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- Usually acquired by inhalation of airborne bacilli from a person with active tuberculosis (TB). Bacilli multiply in alveoli and spread via macrophages, lymphatics, and blood. 3 possible outcomes:
- Eradication: Tissue hypersensitivity halts infection within 10 weeks.
- Primary infection
- Latent infection: Asymptomatic with positive purified protein derivative (PPD), negative chest radiograph, noninfectious
- Active tuberculosis:
- Occurs from primary infection or reactivation of latent infection
- Affects 10% of infected individuals without preventive therapy
- Risk increases with immunosuppression: Highest the first 2 years after infection
- Well-described forms: Pulmonary (85% of cases), miliary (disseminated), meningeal, abdominal
- Abdominal TB more common in the 3rd or 4th decade in women.
- Children <4 years have higher risk for disseminated TB (lower-lobe infections more common); also more likely to have immediate clinical or radiographic signs. Treat with 4 drugs.
- Older children and adolescents develop upper-lobe infiltrates; can re-occur in adulthood.
- Worldwide: 128 cases/100,000 population
- US: 4.1 cases/100,000 population (1)
- Worldwide: 178 cases/100,000 population
- United States: 4.8 cases/100,000 population (1)
- For infection:
- Homeless, minority, institutionalized settings; close contact with infected individual; persons from areas with high incidence of active tuberculosis (Asia, Africa, Latin America, former Soviet Union states); health care workers; frequent or prolonged visits to high prevalence areas; and populations with locally increased incidence (e.g., medically underserved, low income, substance abusers)
- For development of disease once infected:
- HIV; lymphoma; silicosis; diabetes mellitus; chronic renal failure; cancer of head, neck, or lung; children <5 years of age; malnutrition; systemic corticosteroids, immunosuppressive drugs; IV drug abuse, alcohol abuse, cigarette smokers; <2 years since infection with Mycobacterium tuberculosis; history of gastrectomy or jejunal bypass; <90% of ideal body weight; conversion from negative to positive tuberculin skin test (PPD) or interferon-gamma release assay (IGRA) within previous 2 years
- Treat latent infection and report to health department for identification, and testing and treatment of all close contacts.
- Bacille Calmette Guérin (BCG) vaccine: Live attenuated M. bovis, prevents 50% of pulmonary disease and 80% of meningitis and miliary disease in children.
- Used more in countries with endemic TB. In the US, consider BCG for children with negative PPD and HIV tests with unavoidable high risk and for health care workers at high risk for drug-resistant infection.
Cell-mediated response by the body causes activated T lymphocytes and macrophages to form a “granuloma” that limits organism replication. Destruction of the macrophages produces early “solid necrosis.” In 2–3 weeks, “caseous necrosis” develops, establishing latency. In people with intact immunity, granuloma undergoes “fibrosis” and calcification; in people with less effective immune systems, primary progressive tuberculosis develops.
M. tuberculosis, M. bovis, or M. africanum
Commonly Associated Conditions
HIV infection (emphasize direct observed therapy [DOT]); most recommendations remain the same.