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Tuberculosis (TB) is an infection caused by the organism Mycobacterium tuberculosis. TB is a global disease that has important implications for affected neonates who can acquire the disease in either the postnatal period or, more rarely, through congenital transmission from an infected mother.
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The World Health Organization (WHO) estimates that there were 9.4 million incident and 14 million prevalent cases of TB in 2009. A total of 3.3 million (35%) of the new cases in 2009 occurred in women. The majority of incident cases in 2009 occurred in Asia (55%) and Africa (30%). An estimated 1.1 million (12%) of cases occurred in human immunodeficiency virus (HIV)–positive individuals. Approximately 1.7 million people died of TB in 2009. The Centers for Disease Control and Prevention (CDC) reports 11,545 incident cases of TB in the United States in 2009, with 59% of cases occurring in foreign-born individuals. Although the exact incidence of neonatal cases is unknown, <200 cases of congenital TB have been reported in the English literature.
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M. tuberculosis is transmitted via inhalation of airborne droplet nuclei. Alveolar macrophages engulf M. tuberculosis, which then spreads through the lymphatic system to hilar lymph nodes. The infection can either be contained or lead to primary progressive TB. Granulomas containing the mycobacterium form via cell-mediated immunity within 2–8 weeks in most individuals. Infected macrophages interact with T lymphocytes to release cytokines that promote phagocytosis of M. tuberculosis and granuloma formation. Children under the age of 5 years and immunosuppressed individuals lack host immunity and therefore develop active primary progressive disease in the lung parenchyma and hilar lymph nodes. In these individuals, the characteristic fibrous granuloma capsule is disrupted, and liquefactive necrosis of the central caseous material occurs. The necrotic material can then flow into adjacent vasculature and disseminate systemically or to adjacent bronchi and spread externally via respiratory droplets. Immunosuppression and malnutrition are risk factors for reactivation of latent infection.
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Vertical transmission to the fetus can occur hematogenously or transplacentally. Hematogenous spread results in lesions in the liver and periportal lymph nodes or lungs. The increase in oxygenation and circulation in the postnatal period activate the replication of the bacilli. M. tuberculosis has been identified in amniotic fluid, and transmission of TB can occur via aspiration of infected fluid.
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The highest risk of transmission to newborns occurs via respiratory transmission from untreated mothers during the postnatal period. Maternal extrapulmonary TB, such as miliary TB or tuberculous endometritis, increases the risk of congenital infection. Maternal treatment for 2–3 weeks in the antenatal period reduces the risk of postnatal infection. HIV is a risk factor for maternal TB, which in turn increases the risk of mother to child transmission of HIV. Living in endemic areas or crowded conditions also increases the risk of TB.