Infectious Disease Pathology
Case 2 -
Rhodococcus Equi Pneumonia
as the Presenting Feature in a HIV Positive Aids Patient
Margie A. Scott
Central Arkansas Veterans Healthcare System and
University of Arkansas for
Little Rock, Arkansas
Click on each slide thumbnail image for an enlarged view
A 34-year old divorced mechanic presented to the
emergency room with a 3 to 4 week history of malaise, fever, non-productive cough, left sided chest pain
and night sweats. The patient recalls recent exposure to a friend's child who was diagnosed with
"walking pneumonia." Past medical history was positive only for degenerative joint disease of the knee
(ACL tear) and two bouts of "pink eye" in the past year. The patient lives in a rural area and enjoys
outdoor activities such as hunting, rodeos and horseshows, but recalls no unusual exposures to wildlife,
ticks or insects. The patient reports being HIV and PPD negative at time of military discharge. On
physical examination there was mild fever of 99.9 F, BP 123/73, pulse 120, respiratory rate 20, and chest
was clear to auscultation. Laboratory: WBC 7.3, Hgb 13.3, Hct 38.8, Plt 251, Basic metabolic panel and
UA were normal. Chest X-ray was positive for a LUL infiltrate, 6.7cm maximum dimension with focal
cavitation, very suspicious for TB. The patient was admitted with respiratory isolation precautions,
started on broad-spectrum antibiotic therapy, a PPD and control were placed and the infection control
team was notified. Blood and sputum samples were collected for bacterial, fungal and mycobacterial
cultures. Serologic studies for Hepatitis C, Hepatitis B and HIV were obtained. The patient left the
hospital AMA on hospital day 2, with the PPD and control noted as "no induration." The patient returned
to the emergency room one week later with progressive respiratory symptoms and was admitted for pulmonary
consultation, CT scan and bronchoscopy. The TB skin test was noted as "negative" by the patient. AFB
smears were negative and previously obtained cultures were negative to date. CT scan confirmed the LUL
density with three areas of early cavitation as well as a focal infiltrate in the RUL and mild hilar
adenopathy. No additional abnormalities were identified. A transbronchial biopsy and BAL were obtained
Case 2 - Figure 1 - Chest radiograph showing a left upper lobe infiltrate with focal cavitation. Read as suspicious for tuberculosis
Case 2 - Figure 2 - Transbroncial biopsy was non-contributory and does not reflect lesional sampling.
Case 2 - Figure 3 - BAL cell block, inflammatory process with a predominance of histiocytes and neutrophils.
Case 2 - Figure 4 - BAL cell block, some histiocytes show eosinophilic cytoplasm. (Gram stain, not shown, demonstrated rare gram-positive coccobacilli of Rhodococcus equi.
Histologic & Culture Findings
Sections of the
transbronchial biopsy are non-contributory, and do not reflect lesional sampling. The BAL cell block
shows a distinct inflammatory process with a predominance of histiocytes and neutrophils. Scattered
histiocytes have an enhance eosinophilic cytoplasmic appearance on Hematoxylin&Eosin which increases
with Periodic-Acid-Schiff staining. Scattered intracellular gram positive coccobacilli are identified;
acid fast stains are negative. Follow-up cultures were positive for Rhodococcus
equi with successful isolation and identification from the Lowenstein-Jensen slant. Sensitivities
performed on 5% RBC enriched Mueller-Hinton agar using standard Kirby-Bauer method, show the organism to
be sensitive to rifampin, vancomycin, erythromycin, tetracycline, imipenem, cefepime, clindamycin,
tobramycin, amikacin, ciprofloxicin, levofloxicin and sulfa/trimethoprim. (noted in report: There are
no NCCLS standards for sensitivity testing of this organism; sensitivities are compared to those reported
within the literature; antimicrobial therapy should be coordinated with infectious disease specialist
The patient was evaluated for suspected HIV
infection based on clinical history and presentation. The HIV positive screen was confirmed by western
blot analysis and subsequent viral load studies demonstrated 533,000 viral copies per ml (Log 10 =
5.73c/ml). Lymphocyte subset studies demonstrated an absolute lymphocyte count of 962 (ref: 1000 –
4000); CD 8 suppessors = 64%; CD 4 helpers = 11% with a markedly decreased absolute CD4 cell count of 105
(ref 500 – 1500). HIV Genotyping studies indicted possible resistance to Indinavir (PI), saquinavir
(PI), ritonavir (PI), nelfinavir (PI), lamivudine (NRTI), sidovudine (NRTI). The patient was started on
lamivudine (NRTI), stavudine (NRTI) and efavirenz (NNRTI) as initial therapy for his HIV infection. He
was started on levofloxicin for his Rhodococcus equi pneumonia and
prophylactic bactrim was initiated. The patient noted improvement of clinical respiratory symptoms
within one week of initiating antibiotics. The patient responded with a greater than two-fold log
decrease in HIV viral load and a follow-up chest X-ray performed three months after presentation,
demonstrated near complete clearing of the air space disease in the Left Upper Lobe. The patient is
tolerating anti-viral and antibiotic therapy well and is to complete a six month course of levofloxicin
before considering discontinuation of this antibiotic for his Rhodococcus
pneumonia as the presenting feature in a HIV positive AIDS patient.
Rhodococcus equi (previously Corynebacterium
equi) is an opportunistic gram positive weakly acid-fast coccobacillus that resides in the soil
and is usually spread to it intended host, horses, via the respiratory route and is known as a common
cause of foal pneumonia. Although occasional cases of human direct inoculation infection of the eyes,
skins, oral mucosa have been described in the immunocompetent host, these tend to be mild and
self-limited. Since the first reported human infection with Rhodococcus
equi in 1967, it has become increasingly recognized as an important pathogen in the
immunocompromised host. Rhodococcus equi pneumonia characteristically
presents with fever, chills, malaise, chronic cough and necrotizing cavitary pneumonia on Chest X-ray.
The cavitary lesions vary at presentation from 1 to 8cm in diameter. The clinical danger is its mimicry
of tuberculosis and the identification of acid fast bacilli on respiratory specimens obtained for TB
culture may result in the initiation of anti-tuberculous therapy. Other sites of infection and
presentations documented in the literature include: soft tissue mass, pleural effusion, empyema, pelvic
abscess, brain abscess, subcutaneous abscess, paraspinal abscess, osteomyelitis, lymphadenitis,
endophthalmitis and fever of unknown origin. Widespread dissemination of infection at autopsy in the
immunocompromised host is well documented. As clinicians become more aware of this potential pathogen of
the immunocompromised host, outcomes have improved with long term multi-drug, antimicrobial therapy and
close follow-up for resolution of symptoms. Rhodococcus equi, in fact, may
be the initial presentation of a patient with undiagnosed HIV infection and AIDS.
Histologically, Rhodococcus equi pneumonia is characterized by a dense
mixed histiocytic and neutrophilic infiltrate, without formation of giant cells or distinct granulomas.
The histiocytes become filled with organisms and take on a brightly eosinophilic appearance in routine
Hematoxylin-Eosin stains. These histiocytes further intensify with Periodic-Acid-Shiff (PAS) staining,
similar in appearance to the infected histiocytes of Whipple's disease. The organisms stain well with
gram stain and silver precipitate stains such as Gomori-methenamine silver, and variably with acid fast
stains. In patients with ongoing chronic infection and/or inadequate antimicrobial therapy, the
histiocytes become the predominant cell type and may take on a "spindle cell pseudotumor" appearance with
the formation of intracytoplasmic laminated inclusions, Michaelis-Gutmann bodies. The spindle cell
variant morphology occurring in sites such as a soft tissue mass with minimal necrosis can have a
striking pseudosarcomatous appearance. In this setting, malacoplakia with numerous Michaelis-Gutmann
bodies help call the attention of the pathologist to the infectious nature of the lesion.
Recognition of culture isolate is often difficult in routine bacterial cultures of respiratory
specimens, as the gram stain morphology is 'diphtheriod' in nature and may be dismissed as 'a
contaminant' in its early growth stages seen at 72 hours among other respiratory organisms. If isolation
occurs during routine bacterial culture work-up, it is usually because the organism grows as a single
isolate from a sterile site such as blood, cerebral spinal fluid or deep tissue lesions. It is much
easier to recognize this respiratory pathogen as it grows during attempts for mycobacterial isolation.
The characteristic salmon-pink colony color is striking in contrast to Lowenstein-Jensen and Sabouraud
media; acid fast staining of fresh isolates quickly highlights the acid fast nature of this
non-mycobacterial pathogen. It is important that technologists recognize the clinical significance of
this opportunistic pathogen and complete the work-up and identification of this pathogen.
Both internists and pathologists should consider Rhodococcus equi within
the differential of the immunocompromised patient presenting with chronic cavitary pneumonia,
particularly when cultures fail to identify Mycobacteria, Nocardia, Pseudomonas,
enterobacteriacea such as Salmonella and fungal organisms.
- Allen U, Niec A, Kerem E, et al. Rhodococcus equi pneumonia in a child with leukemia. Ped Infect Dis J. 1989; 8:656-658.
- Asoh N, Watanabe H, Fines-Guvon M, et al. Emergence of rifampin-resistant Rhodococcus equi with several types of mutations in the rpoB gene among AIDS patients in northern Thailand. J Clin Microbiol 2003; 41(6):2337-2340.
- Aviram G, Fishman JE, Sagar M. Cavitary lung disease in AIDS: etiologies and correlation with immune status. AIDS Patient Care STDS. 2001; 15(7):353-361.
- Berg R, Chmel H, Mayo J, Armstrong D. Corynebacterium equi infection complicating neoplastic disease. Am J Clin Pathol 1977; 68:73-77.
- Casado JL, Navas E, Frutos B, Moreno, et al. Salmonella lung involvement in patients with HIV infection. Chest 1997; 112(5):1197-1201.
- Doig C, Gill M, Church D. Rhodococcus equi: an easily missed opportunistic pathogen. J.Scand J. Infect Dis 1991; 23:1-6.
- Emmons W, Reichwein B, Winslow DL. Rhodococcus equi infection in the patient with AIDS: Literature review and report of an ususual case. Rev Infect. Dis. 1991; 13:91-96.
- Fierer J, Wolf P, Seed L. et al. Non-pulmonary Rhodococcus equi infections in patients with acquired immune deficiency syndrome (AIDS). J Clin Path 1987; 40:556-558.
- Gallant JE, Ko AH. Cavitary pulmonary lesions in patients infected with human immunodeficiency virus. Clin Infect Dis 1996; 22(4):671-682.
- Golub B, Falk G, Spink W. Lung abscess due to Corynebacterium equi. Report of the first human infection. Ann Intern Med 1967; 66:1174-1177.
- Hamrock D, Azmi FH, O'Donnell E, et al. Infection by Rhodococcus equi in a patient with AIDS: histologic appearance mimicking Whipple's disease and Mycobacterium avium-intracellulare infection. J Clin Pathol. 1999; 52(1):68-71.
- Lachman MR. Cytologic appearance of Rhodococcus equi in bronchoalveolar lavage specimens. A case report. Acta Cytol 1995: 39(1):111-113.
- Lo A, Stratta RJ, Trofe J, et al. Rhodococcus equi pulmonary infection in a pancreas-alone transplant recipient: consequences of intense immunosuppression. Transpl Infect Dis 2002; 4(1):46-51.
- Magnusson H. Pyaemia in foals caused by Corynebacterium equi. Vet Res 1938; 50:1459-1468.
- McNeil MM, Brown JM. Distribution and antimicrobial susceptibility of Rhodococcus equi from clinical specimens. Eur J Epidemiol 1992; 8:437-443.
- Munoz P, Burillo A, Palomo J, et al. Rhodococcus equi infection in transplant recipients: case report and review of the literature. Transplantation 1998; 65(3):449-453.
- Nordmann p, Rouveiz E, Geunounou M, Nicolas M. Pulmonary abscess due to a rifampicin/fluoroquinolone resistance Rhodococcus equi strain in a HIV infected patient (letter). Eur J Clin Microbiol Infect Dis 1992; 11:557-558.
- Prescott JF. Epidemiology of Rhodococcus equi infection in horses. Vet Micro 1987;14:211-214.
- Sanz-Moreno J, Flores-Segovia J, Olmedilla-Arregui G, et al. Rhodococcus equi pneumonia: highly active antiretroviral therapy helps but does not cure lung inf. AIDS 2002; 16:509-611.
- Schuster MG, Norris AH. Community-acquired Pseudomonas aeruginosa pneumonia in patients with HIV infection. AIDS 1994; 8(10):1437-1441.
- Scott MA, Graham BS, Verrall R et al. Rhodococcus equi – An increasingly recognized opportunistic pathogen – Report of 12 cases and review of 65 cases in the literature. Am J Clin Pathol 1995; 103 (5): 649-655.
- Scannell K, Portoni E, Finkle H, Rice M. Pulmonary malacoplakia and Rhodococcus equi infection in a patient with AIDS. Chest 1990; 97:1000-1001.
- Torres-Tortosa M, Arrizabalaga J, Villanueva JL, et al. Prognosis and clinical evaluation of infection caused by Rhodococcus euqi in HIV-infected patients: a multicenter study of 67 cases. Chest 2003; 123(6):1970-6.
- Wang H, Tollerud D, Danar D, et al. Another Whipple-like disease in AIDS? (letter) NEJM 1986; 413:1577-1578.
- Weingarten JS, Huang DY, Jackman JD Jr. Rhodococcus equi pneumonia. An unusual early manifestation of the acquired immunodeficiency syndrome (AIDS). Chest 1988; 94(1):195-196.