Nontuberculous mycobacterial (NTM) lung disease is a chronic respiratory infection caused by a diverse group of bacteria found ubiquitously in the environment. Unlike the organism that causes Tuberculosis, NTM are generally not spread from person to person, instead infecting individuals whose lungs are already susceptible. This progressive condition can be debilitating, causing damage to the lung tissue that affects breathing over time. Because the symptoms are often similar to other respiratory illnesses, diagnosis can be challenging and frequently delayed, requiring a long-term commitment to specialized, multi-drug treatment plans.
Non-Tuberculous Mycobacteria: Source and Infection
Nontuberculous mycobacteria are a family of over 200 species of bacteria that exist naturally in the world around us. These organisms thrive in soil, dust, and particularly in water sources, including natural bodies of water and household plumbing systems. The most common cause of NTM lung disease globally is the Mycobacterium avium complex (MAC), but other species like Mycobacterium abscessus also cause significant disease. Exposure to these bacteria is a daily occurrence for most people, but only a small fraction develop a progressive infection.
Infection occurs when a susceptible person inhales aerosolized particles containing the bacteria, such as those generated by showerheads, hot tubs, or contaminated household water. The NTM strains isolated from patients are often genetically identical to those found in their home water supply. The bacteria form resilient clusters called biofilms on surfaces, making them difficult to eliminate from engineered water systems, which serve as a reservoir for human exposure. NTM species are categorized by their growth rate, such as MAC being “slow growers” and M. abscessus being “rapid growers,” a distinction that influences the severity and treatment of the disease.
Recognizing the Signs and Identifying Risk Factors
The signs of NTM lung disease can be subtle and mimic those of more common respiratory conditions, contributing to difficulty in early recognition. The most frequently reported symptom is a persistent, chronic cough, which may or may not produce mucus or blood. Other generalized symptoms include profound fatigue, unexplained weight loss, and night sweats. As the infection progresses, individuals may also experience shortness of breath and chest discomfort.
A person’s underlying health status is the most significant factor determining susceptibility to NTM lung disease. Individuals with pre-existing structural lung diseases are at the highest risk, particularly those with bronchiectasis, chronic obstructive pulmonary disease (COPD), or cystic fibrosis. These conditions compromise the lungs’ natural ability to clear mucus and pathogens, allowing the NTM to establish an infection. Advanced age and any condition that weakens the immune system also increase the risk.
Disease Patterns
Some individuals develop a form of the disease characterized by small nodules and widening of the airways (bronchiectasis), often noted in older, thin, non-smoking women without a prior history of lung disease. Conversely, a more severe, destructive form that creates cavities in the lung tissue is often seen in older men with a history of smoking and COPD.
Confirming the Diagnosis
The diagnosis of NTM lung disease requires satisfying three distinct criteria: clinical, radiographic, and microbiological evidence. The clinical criterion is met by the presence of non-specific symptoms, such as chronic cough, fatigue, and weight loss, which prompt the initial investigation. The second criterion involves radiographic findings, typically obtained through a chest X-ray or, more commonly, a high-resolution computed tomography (CT) scan. These images reveal characteristic changes in the lung structure, such as nodules, cavities, or the abnormally widened airways associated with bronchiectasis.
The final step is microbiological confirmation, which involves culturing the bacteria from respiratory samples. Because NTM are common in the environment, a single positive sample could be contamination, meaning the patient is only colonized and not truly infected. To confirm active disease, guidelines require at least two separate positive sputum cultures, or a positive culture from a bronchial washing collected via bronchoscopy. This requirement ensures the organism is actively causing disease.
The laboratory must identify the specific NTM species, as this significantly influences the treatment strategy. A thorough diagnostic workup is also performed to rule out other conditions with similar presentations, especially Tuberculosis. The process of collecting multiple samples and waiting for the slow-growing bacteria to culture often contributes to the diagnostic delay experienced by patients.
Management and Treatment Regimens
The management of NTM lung disease is prolonged and complex, presenting a significant burden due to the treatment duration and potential for side effects. Treatment is generally reserved for patients who meet all three diagnostic criteria and whose disease is progressive or severe. The goal of therapy is to eradicate the bacteria and ensure sputum cultures remain negative. This requires a sustained commitment to medication, typically lasting for at least 12 months after the first negative culture is achieved, often translating to 18 to 24 months of total treatment.
A multi-drug regimen is the standard of care for most NTM infections, especially those caused by MAC. It typically centers on a macrolide antibiotic combined with two or more other drugs, such as rifampin and ethambutol. The combination of drugs is necessary to overcome the bacteria’s natural resistance and prevent further drug resistance. The specific regimen depends on the identified species; for example, M. abscessus requires particularly aggressive and sometimes intravenous treatment.
Drug susceptibility testing (DST) is performed to determine which medications are most effective against the specific strain of NTM, which is helpful in guiding treatment for rapid-growing mycobacteria. Given the length of the treatment, drug side effects are a major concern and require close monitoring. Non-pharmacological management also plays an important role, including airway clearance techniques, specialized physiotherapy, and nebulized hypertonic saline, recommended to help clear the infected mucus from the lungs.