Mycobacterium avium

Mycobacterium avium is a worldwide opportunistic non-tuberculous pathogen (NTM). Mycobacterium avium is detected in dust, dirt, fresh and salt water, birds, pigs, sheep, cattle, milk and eggs.The pathogen (see also MAC complex) is one of the most commonly identified nontuberculous mycobacteria (NTM) (Xu X et al. 201).
Mycobacterium avium can cause a wide range of clinical conditions; most common are infections of the lung followed by lymphadenitis in children, skin diseases, and other extrapulmonary or disseminated infections in severely immunocompromised patients. MAC infections are also possible in immunocompetent hosts.

Combination antiretroviral therapy (cART) has changed the epidemiology of Mycobacterium avium. Between the pre- and post-cART periods, a significant decline in the incidence of disseminated M. avium complex (DMAC) infections was observed. between 1990 and 1996, DMAC diagnoses had more than doubled. During this time, DMAC prevalence in people living with AIDS (PLHA) reached 20-23% overall in developed countries and >40% in groups with CD4 cell counts <10 cells/mm3. Currently, the incidence of DMAC in PLHA is two events per 1000 patient-years. Fundamentally, however, disseminated Mycobacterium avium complex infections, still pose a significant clinical threat with a high mortality rate if therapy is inadequate (Marochi-Telles JP et al. (2020).

Treatment may be difficult as it depends on several factors, including the causative organism, the immunological status of the patient, and the extent of the disease.

Surgical removal of individual lesions, if necessary.

The optimal treatment regimen for disseminated MAC is clarithromycin, ethambutol +/rifabutin; whether a three-drug regimen alone would be appropriate in this situation is not known.

The optimal duration of treatment is also unknown, but 6- to 12-month chemotherapy is usually recommended.

Specific treatment of IFN-γ autoantibodies associated with NTM infections is not codified and requires prolonged treatment with multiple drugs.

The use of immunomodulation strategies is still debated, and long-term suppressive treatment should be considered for persistently high levels of neutralizing antibodies (Xu X et al. 2019)

Mycobacterium avium complex (MAC) is a grouping of slow-growing mycobacteria based on molecular biology criteria. Members of this complex should have corresponding levels in at least two of the following target molecules against either M. avium ATCC 25291T or Mycobacterium intracellulare ATCC 13950T:

• >99.4 % sequence identity for the full 16S rRNA gene, >98.7 % for the partial (5') 16S rRNA gene, >97.3 % for hsp65 and >94.4 % for rpoB region V.
• A >97. 5% value in concatenated analyses of >2500 bp encompassing 16S rRNA, hsp65, and rpoB gene sequence data or ≥85% average nucleotide identity with M. avium ATCC 25291T or M. intracellulare ATCC 13950T based on whole-genome sequencing data is recommended.

According to this molecular definition, the complex currently consists of 12 validly published species:

• Mycobacterium avium
• Mycobacterium intracellulare
• Mycobacterium chimaera
• Mycobacterium colombiense
• Mycobacterium arosiense
• Mycobacterium vulneris
• Mycobacterium boucheduronense
• Mycobacterium timonens
• Mycobacterium marseillnse
• Mycobacterium yongoense
• Mycobacterium paraintracellulare
• Mycobacterium lepraemurium.

1. Heifets LB, Isemann MD (1990) Choice of antimicrobial agents for M. avium disease based on quantitative tests of drug susceptibility. N Engl J Med 322:419-420

2. Marochi-Telles JP et al (2020) Disseminated Mycobacterium avium on HIV/AIDS: Historical and Current Literature Review. AIDS Rev 22:9-15.

3. Mattoo R (2012) Targeting emerging Mycobacterium avium infections: perspectives into pathways and antimicrobials for future interventions. Future Microbiol16:753-764.

4. To K et al. (2020) General Overview of Nontuberculous Mycobacteria Opportunistic Pathogens: Mycobacterium avium and Mycobacterium abscessus. J Clin Med 9:2541.

5. van Ingen J et al. (2018) A definition of the Mycobacterium avium complex for taxonomical and clinical purposes, a review. Int J Syst Evol Microbiol 68:3666-3677.

6. Xu X et al. (2019) Chronic Mycobacterium avium skin and soft tissue infection complicated with scalp osteomyelitis possibly secondary to anti-interferon-γ autoantibody formation. BMC Infect Dis 19:203.