Rhodococcus equi Infection Workup

Updated: Dec 03, 2018
  • Author: Indira Kedlaya, MD; Chief Editor: Thomas E Herchline, MD  more...
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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.

In the series reported by Torres-Tortosa et al, R equi was isolated from sputum in 52.2% of cases, blood cultures in 50.7% cases, and bronchoscopy in 31.7% of cases. [8]


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. Torres-Tortosa et al reported that chest radiographic findings were abnormal in 97% of patients with HIV infection. [8]

Chest radiograph of a patient with Rhodococcus equ Chest 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 equ Chest 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

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 i Chest CT scan of a patient with Rhodococcus equi infection demonstrating nodular infiltrates.

Other considerations

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.


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.



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.


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.