DESCRIPTION

• Active tuberculosis (TB) infection caused by Mycobacterium tuberculosis
  • Primary infection or reactivation of latent infection; risk increases with immunosuppression: highest risk first 2 years after infection; reactivation risk increases with comorbid disease (e.g., HIV, diabetes).
  • Well-described forms: pulmonary (85%), miliary (disseminated), meningeal, abdominal, lymphadenitis (scrofula)
• Usually acquired by inhalation of airborne bacilli from an individual with active TB; bacilli multiply in alveoli and spread via macrophages, lymphatics, and blood. Outcomes include: eradication (tissue hypersensitivity stops infection; primary TB; latent TB (LTBI).

EPIDEMIOLOGY

ETIOLOGY AND PATHOPHYSIOLOGY

• Causative organisms: Mycobacterium tuberculosis (mostly); less likely M. bovis or M. africanum
• Spread by aerosol droplets and reach alveolar space; alveolar macrophages ingest and migrate.
• Cell-mediated response by activated T lymphocytes and macrophages forms a granuloma (“tubercle”) that limits bacterial replication. If replication continues, the tubercle grows with spread to regional lymph nodes. An expanding tubercle within the lung parenchyma with regional lymph node enlargement is called a Ranke complex.
• As the infection is contained, destruction of the macrophages produces early “solid necrosis.” In 2 to 3 weeks, “caseous necrosis” develops and LTBI ensues. In the immunocompetent, granuloma undergoes “fibrosis” and calcification. In the immunocompromised, primary progressive TB develops. Cavitary lesions may form.

RISK FACTORS

• For latent infection: homeless, correctional facility, close contact with infected, living in area with high incidence of active TB, health care worker; medically underserved, low income, substance use
• For development of disease once infected: renal failure; lymphoma; silicosis; diabetes; cancer of head, neck, or lung; children <5 years old; malnutrition; systemic corticosteroids; HIV; immunosuppressive drugs; IV drug use, alcohol misuse, cigarette smokers; <2 years since infection with M. tuberculosis; <90% ideal body weight

GENERAL PREVENTION

• Screen for and treat LTBI. Report active TB to health department; test and treat all close contacts. Without treatment LTBI will progress to active TB disease in 5–10%.
• Bacillus Calmette-Guérin (BCG) vaccine: used primarily in endemic countries; high-risk children, negative PPD; ongoing exposure. Not recommended in US.

COMMONLY ASSOCIATED CONDITIONS

Immunosuppression; HIV coinfection; malignancy