Mycobacterium avium-intracellulare, commonly referred to as MAC, denotes a group of closely related bacterial species within the genus Mycobacterium. This complex primarily includes Mycobacterium avium and Mycobacterium intracellulare. MAC is classified as a nontuberculous mycobacterium (NTM), distinguishing it from Mycobacterium tuberculosis, which causes tuberculosis.
Sources and Transmission
MAC bacteria are ubiquitous, found extensively in natural water sources, soil, and dust. They are also common residents in treated water systems, including household plumbing, showerheads, hot tubs, and spas. Human infection primarily occurs through inhaling aerosolized water droplets or dust particles containing bacteria. Less commonly, ingesting contaminated water or food can also lead to exposure. MAC is not considered contagious.
Populations at Risk
Not everyone exposed to MAC bacteria develops an infection. Individuals with pre-existing structural lung diseases face an increased risk of pulmonary MAC infection, including conditions such as bronchiectasis, chronic obstructive pulmonary disease (COPD), and cystic fibrosis.
Another group at elevated risk comprises those with compromised immune systems. People with advanced HIV/AIDS, particularly when their CD4 cell counts fall below 50 cells/µL, are highly susceptible to MAC infections. Organ transplant recipients and individuals undergoing chemotherapy also have weakened immune defenses, making them more vulnerable.
A distinct demographic susceptible to pulmonary MAC infection includes older, slender women, a presentation sometimes referred to as “Lady Windermere syndrome”. These individuals may not have a history of traditional lung disease or immune compromise.
Manifestations of Infection
MAC infection can present in several ways, depending on the affected body system and the individual’s overall health. Pulmonary disease is the most frequently observed form, primarily affecting the lungs. Symptoms often develop slowly over weeks to months and include a persistent cough (which may be productive), fatigue, shortness of breath, unintentional weight loss, and night sweats.
Disseminated disease, where the infection spreads throughout the body, almost exclusively affects individuals with severely weakened immune systems, such as those with advanced HIV/AIDS. This systemic form can cause persistent high fever, anemia due to bone marrow involvement, chronic diarrhea, and abdominal pain.
Lymphadenitis, characterized by swollen lymph nodes, is another manifestation of MAC infection, most commonly observed in young children. These swollen nodes typically appear in the neck region. This form of infection is generally localized.
Diagnosis and Medical Evaluation
Confirming a MAC infection requires careful evaluation, as its environmental presence means detection alone does not always indicate active disease. Imaging studies are a routine part of the evaluation. Chest X-rays sometimes reveal abnormalities, but high-resolution computed tomography (CT) scans of the chest are more sensitive and can identify characteristic changes in the lungs, such as multifocal bronchiectasis or small nodules, which suggest MAC pulmonary disease.
Microbiological evaluation is central to diagnosis, particularly for pulmonary infections. Multiple sputum (phlegm) samples are collected on separate days; at least two positive cultures for MAC are generally required to confirm an active pulmonary infection. For suspected disseminated disease, blood cultures are performed to isolate the bacteria. Molecular tests, such as PCR, can also aid in faster identification once mycobacteria are detected.
In some instances, when sputum samples are inconclusive or if the infection is suspected in other tissues, a biopsy may be necessary. This involves taking a tissue sample from an affected lymph node or lung lesion for microscopic examination and culture.
Treatment Approaches
Treating a confirmed MAC infection is a lengthy and complex process. Therapy consistently involves a combination of multiple antibiotics taken simultaneously. This multi-drug approach, typically including a macrolide (like azithromycin or clarithromycin), ethambutol, and a rifamycin (such as rifabutin or rifampin), is employed to enhance effectiveness and minimize the development of drug resistance.
The duration of antibiotic therapy is extensive, usually continuing for a minimum of 12 months after sputum cultures consistently test negative for MAC. This means the total treatment period often extends to 18 months or even longer, depending on the individual’s response and the resolution of symptoms. For severe or resistant cases, additional medications such as parenteral aminoglycosides like amikacin may be incorporated into the regimen.
Adherence to the prescribed medication regimen for the full duration is paramount for successful treatment outcomes. Consistent and complete medication intake helps ensure the infection is eradicated and reduces the likelihood of relapse. Interruptions or early discontinuation of therapy can lead to treatment failure and the emergence of drug-resistant strains.