Tuberculosis in humans and its epidemiology, diagnosis and treatment in the United States [Serialised article. Tuberculosis: a re-emerging disease in animals and humans. Number 2 in the series]

Tuberculosis (TB) is a pulmonary and systemic disease caused by Mycobacterium tuberculosis complex species. TB is spread from person to person by airborne transmission. Several factors determine the probability of transmission, including the infectiousness of the source patient and the nature of the environment where exposure occurs. This initial infection (primary TB) rapidly progresses to disease in some persons (especially children and immunocompromised persons), but resolves spontaneously in most individuals. This condition in which the organism lies dormant is known as latent TB infection (LTBI). In the United States, the diagnosis of LTBI is made with either the tuberculin skin test or an interferon-gamma release assay. LTBI is treated with isoniazid (INH; usually for 9 months) to prevent progression to TB disease. Up to 5% of immunocompetent persons will progress to TB disease at some time in the future, even decades after infection, if they are not treated for LTBI. Pulmonary TB disease is diagnosed using a combination of chest radiography and microscopic examination, culture and nucleic acid amplification testing of sputum. Treatment of drug-susceptible TB consists of at least 6 months of an INH and rifampin-containing regimen (with ethambutol and pyrazinamide for the first 2 months). In the United States, drug-resistant TB is relatively rare (approximately 1% of all patients), and is treated with an 18–24 month individualized regimen based on drug susceptibility test results.