Transmission of M. tuberculosis is from person to person, usually by respiratory droplets that become airborne when the ill individual coughs, sneezes, laughs, sighs, or breathes. Infected droplets dry and become droplet nuclei, which may remain suspended in the air for hours, long after the infectious person has left the environment.
Several patient-related factors are associated with an increased chance of transmission. Of these a positive acid-fast smear of the sputum most closely correlates with infectivity. Children with primary pulmonary tuberculosis disease rarely, if ever, infect other children or adults. Tubercle bacilli are relatively sparse in the endobronchial secretions of children with primary pulmonary tuberculosis, and a significant cough is usually lacking. When young children cough, they rarely produce sputum, lacking the tussive force necessary to project and suspend infectious particles of the requisite size. Hospitalized children with suspected pulmonary tuberculosis are placed initially in respiratory isolation. Most infectious patients become noninfectious within 2 weeks of starting effective treatment, and many become noninfectious within several days.
In North America, tuberculosis rates are highest in for-eign-born persons from high-prevalence countries, residents of prisons, residents of nursing homes, homeless persons, users of illegal drugs, persons who are poor and medically indigent, health care workers, and children exposed to adults in high-risk groups. Among U.S. urban dwellers with tuberculosis, persons with acquired immunodeficiency syndrome (AIDS) and racial minorities are overrepresented. Most children are infected with M. tuberculosis from household contacts, but outbreaks of childhood tuberculosis centered in elementary and high schools, nursery schools, family day care homes, churches, school buses, and stores still occur. A high-risk adult working in the area has been the source of the outbreak in most cases.
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Latent tuberculosis describes the asymptomatic stage of infection with M. tuberculosis. The tuberculin skin test (TST) is positive, but the chest radiograph is normal, and there are no signs or symptoms of illness. Tuberculosis disease occurs when there are clinical signs and symptoms or an abnormal chest radiograph. The term tuberculosis usually refers to the disease. The interval between latent tuberculosis and the onset of disease may be several weeks or many decades in adults. In young children, tuberculosis usually develops as an immediate complication of the primary infection, and the distinction between infection and disease may be less obvious.
Primary pulmonary tuberculosis in older infants and children is usually an asymptomatic infection. Often the disease is manifested by a positive TST with minimal abnormalities on the chest radiograph, such as an infiltrate with hilar lymphadenopathy or Ghon complex. Hilar lymphadenopathy may compress the bronchi or trachea malaise, low-grade fever, erythema nodosum, or symptoms resulting from lymph node enlargement may occur after the development of delayed hypersensitivity.
Lymphadenopathy is common in primary pulmonary disease. Hilar lymphadenopathy may compress the bronchi or trachea. The most common extrathoracic sites of lymphadenitis are the cervical, supraclavicular, and submandibular areas (scrofula).
Progressive primary disease is characterized by a primary pneumonia that develops shortly after initial infection. Progression to pulmonary disease, disseminated miliary disease, or progression of central nervous system (CNS) granulomas to meningitis occurs most commonly in the first year of life.
Tuberculous pleural effusion, which may accompany primary infection, generally represents the immune response to the organisms and most commonly occurs in older children or adolescents. Pleurocentesis reveals lymphocytes and an increased protein level, but the pleural fluid usually does not contain bacilli.