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Mycobacterium avium subsp. intracellulare
Mycobacterium avium subsp. intracellulare bacteria in lymph tissue.
Scientific classification
Kingdom: Bacteria
Phylum: Actinobacteria
Order: Actinomycetales
Family: Mycobacteriaceae
Genus: Mycobacterium
Species: M. avium
Subspecies: M. a. intracellulare

Mycobacterium avium complex (MAC) is a group of genetically related bacteria belonging to the genus Mycobacterium. It includes Mycobacterium avium subspecies avium (MAA), Mycobacterium avium subspecies hominis (MAH), and Mycobacterium avium subspecies paratuberculosis (MAP).

Historically, MAC has also included Mycobacterium avium intracellulare (MAI) – a distinct species of bacteria.


Sources and symptoms

MAC bacteria are common in the environment and cause infection when inhaled or swallowed. Symptoms of MAC diseases are reminiscent of tuberculosis. They include fever, fatigue, and weight loss. Many patients will have anemia and neutropenia if bone marrow is involved. Pulmonary involvement is similar to TB, while diarrhea and abdominal pain are associated with gastrointestinal involvement. MAC should always be considered in a person with HIV infection presenting with diarrhea. Recently M. avium has been found to deposit and grow in bathroom shower heads from which it may be easily aerosolized and inhaled.[1]

The various subspecies are prevalent in different areas:

Mycobacterium avium infection in children

The M. avium and M. haemophilum infection in children form a distinct clinical entity, not associated with abnormalities of the immune system. M. avium typically causes swelling of the lymph nodes of the neck that is usually unilateral. The treatment of choice is surgical excision of the affected lymph nodes,[2] with antibiotic treatment (usually clarithromycin and rifabutin for 18 to 24 months) reserved for those patients who cannot have surgery.

MAC in patients with HIV infection

MAC in patients with HIV disease is theorized to represent recent acquisition rather than latent infection reactivating (which is the case in many other opportunistic infections in immunocompromised patients).

The risk of MAC is inversely related to the patient's CD4 count, and increases significantly when the CD4 count decreases below 50 cells/mm³. Other risk factors for acquisition of MAC infection include using an indoor swimming pool, consumption of raw or partially cooked fish or shellfish, bronchoscopy and treatment with granulocyte stimulating factor.

Disseminated disease was previously the common presentation prior to the advent of highly active antiretroviral therapy (HAART). Today, in regions where HAART is the standard of care, localized disease presentation is more likely. This generally includes a focal lymphadenopathy/lymphadenitis.



Diagnosis can be achieved through blood cultures, or cultures of other bodily fluids such as sputum. Bone marrow culture can often yield an earlier diagnosis, but is usually avoided as an initial diagnostic step because of its invasiveness.


Treat prophylactically with Azithromycin.

Post-infection, treatment involves a combination of anti-tuberculosis antibiotics. These include:

MAC in immunocompetent patients

Please see information on nontuberculous mycobacteria (NTM) disease.


  1. ^ Showerheads may harbor bacteria dangerous to some By RANDOLPH E. SCHMID, AP Science Writer Randolph E. Schmid, Ap Science Writer – Mon Sep 14, 9:19 pm ET
  2. ^ Lindeboom JA, Kuijper EJ, van Coppenraet ESB, Lindeboom R, Prins JM (2007). "Surgical excision versus antibiotic treatment for nontuberculous mycobacterial cervicofacial lymphadenitis in children: A multicenter, randomized, controlled trial". Clin Infect Dis 44: 1057–64. doi:10.1086/512675. 


  • Diagnosis and treatment of disease caused by nontuberculous mycobacteria. This official statement of the American Thoracic Society was approved by the Board of Directors, March 1997. Medical Section of the American Lung Association. Am J Respir Crit Care Med 1997; 156:S1.
  • Teirstein, AS, Damsker, B, Kirschner, PA, et al. Pulmonary infection with MAI: Diagosis, clinical patterns, treatment. Mt Sinai J Med 1990; 57:209.

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