What is Mycobacterium avium complex (MAC)?

• Mycobacterium avium complex is a group of related acid-fast organisms that grow only slightly faster than M. tuberculosis and can be divided into a number of serotypes.
• Some cause disease in birds; others cause disease in mammals but not birds.
•  They are found in soil and water and in infected animals.

Describe the pathogenesis of infection with MAC in persons infected with HIV.

• Asymptomatic colonization after ingestion or inhalation precedes infection.
• Symptomatic localized infection can occur either in the lung or the G1 tract.
•  The GI tract is probably the most common portal of entry. 
• Focal pneumonias are uncommon in AIDS and dissemination usually ensues.
• Localized GI infection can occur from esophagus to rectum but the duodenum is most common.
•  Dissemination usually involves many organs-- most commonly the blood, bone marrow, liver, spleen. and lymph nodes; but the organism has been recovered from the eye, brain, meninges, CSF. skin, tongue. heart, lung, stomach, thyroid, breast, parathyroid, adrenals, kidney, pancreas, prostate, testis and urine.
•  Microscopically, tissues are filled with large numbers of distended histiocytes that on Ziehl-Neelson staining are packed with acid-fast bacilli.
• The histologic picture in disseminated MAC disease is similar to that seen in lepromatous leprosy and reflects an inability of the host to mount an efftective cell-mediated immune response which would be manifest as a granulomatous reaction.
• Phagocytosis by macrophages in patients with AIDS appears to be intact but intracellular killing does not occur.

What are the clinical manifestations of infection with MAC in persons with AIDS? 

• Fever and weight loss are characteristic with chronic diarrhea, abdominal pain and signs of extrahepatic biliary obstruction occurring less commonly.
•  Severe anemia is common. 
• Rarely reported are localized pneumonia, endobronchial lesions, arthritis, skin lesions and endophthalmitis.

How is infection with MAC diagnosed? What specimens are useful for culture?

• Diagnosis is established by recovery of the organism from a normally sterile site but blood, bone marrow, lymph node, and liver are most common.
• Blood cultures are set up in specialized liquid medium with radiometric detection systems developed specifically for mycobacteria.
• DNA probes are used to rapidly identify any growth of acid- fast- positive organisms.
• Tissue samples show infiltration with swollen macrophages containing large numbers of mycobacteria on acid-fast staining.

Is infection with MAC treatable? What medications are used in its treatment? For prevention?

• Yes, initial antimicrobial treatment should be with clarithromycin (high dose) or azithromycin plus ethambutol.
•  Additional drugs may be used but recent results of clinical trials have shown that addition of clofazimine to a two drug regimen containing one of the macrolides plus ethambutol provided no benefit.
• Besides clofazimine, amikacin , rifampin, rifabutin ciprofloxacin can all be used. AT LEAST TWO DRUGS SHOULD BE USED TO PREVENT EMERGENCE OF RESISTANT MUTANTS.
• Clinical response may require 2 - 8 weeks and therapy should be continued for life. PREVENTIVE THERAPY in patients at high risk (T-cell counts <50 -100) with rifabutin or clarithromycin or azithromycin being recommended for lifelong prophylaxis.