Rhodococcus equi Workup

  • Author: Indira Kedlaya, MD; Chief Editor: Burke A Cunha, MD   more...
 
Updated: Nov 21, 2011
 

Laboratory Studies

  • CBC count: This is important for evaluation of leukocytosis, anemia, and neutropenia.
  • Chemistry panel
  • HIV screening tests: All patients with R equi infection should undergo screening for HIV because more than half of reported cases involve patients infected with HIV.
  • Blood cultures (including lysis centrifugation blood cultures for fungi and mycobacteria): The distinctive salmon-colored colonies of R equi may not appear for 4-7 days. Any growth of diphtheroids should be viewed with suspicion. Consider blood cultures also in localized infections. In patients infected with HIV, the rate of positive blood culture results is 83-100%. In immunocompetent patients, blood cultures yield positive results in about 30% of patients.
  • Sputum Gram stain and culture: In patients infected with HIV who have pulmonary involvement, the rate of positive sputum culture results may be 60-100%. A positive sputum culture result may be found in only about 35% of immunocompetent patients.
  • Stool culture: Obtain a stool culture in a patient infected with HIV who has diarrhea.
  • Depending on the site of infection, obtain specimens for culture from other infected sources, such as abscess, eye drainage, and cerebrospinal fluid.
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Imaging Studies

  • Chest radiography: Consider chest radiography even in patients with extrapulmonary R equi infections.
    • Multiple nodular infiltrates (as seen in the image below) are the usual findings of R equi infection. In patients infected with HIV, R equi infections have a preference for the upper lobes. Upper lobes were involved in 55% and lower lobes in 35%. In immunocompetent patients, R equi infections have no definite predilection for any particular lobe. Chest radiograph of a patient with Rhodococcus equChest radiograph of a patient with Rhodococcus equi infection showing multiple nodular infiltrates.
    • If untreated, nodular infiltrates are followed by cavitation (as seen in the image below). Approximately 54-77% of all patients with R equi infection demonstrate cavitation. Cavitation is more common in patients infected with HIV (about 67-77%). Chest radiograph of a patient with Rhodococcus equChest radiograph of a patient with Rhodococcus equi infection demonstrating cavitation of pulmonary nodules.
    • Other findings of R equi infection on chest radiography include interstitial pneumonia, abscesses, and pleural effusion. Cavities observed with R equi infection are thick-walled and may demonstrate air-fluid levels, indicating progression to abscess formation.
  • CT scanning of the thorax is more sensitive and may show more nodules (as seen in the image below) and cavitation than are observed on a plain radiograph. Chest CT scan of a patient with Rhodococcus equi iChest CT scan of a patient with Rhodococcus equi infection demonstrating nodular infiltrates.
  • Plain radiographs in osteomyelitis may demonstrate an osteolytic lesion. CT scan and MRI study may demonstrate a mass with a necrotic center. Appropriate imaging is also necessary in cases of meningitis, brain abscess, and abdominal infections.
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Other Tests

  • Microbiological characteristics
    • R equi is cultured easily in ordinary nonselective media. Large, smooth, irregular, mucoid colonies appear within 48 hours. The salmon-colored pigment rarely appears before day 4.
    • R equi is a facultative, intracellular, nonmotile, non–spore-forming organism. Gram stain shows pleomorphic gram-positive coccobacilli. The bacteria may be coccoid in solid media, but, in liquid media, they form long rods. The organism may also be inconsistently acid-fast with Ziehl-Nelson staining, depending on the culture media. It may be distinguished from mycobacterial genera with the 14-day arylsulfatase test.
    • R equi is nonfermenting (distinguishing it from pathogenic corynebacteria), gelatinase negative, catalase positive, usually urease positive, and oxidase negative.
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Procedures

  • Bronchoscopy with washings and bronchoalveolar lavage (BAL) are other diagnostic procedures that may be helpful in diagnosing R equi infections. Reviews of R equi infection have reported that specimens obtained with bronchial washings or BAL showed positive results in 46%-66% of patients infected with HIV.
  • Other diagnostic procedures that may be necessary in R equi pneumonia include aspiration of pleural fluid, transthoracic needle biopsy, and open lung biopsy.
  • Likewise, depending on the site of infection, other procedures may provide the diagnosis. These may include lumbar puncture, biopsy, aspiration of abscess, joint aspiration, bone marrow biopsy, and vitrectomy for endophthalmitis.
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Histologic Findings

The ability of R equi to persist in and destroy macrophages is the basis of its pathogenesis. The typical pattern is a necrotizing granulomatous reaction dominated by macrophages filled with granular cytoplasm that shows positive results on periodic acid-Schiff stain and contains large numbers of coccobacilli.

Malakoplakia is an unusual inflammatory disorder with accumulation of characteristic histiocytes with calcified lamellar cytoplasmic bodies (Michaelis-Gutman bodies). Malakoplakia was initially described in lower urinary tract infections, and Escherichia coli is the organism most often implicated. Pulmonary R equi infections in immunocompromised hosts may have this typical histopathological finding. If malakoplakia is found in pulmonary infections, strongly suspect R equi infection. Although malakoplakia in immunocompromised patients is mostly found in pulmonary infections, it also has been demonstrated in subcutaneous infections and abscesses.

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Contributor Information and Disclosures
Author

Indira Kedlaya, MD  Provider, Sunshine Medical Group, PLLC

Indira Kedlaya, MD is a member of the following medical societies: American College of Physicians-American Society of Internal Medicine

Disclosure: Nothing to disclose.

Coauthor(s)

Michael Ing, MD  Chief, Section of Infectious Disease, JL Pettis Memorial VA Medical Center, Assistant Professor, Department of Internal Medicine, Loma Linda University School of Medicine

Michael Ing, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of HIV Medicine, American College of Physicians-American Society of Internal Medicine, American Society for Microbiology, American Society of Tropical Medicine and Hygiene, HIV Medicine Association of America, Infectious Diseases Society of America, International AIDS Society, Royal Society of Tropical Medicine and Hygiene, and Society for Healthcare Epidemiology of America

Disclosure: Nothing to disclose.

Specialty Editor Board

Larry I Lutwick, MD  Professor of Medicine, State University of New York Downstate Medical School; Director, Infectious Diseases, Veterans Affairs New York Harbor Health Care System, Brooklyn Campus

Larry I Lutwick, MD is a member of the following medical societies: American College of Physicians and Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD  Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Aaron Glatt, MD  Professor of Clinical Medicine, New York Medical College; President and CEO, Former Chief Medical Officer, Departments of Medicine and Infectious Diseases, St Joseph Hospital (formerly New Island Hospital)

Aaron Glatt, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physician Executives, American College of Physicians, American College of Physicians-American Society of Internal Medicine, American Medical Association, American Society for Microbiology, American Thoracic Society, American Venereal Disease Association, Infectious Diseases Society of America, International AIDS Society, and Society for Healthcare Epidemiology of America

Disclosure: Nothing to disclose.

Eleftherios Mylonakis, MD  Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital

Eleftherios Mylonakis, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Society for Microbiology, and Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Chief Editor

Burke A Cunha, MD  Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital

Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and Infectious Diseases Society of America

Disclosure: Nothing to disclose.

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Chest radiograph of a patient with Rhodococcus equi infection showing multiple nodular infiltrates.
Chest radiograph of a patient with Rhodococcus equi infection demonstrating cavitation of pulmonary nodules.
Chest CT scan of a patient with Rhodococcus equi infection demonstrating nodular infiltrates.
 
 
 
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