Pneumonia due to Rhodococcus equi
Brief history of the Present Illness:
A 72 year old immunocompromised gentleman presented with a 5 month history of productive cough. It was associated with exertional dyspnea, drenching night sweats, and loss of energy. He also reported losing about 17 lbs during the course of his illness. He denied hemoptysis, chest pain, fever or chills. He denied any sick contacts or known exposure to tuberculosis.
Past Medical History
He had history of type 1 diabetes mellitus and underwent living unrelated donor kidney and pancreas transplantation 15 years prior to his current presentation and was on an immunosuppressive regimen consisting of tacrolimus, mycophenolate mofetil, and prednisone.
Prednisone, tacrolimus, mycophenolate mofetil,
He was a resident of Michigan and reported traveling with his wife about 5 months prior to New Mexico, California, Nevada and Arizona. He did not have any known exposure to farm animals.
The patient appeared well. His blood pressure was 123/57 mmHg, pulse rate 84 beats per min, temperature 37.8 degree Celsius and respirations 20 breaths per minute. Chest auscultation revealed crackles in left mid chest. Remainder of the examination was normal.
Hemoglobin was 9.9 g/dL (reference range 13.5-17.5) with a leukocyte count of 5.7 x 10(9)/L (reference range: 3.5-10.5 x 10(9)/L). Creatinine was 0.8 mg/dL (reference range 0.8-1.3 mg/dL).
Serum (1, 3)beta-D-glucan was within normal limits. Histoplasma antibody, urine Histoplasma antigen, aspergillus antigen, coccidioides antibody, blastomyces antibody, and cryptococccal antigen were negative. Blood cultures, including a fungal/mycobacterial blood culture, were negative. Sputum bacterial culture yielded normal flora.
Chest X-ray showed a 5-cm mass in the left upper lobe (Figure 1).
Clinical Course Prior to Diagnosis
A CT scan of the chest showed a mass adjacent to the left hilum measuring 3.6 x 3.4 x 5.2 cm (Figure 2). A subsequent PET scan showed a hypermetabolic mass in the posterior and superior lateral component of the left upper lobe, suspicious for bronchogenic carcinoma.
Bronchoscopy with BAL and biopsy was performed. . Histopathologic examination revealed acute and chronic inflammation with focal non-caseating granulomatous inflammation.
- Histoplasma capsulatum
- Coccidioides immitus
- Blastomyces dermatitidis
- Non-tuberculous mycobacterium (e.g. M. kansasii, M. avium intercellulare)
- Rhodococcus equi
- Nocardia spp.
- Bronchogenic carcinoma
Diagnostic Procedure(s) and Result(s):
Mycobacterial cultures from BAL grew rhodococcus equi after 18 days of incubation. Susceptibility testing showed that the organism was susceptible to imipenem, meropenem, moxifloxacin, clarithromycin, tobramycin, vancomycin and rifampin. Resistance was noted to amoxicillin/clavulanate, cefepime, doxycycline, tobramycin, and TMP/SMX.
The patient was treated with a combination of meropenem, vancomycin and azithromycin for 5 months followed by secondary prophylaxis with oral azithromycin to be continued indefinitely. A follow up CT scan of the chest after 5 months of therapy showed improvement of the left upper lobe lesion (Figure 3).
Brief Discussion of Differential/Major Teaching points of case:
Rhodococcus equi (R.equi), previously known as Corynebacterium equi, is a gram positive coccobacillus known to be a zoonotic pathogen and is being increasingly recognized as a human pathogen. It was first isolated from foals with pneumonia in 1923 and first case of human infection was reported in 1967(1, 2). It is commonly found in soil in areas where herbivores graze. Exposure to contaminated soil via inhalation or ingestion is the major route of acquisition of infection. R. equi is an intracellular pathogen and has the ability to persist in macrophages, where it evades phagocytosis(3). The major immune defense against R. equi is cell mediated and impairment of cell mediated immunity is an important risk factor for the development of infection.
It is known to cause opportunistic infections such as pneumonia and bacteremia in immunosuppressed individuals, including those with late stage HIV infection and transplant recipients. Cavitary pneumonia may be seen in up to one half of infected patients with symptoms primarily consisting of productive cough and hemoptysis. Extra-pulmonary infections may occur with or without concurrent pulmonary infection and include bacteremia, skin and soft tissue infections, and brain abscess. Although R.equi pneumonia has been described in immunocompetent individuals, it is extremely uncommon(4, 5).
Diagnosis is made by culture of the clinical specimen. R. equi readily grows on nonselective culture media under aerobic conditions at 37oC. R. equi is weakly acid fast due to presence of mycolic acid in the cell wall. The name Rhodococcus comes from the red colored colonies seen after 4-7 days of incubation. It appears as gram positive rods in liquid media but is more coccoid in solid media or clinical specimen. Histopathology typically shows necrotizing granulomatous reaction similar to that elicited by mycobacterium and nocardia species.
Treatment usually includes a macrolide or fluoroquinolone in combination with rifampin or two of the following: vancomycin, linezolid, imipenem or an aminoglycoside. Duration of treatment is variable and is determined by clinical and radiological improvement. In patients with ongoing immunosuppressive conditions, secondary prophylaxis should be considered after initial therapy. In our patient, rifampin was avoided due to the risk of interaction with his immunosuppressive medications.
Rhodococcus equi pnemonia
- Golub B, Falk G, Spink WW. Lung abscess due to Corynebacterium equi. Report of first human infection. Annals of internal medicine. 1967;66(6):1174-7.
- Muscatello G, Leadon DP, Klayt M, Ocampo-Sosa A, Lewis DA, Fogarty U, et al. Rhodococcus equi infection in foals: the science of 'rattles'. Equine veterinary journal. 2007;39(5):470-8.
- Prescott JF. Rhodococcus equi: an animal and human pathogen. Clinical microbiology reviews. 1991;4(1):20-34.
- Linares MJ, Lopez-Encuentra A, Perea S. Chronic pneumonia caused by Rhodococcus equi in a patient without impaired immunity. The European respiratory journal. 1997;10(1):248-50.
- Kedlaya I, Ing MB, Wong SS. Rhodococcus equi infections in immunocompetent hosts: case report and review. Clinical infectious diseases : an official publication of the Infectious Diseases Society of America. 2001;32(3):E39-46.
Figure #, location of image, type of image, legend
- Chest X-ray- Chest x-ray shows a 5-cm mass in the left upper lobe.
- CT Chest: Consolidative mass measuring 10.7 x 9.1 cm in the left upper lobe, containing a few foci of internal air
- CT Chest: Improvement in the left upper lobe lesion after 5 months of therapy.
Please indicate which photo or figure you think is most representative of the case.
M. P. Wilhelm, None