Mycobacterium avium Complex

Etiology

M. avium complex is a nonmotile, non-spore-forming, gram-positive acid-fast bacillus. M. avium complex is a nonchromogen and slow-growing and takes about 10 to 20 days to develop mature colonies. M. avium complex belongs to class III of the Runyon classification. M. avium grows best at 34.5 C, and Mycobacterium intracellulare grows best at 31.5  C. However, M. avium complex components can grow between 28 C to 38.5 C. Most M. avium can survive 49 C. Whereas only 10% of M. intracellulare survive a temperature of 49 C. Mycobacterium scrofulaceum is similar to Mycobacterium avium complex in the biochemical properties and the same group. M. avium is composed of 4 named subspecies; these are M. avium subspecies Avium (2 strains), M. avium subspecies silvaticum, M. avium subspecies paratuberculosis, and M. avium subspecies hominissuis. It appears that the M. avium subspecies avium is responsible for pulmonary infections, and the M. avium subtype hominissuis appears to be gastrointestinal in origin.[4] M. avium subspecies paratuberculosis was identified in ruminants as a causative agent for Johne disease. Some have hypothesized and debated for decades that M. avium subspecies paratuberculosis is the etiological agent of Crohn disease in humans without strong evidence. A recent publication found a high prevalence of M. avium subspecies paratuberculosis antibodies in inflammatory bowel diseases when compared to patients with noninflammatory bowel diseases, 64% vs. 9.7%. Live M. avium subspecies paratuberculosis was only isolated from 2 Crohn disease patients. M. avium subspecies paratuberculosis has also been reported to trigger autoimmunity, including multiple sclerosis and type 1 diabetes mellitus.

In 2004, Mycobacterium chimaera was identified as having a unique genetic composition similar to that of the Mycobacterium avium complex but different from both M. avium and M. intracellulare. Recently, contamination of the heater-cooler system during cardiac surgery resulted in some device-related infections due to M. chimaera. The biochemical reactions with Mycobacterium avium complex are catalase-positive, negative for niacin, nitrate reduction, and tween hydrolysis. The biochemical methods are inadequate for clinical use. Currently, molecular techniques have been applied, and a Mycobacterium avium complex polymerase chain reaction multiplex has been developed, which can detect the individual components of the Mycobacterium avium complex. Restriction fragment length polymorphism and multilocus sequence typing are some other ways to identify and diagnose Mycobacterium avium complex infections.[5][6][4]

Epidemiology

Mycobacterium avium complex is ubiquitous and reported in the Americas, Asia, and Europe. There are pockets of high prevalence throughout the world. In the United States, the prevalence varies from 1.4 to 6.6 per 100,000 population, but no endemic area has been recognized. Recently, a trend towards increasing Mycobacterium avium complex infections has been identified. A seasonal trend in mycobacterial infections around the incidence cycle of 12 months, with peaks in late winter/spring and troughs in autumn, was noted. Women had a higher prevalence, up to 1.6 fold relative to men. A study done in Australia has shown a higher male prevalence (OR 2.1). The differences may be due to the nature of pulmonary involvement. Women have increased predilection for nodular/bronchiectatic disease, also known as Lady Windermere syndrome. Mycobacterium avium complex has been isolated from the environment, including soil, aerosolized water, bathrooms, house dust, birds, farm animals, hot water systems, cigarette components, and house dust. The ecological niche of this organism has not been identified.[7][8]

Pathophysiology

MAC is acquired by inhalation and can also be ingested into the gastrointestinal tract, which adheres to the mucosal epithelial cells and infects the macrophages. From the submucosal tissues and lymph nodes, the organism is carried by lymphatics to the rest of the body. In most people, disseminated MAC infection occurs when the CD4 count is less than 50 cells per microliter. In patients who do not have HIV, the most important risk factor for MAC infection is underlying lung disease. MAC has also been associated with bronchiectasis and a hypersensitivity pneumonitis-like reaction.[9]

History and Physical

Mycobacterial infections, including Mycobacterium avium complex infections, can be categorized into several clinical patterns, including pulmonary disease, skin and soft tissue infections, musculoskeletal infections, disseminated disease, catheter-associated disease, and lymphadenitis. Pulmonary disease is the most common presentation. Mycobacterium avium complex infections occur in both immunocompetent and immunosuppressed patients. M. avium is the most frequent organism in HIV and immunosuppressed patients; about 40% of pulmonary infections in immunocompetent patients can be due to M. intracellulare. With the addition of new members to the Mycobacterium avium complex, the extent of involvement of each organism within the Mycobacterium avium complex is unknown.

Risk factors for Mycobacterium avium complex pulmonary disease are pneumoconiosis, chronic obstructive pulmonary disease, cystic fibrosis, other chronic lung diseases, persons with thoracic and skeletal abnormalities such as severe scoliosis, straight back syndrome, patients with mitral valve prolapse, CD4 less than 50 in AIDS patients, low CD4 in lymphoreticular malignancies, elderly women who suppress cough, immunosuppression post-transplant and in patients with deficiency in IFN-gamma production as well as IFN-gamma receptor deficiency. Siblings of an index patient have a much higher prevalence of Mycobacterium avium complex infection than the general population. There are no known risk factors for cutaneous Mycobacterium avium complex infection and cervical lymphadenitis due to Mycobacterium avium complex.

The Mycobacterium avium complex pulmonary disease can be radiologically classified into fibro-cavitary and nodular bronchiectatic types. Case reports for atypical presentations such as pulmonary nodules, pleurisy, multiple cavitary nodules, and pleural effusion with hydropneumothorax have been published. Pleural involvement may be seen in 5% to 15% of patients. Symptoms in immunocompetent patients are nonspecific, with chronic cough as the most frequent symptom. Fever and hemoptysis are not as frequent as in tuberculosis and are uncommon in HIV patients. Fibro-cavitary disease is more common in Caucasian men, occurs in the setting of underlying structural lung disease, and presents with worsening cough, hemoptysis, and constitutional symptoms. The bronchiectasis with centrilobular nodules has a predilection for the middle lobe and the lingula. In the fibro-cavitary forms, the cavities are thin-walled with a predilection to the upper lobes. Elsewhere, there may be tree-in-bud opacities, suggesting endobronchial spread.

A chest x-ray may not show bronchiectasis very well. A high-resolution computed tomography (HRCT) is more sensitive to changes such as bronchiectasis, small nodules, tree-in-bud appearance, ground-glass opacities, and pleural thickening.  Upper lobe cavitation is less common than pulmonary tuberculosis, and middle lobe bronchiectasis is more frequent in Mycobacterium avium complex pulmonary infections. HRCT may show a feeding bronchus sign, which suggests that peribronchial nodules due to Mycobacterium avium complex infection evolve into focal cystic bronchiectasis and manifest as cavitary lesions; in this regard, the cavities are different from tuberculosis, where cavities are due to caseous necrosis of lung parenchyma. In the nodular-bronchiectatic form, the nodules are smaller than 5 mm to 10 mm and closely associated with areas of bronchiectasis. Both radiologic forms of Mycobacterium avium complex pulmonary infections are prevalent equally, but the nodular bronchiectatic form is more common in elderly Caucasian women, who suppress a cough, while the fibro-cavitary disease is more common in white men with underlying chronic lung disease. Mycobacterium avium complex is also the most frequent nontuberculosis NTM cause of cervical lymphadenitis commonly seen in children. Hypersensitivity pneumonitis-like presentation can occur. Initially thought to be an allergic reaction only, the current opinion may be shifting towards both infection and inflammation. Disseminated infections with Mycobacterium avium complex occur in the setting of AIDS and immunosuppression.

Evaluation

Infection with Mycobacterium avium complex may be asymptomatic. Colonization of the pulmonary tract without infection is unproven in the NTM diseases. Contamination of a sputum sample is possible; therefore, more than 1 sample is required. Symptoms are non-specific, and the differential diagnosis is wide. Microbiologic isolation is required to make the diagnosis. Per the Infectious Disease Society of America (IDSA) recommendations, a minimum evaluation of a patient suspected of nontuberculous mycobacterial infection should include a radiologic, microbiological, and clinical evaluation. Clinically, the exclusion of pulmonary tuberculosis is important. Clinical evaluation of underlying diseases, risk factors for mycobacterial diseases, and the ability to tolerate prolonged multidrug therapy should be undertaken. While the Mycobacterium avium complex is not a part of the lung microbiome, there is no need to treat all patients with sputum-positive for Mycobacterium avium complex. Selecting patients for therapy is a clinical calculus combining microbiologic, radiologic, and clinical criteria. The therapy is long, and there is significant potential for adverse drug reactions.[10][11]

Treatment / Management

The macrolide antibiotic is the backbone of therapy for Mycobacterium avium complex infections. The Infectious Disease Society of America recommended triple antibiotic therapy for fibrocavitary and severe nodular bronchiectatic disease. For moderate to mild disease, dual antibiotic therapy is sufficient. Observation is reasonable, but generally, Mycobacterium avium complex pulmonary infections are progressive, and eventually, a patient has indications for therapy. In this situation, expert opinion suggests that sputum should be checked once in 3 months and radiological evaluation in 6 months. A chest x-ray may be sufficient for the fibro-cavitary disease, but HRCT is needed to assess nodular bronchiectatic disease. Risk factors for progressive disease are cavitary disease, low body mass index, older age, and co-morbidities. The risk of progression must be weighed against the potential risk of treatment.[12][13][14](B2)

The medications used to treat Mycobacterium avium complex infections are macrolide, clofazimine, rifampin, rifabutin, ethambutol, fluoroquinolone, linezolid, and aminoglycosides. There is no proven correlation between in-vitro susceptibility and clinical response. The only susceptibility to macrolide and amikacin may be useful clinically. If the patient is intolerant to first-line drugs, then susceptibility testing to secondary medications may be of some value. The goal of therapy is to have culture negativity for 12 months. Sputum conversion takes 3 to 6 months. Close monitoring for drug intolerance is required. At the initiation of therapy, a baseline audiogram, electrocardiogram, an eye exam for visual acuity and color discrimination, a complete blood count, and a comprehensive metabolic panel must be obtained.

For severe and fibrocavitary disease, a typical 3-drug regimen consists of daily oral azithromycin, rifampin, and ethambutol; intermittent dosing is inadequate in this situation. For severe fibrocavitary disease, parenteral aminoglycoside can be used in the initial phase (first 8 to 16 weeks of therapy) as a fourth agent, but there is no proven benefit. In patients who cannot tolerate parenteral aminoglycoside, inhaled amikacin can be used. For severe localized cavitary disease, surgical resection after the sputum becomes negative can be considered. Surgery is not without risk of formation of a bronchopleural fistula. For mild to moderate nodular bronchiectatic disease, intermittent dosing of the 3-drug regimen can be used. No prospective studies evaluate the efficacy of the macrolide-containing 3-drug or 2-drug regimens. However, long-term sputum conversion rates of 86% have been documented. Small randomized controlled trials comparing 3-drug and 2-drug regimens showed higher treatment failure with 2-drug therapy. The macrolide-containing regime was not evaluated in this trial.

The key to successful therapy is close monitoring for disease worsening and drug-related toxicity. Patients should be followed once every 2 months while on therapy. Introduce 1 drug at a time and a lower dose and increase to a therapeutic dose over 2 to 5 days. Therapeutic drug monitoring is of no proven value. Azithromycin peak levels can be measured in malabsorption or treatment failure or if the dose is low. A peak level of greater than 0.4 mcg/mL was independently associated with a favorable microbiologic response. Rifampin can decrease macrolide levels. A study by Koh WJ et al in 101 patients showed no correlation between clarithromycin level and favorable microbiologic response.

Differential Diagnosis

The differential diagnosis for Mycobacterium avium complex infections includes the following:

• HIV
• Lymphoma
• Fungal infections
• Hypersensitivity pneumonitis
• Sarcoidosis
• Tuberculosis

Complications

The complications that can manifest with Mycobacterium avium complex infections are as follows:

• Severe wasting
• Hepatomegaly
• Splenomegaly
• Generalized lymphadenitis
• Synovitis, tenosynovitis