Mycobacterium avium complex infection

Mycobacterium avium complex (MAC) infection is a bacterial disease caused by common germs found in water, soil, dust, and food. While almost everyone is exposed to these bacteria, they typically only cause illness in people with weakened immune systems or existing lung disease.

Table of contents

• What is Mycobacterium avium complex?
• Other names for this infection
• The bacteria that cause MAC
• Where MAC bacteria are found
• Who gets MAC infection?
• Types of MAC infection
• Symptoms of MAC infection
• How MAC is diagnosed
• Treatment of MAC infection
• What to expect with MAC

What is Mycobacterium avium complex?

Mycobacterium avium complex (MAC) is a group of bacteria that belongs to a larger family called nontuberculous mycobacteria (NTM), which are bacteria related to tuberculosis but different from it^[1]. MAC consists primarily of two bacterial species: Mycobacterium avium and Mycobacterium intracellulare. These two species are so similar that laboratories cannot tell them apart without special genetic testing^[1].

MAC is the most common cause of nontuberculous mycobacterial infections in humans worldwide^[1][6]. The respiratory system is the most common site of infection, although MAC can affect other parts of the body as well^[1].

Mycobacterium avium-intracellulare (MAI), MAC, MAI

Other names for this infection

Mycobacterium avium complex infection is also known as Mycobacterium avium-intracellulare (MAI) infection^[2][4]. When the infection spreads throughout the body, it may be called disseminated MAC or DMAC^[2][4].

The bacteria that cause MAC

MAC bacteria are nonmotile, non-spore-forming, gram-positive acid-fast bacilli^[1]. This means they have a special waxy coating that makes them resistant to many standard antibiotics. The bacteria are slow-growing and take about 10 to 20 days to develop mature colonies in laboratory cultures^[1].

These bacteria grow best at specific temperatures: Mycobacterium avium prefers 34.5°C, while Mycobacterium intracellulare grows best at 31.5°C. However, MAC bacteria can grow in temperatures ranging from 28°C to 38.5°C^[1].

A newer member of the Mycobacterium avium complex is Mycobacterium chimaera, which was added to the complex in 2004^[1][5]. Another species, Mycobacterium paraintracellulare, was identified in pulmonary infections in Southeast Asia in 2016^[1].

Where MAC bacteria are found

MAC bacteria are very common in the environment. They have been found in fresh water and salt water worldwide^[1][5]. Common environmental sources include piped plumbing systems in homes and hospitals, bathrooms, hot tubs, aerosolized water, house dust, soil, birds, farm animals, and even cigarette components such as tobacco, filters, and paper^[1].

Because these bacteria are found in water, soil, dust, and food, almost everyone has them in their bodies^[2][4]. However, in people with strong immune systems, the bacteria don’t cause problems.

Who gets MAC infection?

MAC bacteria typically only cause illness in people whose immune systems are weakened or who have existing lung disease^[2][3]. People at higher risk include those with:

• HIV/AIDS, especially when CD4 cell counts drop below 50 cells/mm³^[2][4]
• Chronic lung diseases such as cystic fibrosis, chronic obstructive pulmonary disease (COPD), or bronchiectasis^[5]
• Hairy cell leukemia or other blood cancers^[3]
• Immunosuppression from chemotherapy or organ transplants
• Primary immune deficiency syndromes

Certain lifestyle and health factors also increase risk, including underlying lung disease, heavy smoking, and excessive alcohol consumption^[10].

MAC almost never causes disease in people with CD4 cell counts above 100 cells/mm³^[4]. Up to 50% of people with AIDS may develop MAC infection, particularly when their immune systems are severely weakened^[4].

Types of MAC infection

MAC traditionally causes three main disease syndromes in humans^[10]:

Pulmonary MAC disease affects the lungs and is the most common form of infection^[1]. This type primarily affects people with existing lung damage or disease.

Cervical lymphadenitis is an infection of the lymph nodes, particularly in the neck. This form most commonly affects children under 5 years of age^[10].

Disseminated MAC disease occurs when the infection spreads throughout the body^[2][4]. This serious form typically affects people with severely weakened immune systems, particularly those with AIDS. The infection can spread to lymph nodes, the central nervous system, liver, spleen, bone marrow, intestines, and bones^[3].

A fourth type, called hot-tub lung, is a hypersensitivity pneumonitis (an allergic lung reaction) associated with hot tub use^[10].

• Lungs
• Lymph nodes
• Bone marrow
• Liver
• Spleen
• Intestines
• Central nervous system

Symptoms of MAC infection

The symptoms of MAC infection depend on which part of the body is affected and whether the infection is localized to one area or spread throughout the body^[2][4].

Common symptoms of pulmonary (lung) MAC infection include:

• Chronic productive cough^[10]
• Difficulty breathing or shortness of breath^[10]
• Fatigue^[2]
• Weight loss^[10]

When MAC spreads throughout the body (disseminated infection), symptoms may include^[2][4]:

• High fever or chills
• Night sweats
• Abdominal pain
• Diarrhea
• Weight loss
• Fatigue
• Swollen glands (lymphadenopathy)
• Anemia (fewer red blood cells)

More serious symptoms can include blood infections, hepatitis, and pneumonia^[2].

How MAC is diagnosed

Diagnosing MAC can be challenging because many other infections cause similar symptoms^[2]. Getting the right diagnosis is important for proper treatment. In areas with high rates of tuberculosis, MAC infections are often mistakenly diagnosed and treated as tuberculosis^[3].

Your doctor may order several tests to find MAC bacteria in your body^[2][4]:

Laboratory cultures involve collecting samples from your blood, urine, sputum (thick fluid from your lungs), bone marrow, or tissue. These samples grow in a laboratory for several weeks, then a technician checks them for signs of MAC bacteria^[2].

Acid-fast bacillus (AFB) stains and cultures of sputum specimens are used when pulmonary MAC infection is suspected^[3].

Blood tests can check for problems such as anemia and liver disease^[2].

Imaging tests such as chest X-rays or CT scans of your chest and abdomen can help your doctor see problems with your lymph nodes, liver, or spleen^[2].

Biopsy involves taking a tissue sample and examining it under a microscope^[2].

Diagnosis of pulmonary MAC disease in people without HIV requires repeated isolation of the bacteria from respiratory secretions, along with compatible symptoms and findings on chest imaging^[10].

Treatment of MAC infection

Treatment for MAC depends on where the infection is located, how serious your symptoms are, and other medical conditions you may have^[9]. Because MAC bacteria are slow-growing and some infections go away on their own, your doctor may not start treatment right away if you have a mild infection^[9]. This approach, called watchful waiting, involves regular checkups to monitor how you’re doing.

When treatment is needed, MAC infection cannot be cured with just one antibiotic. You will need to take a combination of antibiotics so your body doesn’t become resistant to any single drug^[2][7].

The standard treatment regimen consists of three antibiotics^[7][13]:

• A macrolide antibiotic: either azithromycin or clarithromycin
• Ethambutol
• A rifamycin: rifampin or rifabutin

In general, MAC infection is treated with at least two or three antimicrobials for at least 12 months^[7][13]. For pulmonary MAC disease, treatment should be maintained for at least 12 months after negative sputum culture conversion (when bacteria no longer appear in sputum samples)^[6][13].

Additional antibiotics that might be needed include amikacin, moxifloxacin, and clofazimine^[2][13]. For lung infections that don’t improve within 6 months, your doctor may recommend an antibiotic you breathe in called amikacin liposome inhalation suspension^[9].

If you have HIV, it’s important to start antiretroviral therapy (ART) right away if you haven’t already. You can do this at the same time as your antibiotic treatment^[9]. You may need to take preventive antibiotics until your immune system gets stronger.

MAC medications can cause side effects such as^[2]:

• Nausea, vomiting, or diarrhea
• Abdominal pain
• Eye problems (with ethambutol)
• Rashes or itching
• Anemia
• Hearing loss
• Numbness in feet
• Headache

Tell your doctor about any side effects that make you feel bad, as they may need to change the type of antibiotic you take.

Airway clearance techniques are important if you have lung involvement. These techniques help clear mucus from your airways and may include breathing exercises, special devices, or physical therapy^[9].

In some cases of difficult-to-treat MAC lung disease, surgical removal of affected lung tissue may improve treatment outcomes. However, surgery should only be performed in well-equipped centers after careful consideration of risks and benefits^[6][13].

What to expect with MAC

Generally, the earlier you are diagnosed with MAC infection, the better your outlook^[12]. However, diagnosis often doesn’t happen right away because the infection can look like pneumonia or other lung conditions.

MAC infection is often hard to get rid of. You will most likely need treatment for a year or more, and the infection sometimes comes back after treatment^[12]. The rates of treatment default and disease recurrence after treatment completion remain high^[6][13].

Your outlook depends on many factors, including^[12]:

• Whether you have other health conditions affecting your lungs or immune system
• The presence of cavitary lung disease (holes in the lungs)
• Your age and general health
• Your nutritional status and body weight
• How extensive the disease is

Important prognostic factors for disease progression include low body mass index, poor nutritional status, presence of cavitary lesions, extensive disease, and positive acid-fast bacilli smear results^[6][13].

One review of studies found that 1 in 4 people with MAC die within 5 years, though the cause of death wasn’t necessarily the infection itself, since many people with MAC have other health conditions^[12].

New therapies, including clofazimine, inhaled amikacin, and bedaquiline, have shown promising results for treating MAC pulmonary disease, especially in patients with treatment failure or macrolide-resistant MAC^[6][13].