposed 2–6 months for immunocompromised hosts, but their individual cases included shorter courses and treatment beyond one year. All ten treated patients were reported cured at 90 days after treatment completion; only seven had R. equi, and this follow-up cannot exclude later relapse.
Lin and colleagues questioned whether older prolonged-treatment recommendations apply to every modern presentation. They proposed that 10–14 days might suffice for selected uncomplicated pneumonia, while longer treatment remains appropriate for cavitary lesions or masses. This is a review-based proposal, not validation of routine short-course therapy.
Persistent clinical or radiological abnormalities require reassessment rather than automatic treatment extension: consider residual damage, ongoing infection, resistance, inadequate source control or another diagnosis. Follow-up imaging is particularly relevant in pulmonary masses, cavitary disease and CNS infection. [2,15]
Source control, immune recovery and relapse
Drain empyema and accessible abscesses where indicated. Surgical resection may be required in selected destructive or refractory pulmonary infections, but is not routine. Two patients in the 2024 series required thoracotomy and decortication.
Remove an implicated vascular catheter when feasible. In the cancer-associated bloodstream study, catheter removal with antimicrobial therapy was associated with successful responses; the transplant cohort also reported no relapse among four catheter-associated cases whose lines were removed. These are observational findings, not a randomised comparison of retention and removal. [3,11]
Optimise immune function where possible, including effective antiretroviral therapy and carefully considered reduction of immunosuppression. The lower HIV-associated mortality observed in the antiretroviral era supports the importance of immune recovery, although historical comparisons remain vulnerable to confounding. [3,4]
Relapse can occur despite prolonged treatment. Menon and colleagues described a renal-transplant patient with recurrent infection despite twelve months of antibiotics, surgery and reduced immunosuppression. This illustrates the difficulty of persistent disease, not a universal expected outcome.
Prevention and secondary suppression
Preserving or restoring immune function is central to prevention. Exposure-reduction advice around dusty livestock environments is reasonable, but direct evidence that such measures prevent human infection is limited. Routine organism-specific primary antimicrobial prophylaxis is not established. In the transplant case–control study, co-trimoxazole prophylaxis was not associated with protection. [2,3]
Secondary suppressive treatment may be considered after active treatment in selected patients with persistent profound immunosuppression, residual disease or recurrent infection. The optimal agent, duration and stopping criteria are not established by controlled studies. An oral agent should be selected according to the isolate, tolerability and interactions, with periodic reassessment of immune recovery and relapse risk. The evidence reviewed here does not justify presenting lifelong suppression as mandatory for every transplant recipient. [1,3,16]
Conclusion
R. equi is a clinically important diagnostic possibility in immunocompromised patients with persistent or cavitating pulmonary disease, but its spectrum includes isolated bloodstream, device-associated and focal extrapulmonary infection. Reliable identification and contextual interpretation prevent both missed disease and unnecessary treatment. Management combines carefully interpreted MICs, individualised antimicrobial therapy, source control and immune recovery. Current evidence supports this integrated approach rather than a universally preferred regimen, duration or prophylaxis strategy. [1–4]
References
- Ranganath N, Mendoza MA, Stevens R, Kind D, Wengenack N, Shah A. Rhodococcus infection: a 10-year retrospective analysis of clinical experience and antimicrobial susceptibility profile. J Clin Microbiol. 2024;62:e01537-23. doi:10.1128/jcm.01537-23.
- Yamshchikov AV, Schuetz A, Lyon GM. Rhodococcus equi infection. Lancet Infect Dis. 2010;10:350–359. doi:10.1016/S1473-3099(10)70068-2.
- Vergidis P, Ariza-Heredia EJ, Nellore A, et al. Rhodococcus infection in solid organ and hematopoietic stem cell transplant recipients. Emerg Infect Dis. 2017;23:510–512. doi:10.3201/eid2303.160633.
- Gundelly P, Suzuki Y, Ribes JA, Thornton A. Differences in Rhodococcus equi infections based on immune status and antibiotic susceptibility of clinical isolates in a case series of 12 patients and cases in the literature. Biomed Res Int. 2016;2016:2737295. doi:10.1155/2016/2737295.
- List of Prokaryotic names with Standing in Nomenclature. Species: Rhodococcus equi. https://lpsn.dsmz.de/species/rhodococcus-equi
- List of Prokaryotic names with Standing in Nomenclature. Species: Prescottella equi. https://lpsn.dsmz.de/species/prescottella-equi
- List of Prokaryotic names with Standing in Nomenclature. Genus: Rhodococcus. https://lpsn.dsmz.de/genus/rhodococcus
- Bryan LK, Alexander ER, Lawhon SD, Cohen ND. Detection of vapN in Rhodococcus equi isolates cultured from humans. PLOS ONE. 2018;13:e0190829. doi:10.1371/journal.pone.0190829.
- Yao Y, Zhou R, Wang L, et al. Clinical analysis of 14 cases of acquired immunodeficiency syndrome complicated with Rhodococcus equi infection. Journal of Internal Medicine Concepts & Practice. 2026;21:257–261. doi:10.16138/j.1673-6087.2026.03.11.
- Nickel IC, Boys JA, Dill EA, Weder MM, Krupnick AS, Walters DM. Pulmonary malakoplakia secondary to Rhodococcus equi infection mimicking a lung neoplasm in a lung transplant recipient. Am J Transplant. 2019;19:597. doi:10.1111/ajt.15045.
- Al Akhrass F, Al Wohoush I, Chaftari AM, et al. Rhodococcus bacteremia in cancer patients is mostly catheter related and associated with biofilm formation. PLOS ONE. 2012;7:e32945. doi:10.1371/journal.pone.0032945.
- UK Standards for Microbiology Investigations. Identification of aerobic actinomycetes. ID 10. Issue 2.3. 17 July 2025. Administrative update; last scientific and clinical review 15 January 2015. https://www.rcpath.org/asset/CE08D742-B58E-4E8D-986C43A75AC367C3/
- European Committee on Antimicrobial Susceptibility Testing. When there are no breakpoints? Guidance webpage dated 10 July 2026. https://www.eucast.org/bacteria/clinical-breakpoints-and-interpretation/when-there-are-no-breakpoints/
- Val-Calvo J, et al. International spread of multidrug-resistant Rhodococcus equi. Emerg Infect Dis. 2022;28:1899–1903.
- Lin WV, Kruse RL, Yang K, Musher DM. Diagnosis and management of pulmonary infection due to Rhodococcus equi. Clin Microbiol Infect. 2019;25:310–315. doi:10.1016/j.cmi.2018.04.033.
- Menon V, Gottlieb T, Gallagher M, Cheong EL. Persistent Rhodococcus equi infection in a renal transplant patient: case report and review of the literature. Transpl Infect Dis. 2012;14:E126–E133. doi:10.1111/tid.12008.