Description

• Active tuberculosis (TB)
  • 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% of cases), 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. Three possible outcomes:
  • Eradication: Tissue hypersensitivity halts infection within 10 weeks.
  • Primary TB
  • Latent TB (see “Tuberculosis, Latent (LTBI)”)

Epidemiology

Incidence

• Worldwide (2017): 10.4 million (133 cases per 100,000) population; highest incidence in Asia and Africa. Higher burden countries have approximately 150 cases/100,000 population.
• United States (2020): 7,163 (2.2/100,000); 71% of U.S. cases were in persons born outside of the United States. TB incidence has decreased by an average of 2–3% annually during the previous 10 years.

Prevalence

• Worldwide (2019): World Health Organization estimates 10 million new cases of TB in 2019.
• Mortality
  • Worldwide (2019): 1.4 million deaths due to TB. TB is one of the top 10 leading causes of death worldwide.

Etiology and Pathophysiology

• Mycobacterium tuberculosis, Mycobacterium bovis, or Mycobacterium africanum are causative organisms.
• 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 bacterial replication continues, the tubercle grows with spread to regional lymph nodes. An expanding tubercle within the lung parenchyma combined 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 infection: homeless, correctional facilities, close contact with infected person, living in areas with high incidence of active TB, health care workers; medically underserved, low income, substance abuse
• 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 abuse, alcohol abuse, cigarette smokers; <2 years since infection with M. tuberculosis; <90% of ideal body weight

General Prevention

• Screen for and treat LTBI. Report active TB to health department; test and treat all close contacts.
• Bacillus Calmette-Guérin (BCG) vaccine: More commonly used in endemic countries. In the United States, BCG is generally not recommended and very rarely used for high-risk children with negative PPD and ongoing exposure from parents or close contacts.

Commonly Associated Conditions

Immunosuppression; HIV coinfection; malignancy