Background

Pulmonary nocardiosis is an uncommon pulmonary infection caused by aerobic gram-positive actinomycetes of the genus Nocardia [1]. It occurs mainly as an opportunistic infection in immunocompromised patients, particularly in those with defects in cell-mediated immunity such as patients with human immunodeficiency virus (HIV) infection and those receiving long-term systemic steroids or immunosuppressive agents; however, it can also affect immunocompetent hosts with no underlying disease [2]-[6]. Pulmonary nocardiosis often occurs with chronic lung diseases such as chronic obstructive pulmonary disease (COPD) and bronchiectasis [2]-[4].

Mycobacterium avium complex (MAC) lung disease is becoming more prevalent [7]. It often occurs in immunocompromised hosts such as patients with HIV and in immunocompetent hosts with chronic lung diseases such as COPD and bronchiectasis; however, around 20% of MAC patients have no underlying disease [8]. A subset of patients with MAC lung disease may also experience pulmonary nocardiosis. Previous reports have described a case of pulmonary Nocardia farcinica infection in a patient with Mycobacterium intracellulare infection and bronchiectasis [9] and a case of pulmonary Nocardia asteroides infection in a patient with MAC infection, following allogeneic bone marrow transplantation [10]. Co-infection with N. asteroides and MAC has also been reported in a patient with AIDS [11]. Another report described a case of empyema caused by Mycobacterium tuberculosis, Nontuberculous mycobacteria (species not identified), and N. asteroides isolated from pleural effusion in a middle-aged woman with systemic lupus erythematosus, treated with long-term corticosteroids [12]. There has been no similar report of pulmonary nocardiosis caused by Nocardia cyriacigeorgica in patients with MAC lung disease.

To the best of our knowledge, this is the first report of two cases of pulmonary N. cyriacigeorgica infection with MAC lung disease caused by M. avium, including an immunocompromised 72-year-old man with rheumatoid arthritis (RA), treated with methotrexate for several years (Case 1), and an immunocompetent 53-year-old woman (Case 2).

Case presentation

Case 1 is that of a 72-year-old Japanese man with untreated MAC lung disease caused by M. avium, which was followed 2 years later with a clinical diagnosis of RA; he was referred to a university hospital in 2009. Treatment with methotrexate was initiated for high RA disease activity. Although MAC was continuously isolated from his sputum, he was carefully followed without medical treatment because smear tests were negative, he had no respiratory symptoms, and lung infiltrates were stable. After 3 years of methotrexate therapy, Nocardia species appeared and were continuously isolated from his sputum, which was smear-negative but culture-positive for MAC. His sputum amount increased and chest X-ray imaging and computed tomography (CT) revealed exacerbation of infiltrates in the upper lobes on both sides and in the right middle lobe of the lungs (Figure 1A and B). He had no smoking history and neither cutaneous nor central nervous system nocardiosis. His laboratory findings were almost normal, including normal interferon-gamma release assays (IGRA), with the exception of a high titer of anti-tuberculous glycolipid (TBGL) antigen antibody (19.0 U/mL) and anti-glycopeptidolipid (GPL) core antigen immunoglobulin A (IgA) antibody (4.6 U/mL). He started receiving treatment with trimethoprim/sulfamethoxazole (TMP/SMX) in August 2012. His symptoms and the infiltrates improved after 3 months of therapy (Figure 1C and D).

Figure 1
figure 1

Chest X-ray and chest computed tomography (CT) findings in Case 1. Chest X-ray and chest CT findings examined at the time of therapeutic initiation with trimethoprim-sulfamethoxazole (A and B) and after three months of therapy (C and D).

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Case 2 is that of a 53-year-old Japanese woman referred to a university hospital as an outpatient because of an abnormal shadow upon chest X-ray imaging in 2001. She was a former smoker with a 6-pack-year smoking history (4 cigarettes per day for 30 years). The forced expiratory volume in 1 second/forced vital capacity (FEV1/FVC) ratio was slightly decreased (68.7%). Since her sputum smear and culture were positive for MAC and chest CT revealed widespread bilateral nodular bronchiectatic pulmonary infiltrates, she was initiated on combined therapy with clarithromycin (400 mg/day), rifampicin (600 mg/day), ethambutol (1000 mg/day), and levofloxacin (400 mg/day) shortly after diagnosis in 2001. The sputum MAC culture turned negative 1 year after initiating therapy and she completed 2 years of treatment in 2003. After four treatment-free years, Nocardia species appeared in her sputum in 2007. We continued careful follow-up without medical treatment because she was immunocompetent and showed no symptoms except for a very mild cough; her sputum culture was often negative for Nocardia species and lung infiltrates were stable. Chest X-ray imaging and CT in 2013 revealed an exacerbation of infiltrates in the right middle lobe, a small cavity in the right lower lobe, and centrilobular nodules in the left upper lobe of the lungs (Figure 2C and D) in comparison to images obtained 15 months before exacerbation of the infiltrates (Figure 2A and B). She was admitted to the University Hospital for bronchoscopy. Her physical examination was unremarkable and she had neither cutaneous nor central nervous system nocardiosis. Her laboratory findings were almost normal, including normal IGRA, except for a high titer of anti-TBGL antigen antibody (9.9 U/mL) and anti-GPL core antigen IgA antibody (>10.00 U/mL). Bronchial washing, bronchial curettage, and transbronchial lung biopsy through right B5 and B6 were performed. Nocardia species but not MAC were cultured from samples of the bronchial wash. The pathogen responsible for the exacerbated infiltrates on chest CT was accordingly considered to be Nocardia. The patient is being followed closely; the decision to treat for Nocardia remains pending a change in her status.

Figure 2
figure 2

Chest X-ray and chest computed tomography (CT) findings in Case 2. Chest X-ray and chest CT findings 15 months before the exacerbation of lung infiltrates (A and B) and at the time of exacerbation (C and D).

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In both cases, Nocardia species isolated from sputum were identified as N. cyriacigeorgica by 16S ribosomal RNA gene sequencing. Table 1 shows the minimum inhibitory concentration (MIC) of the indicated antibiotics against N. cyriacigeorgica in each case. Susceptibility testing was performed according to Clinical and Laboratory Standards Institute (CLSI) document M24-A [13]; the specific susceptible breakpoint of TMP/SMX for Nocardia species was ≤2/38 and the specific resistance breakpoint was ≥4/76. The MICs of TMP/SMX for N. cyriacigeorgica were 0.5/9.5 in Case 1 and 8/152 in Case 2. Thus, the N. cyriacigeorgica isolate in Case 1 was regarded as sensitive to TMP/SMX, while the isolate in Case 2 was regarded as resistant.

Table 1 Antimicrobial susceptibilities of Nocardia cyriacigeorgica isolated from sputum cultures
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Discussion

Pulmonary nocardiosis is the most frequent presentation of infection by Nocardia species [1], associated with opportunistic infections in immunocompromised hosts, particularly in those with impaired cell-mediated immunity, although immunocompetent hosts can also present with pulmonary nocardiosis. In spite of the fact that Nocardia infection is relatively rare, early diagnosis and treatment is important because of its poor prognosis in selected cases. A few reports have described pulmonary nocardiosis in patients with nontuberculous mycobacterial lung infection [9]-[12]. Here, we report two cases of pulmonary N. cyriacigeorgica infection in patients with MAC lung disease.

Accumulating evidence suggests there are defined risk factors for Nocardia species infection: HIV infection [5], corticosteroid therapy or use of immunosuppressive agents [5],[6], organ transplantation [6], and diabetes mellitus [3]. Preexisting rheumatic disease treated with immunosuppressant therapy, as in Case 1, is another risk factor for Nocardia species infection [14]. Preexisting pulmonary diseases such as COPD and bronchiectasis are additional risk factors for pulmonary nocardiosis, which occur when bacterial colonization in the lower respiratory tract alters ciliary motility and causes epithelial damage [4]. It is unclear whether preexisting nontuberculous mycobacterium is an independent risk factor for pulmonary nocardiosis, although colonization in the lower respiratory tract may also alter ciliary motility and cause epithelial damage as well as COPD and bronchiectasis. Intrinsic defects in airway clearance may also contribute to the susceptibility to pulmonary nontuberculous mycobacterial infection and nocardiosis. Indeed, a previous report demonstrated that mutations in the cystic fibrosis transmembrane regulator likely contribute to susceptibility and pathogenesis in adults with bronchiectasis and pulmonary nontuberculous mycobacterial infection [15]. Airway clearance therapies are likely to function in synergy with antimicrobial therapy to accelerate resolution of airway infections. In Case 2, the patient had a long-term history of smoking (4 cigarettes per day for 30 years) and the FEV1/FVC ratio was slightly reduced at the initial visit. Although chest CT did not show pulmonary emphysema, she may have a chronic bronchitis form of mild COPD and mild bronchiectasis. Preexisting COPD, bronchiectasis, and MAC lung disease may contribute to the co-infection with Nocardia in Case 2.

The Nocardia species isolated from our patients was N. cyriacigeorgica, a recently identified species confirmed by 16S ribosomal RNA gene sequencing. N. cyriacigeorgica was first classified in 2001 by Yassin et al. as a novel species distinguished by molecular and biochemical analysis from previously identified members of the genus Nocardia such as N. asteroides [16]. A recent study showed that a quarter of the strains from Japan and Thailand that were identified as N. asteroides in fact belonged to N. cyriacigeorgica (27 of 121 cases) [17]. In addition, N. cyriacigeorgica is the most frequently isolated strain in Taiwanese patients with pulmonary nocardiosis (10 of 20 cases) [18]. N. cyriacigeorgica infection may be most common in East and Southeast Asia. Larger populations should be studied to improve our understanding of the characteristics of pulmonary N. cyriacigeorgica infection.

Nocardia species co-infection may be underestimated in patients with MAC lung disease because Nocardia and MAC infection often develop in immunocompromised hosts and in selected immunocompetent hosts with chronic lung disease. Chest CT findings in MAC lung disease, such as nodular bronchiectatic lesions, often resemble those in Nocardia species infection; thus, clinicians may not suspect the relatively rare Nocardia species co-infection. Nocardia species also grow slowly and positive cultures may take up to 4 weeks to appear; thus, clinicians may not obtain positive culture results for Nocardia species unless they proactively suspect Nocardia infection and incubate the samples for longer than normal.

TMP/SMX is widely used in first-line therapy of pulmonary nocardiosis. Amikacin, imipenem, third-generation cephalosporins, minocycline, netilmicin, and amoxicillin-clavulanic acid are also effective against Nocardia isolates in vitro and can be used as alternative antimicrobial agents [4]. However, no randomized prospective controlled trials have been performed to determine the most appropriate therapeutic agent, route of administration, and treatment duration for patients with pulmonary nocardiosis [2]. N. cyriacigeorgica isolated in Case 2 was resistant to TMP/SMX, according to the CLSI definition of susceptibility. TMP/SMX may be excluded as a candidate drug for treatment of pulmonary nocardiosis in Case 2. However, susceptibility test results do not always correlate with clinical outcomes of treatment for Nocardia [19]. In fact, a favorable response was reported in 3 of 4 cases (75%) with TMP/SMX-resistant strains [19]. Therefore, TMP/SMX remains a candidate antimicrobial agent to treat pulmonary nocardiosis in Case 2.

Conclusions

This is the first report of pulmonary nocardiosis due to N. cyriacigeorgica in patients with MAC lung disease. Nocardia species co-infection may be underestimated in patients with MAC lung disease and N. cyriacigeorgica may be the main Nocardia species co-incident with MAC lung disease.

Consent

Written informed consent was obtained from each patient for publication of this Case report. Copies of the written consents are available for review by the Editor of this journal.