MAI lung disease is a chronic respiratory condition caused by infection with the Mycobacterium avium complex (MAC), often referred to as MAI. MAC includes the closely related species M. avium and M. intracellulare. This disease is classified as a Nontuberculous Mycobacteria (NTM) infection. Unlike tuberculosis, MAI lung disease is not contagious from person to person.
The Pathogen and Environmental Sources
The causative agents of MAI lung disease, M. avium and M. intracellulare, are ubiquitous in the environment. They are considered opportunistic pathogens, meaning they primarily cause illness in people with pre-existing conditions or weakened immune systems. These bacteria thrive in various natural and man-made settings, making exposure common for nearly everyone.
The most common environmental reservoirs include soil, dust, and water sources. MAC organisms are frequently isolated from piped plumbing systems, such as household and hospital water supplies, bathrooms, and hot tubs. The bacteria form biofilms, which allow them to adhere to surfaces within these water systems and are believed to be a major source of human infection.
Infection occurs when a person inhales or ingests the bacteria through aerosolized water or dust particles. Activities like taking a shower or sitting near an infected hot tub can aerosolize MAC, allowing the microbes to enter the respiratory tract.
Identifying Symptoms and Vulnerable Populations
Symptoms of MAI lung disease are often non-specific, resembling those of other chronic lung conditions. The most frequent complaints include a chronic, persistent cough, unexplained fatigue, and gradual weight loss. Patients may also experience shortness of breath, night sweats, and occasionally cough up blood.
The disease presents in two primary clinical patterns, each affecting different populations. One form is the fibrocavitary disease, which typically affects older men with a history of heavy smoking or pre-existing structural lung damage like chronic obstructive pulmonary disease (COPD). This presentation involves the formation of thin-walled cavities, often in the upper lobes of the lungs, and tends to cause more pronounced symptoms like weight loss and hemoptysis.
The other common form is the nodular/bronchiectatic disease, which disproportionately affects thin, non-smoking older women, often over the age of 50. This presentation involves multiple small nodules and areas of damaged, widened airways known as bronchiectasis, frequently found in the middle lobe of the right lung. These patients may also have underlying conditions like mild scoliosis or pectus excavatum, and some experts believe a reduced urge to cough may contribute to the infection’s establishment.
Individuals with other underlying lung diseases, such as bronchiectasis or cystic fibrosis, or those with weakened immune systems due to conditions like AIDS, are highly susceptible to developing MAI lung disease.
Diagnostic Procedures and Treatment Regimens
Diagnosing MAI lung disease requires a combination of clinical, radiographic, and microbiological evidence. The process begins with a clinical evaluation of the patient’s symptoms and risk factors. Chest imaging, typically a high-resolution computed tomography (CT) scan, is used to visualize the characteristic lung changes, such as nodules, bronchiectasis, or cavities.
The diagnosis is confirmed through laboratory analysis, which requires isolating the MAC organism from the patient’s respiratory secretions. This is most often done by culturing sputum samples, and repeated positive cultures are generally required to distinguish a true infection from simple environmental colonization. Susceptibility testing is also performed to determine which antibiotics will be most effective against the specific strain of MAC.
Treatment for MAI lung disease is complex, lengthy, and typically involves a multi-drug regimen. The standard approach utilizes a combination of three different antibiotics, most commonly a macrolide (like azithromycin or clarithromycin), ethambutol, and a rifamycin (such as rifampin). Azithromycin is often preferred over clarithromycin due to better patient tolerance and fewer drug interactions.
Therapy is administered for at least 12 months after the patient’s sputum cultures convert to negative. This often results in a total treatment time of 18 to 24 months or more. Due to the potential for side effects from the long-term use of multiple drugs, treatment is sometimes reserved for patients with more severe or progressive disease, such as those with cavitary lesions or weight loss.