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LEARNING OBJECTIVES

Learning Objectives

  • The student will be able to describe the features of latent infection and disease caused by Mycobacterium tuberculosis.

  • The student will be able to recognize characteristic radiographic patterns of the pulmonary disease caused by Mycobacterium spp.

  • The student will be able to explain the epidemiology of nontuberculous mycobacterial diseases in specific clinical conditions including cystic fibrosis, bronchiectasis, COPD, and other structural diseases of the lungs.

  • The student will be able to develop treatment approaches for clinical cases of pulmonary mycobacterial disease.

Mycobacterial diseases of the lung encompass tuberculous and nontuberculous diseases, which have important differences in pathogenesis, transmissibility, therapeutic approach, and public health implications. The incidence of tuberculosis (TB) disease from Mycobacterium tuberculosis began to significantly rise by approximately 20% in the United States after 1985 due to combination of factors; namely, emergence of human immunodeficiency virus (HIV) infection, increased immigration of persons from region of high TB activity and drug resistance, and increased numbers of medically underserved individuals such as drug abusers and homeless populations. Although at least 15 million persons are infected with TB in the United States, the actual number of cases has been decreasing since 1992, primarily due to more effective public health policies, enhanced awareness resulting in earlier diagnosis, and implementation of directly observed therapy (DOT) as standard of care in appropriate settings.

The present incidence of tuberculosis in the United States varies significantly depending on the place of birth of U.S. nationals. It is as high as 20.6 cases per 100,000 persons among foreign-born individuals versus 2.1 cases per 100,000 among U.S. born persons, respectively. In 2007, moreover, foreign-born persons accounted for approximately 60% of new cases in the United States. These data underscore the importance of ongoing overseas screening for tuberculosis with follow-up evaluation after arrival in U.S. bound immigrants. Elimination of tuberculosis in the United States therefore mandates control and prevention of tuberculosis in foreign-born persons.

MYCOBACTERIUM TUBERCULOSIS INFECTION

Development of disease from Mycobacterium tuberculosis (MTB) is the most common global infectious cause of death. Such disease caused by bacteria of the MTB complex preeminently affects the lungs, although other organs are involved in up to one-third of cases. Transmission of TB occurs when a contagious patient cough, sneezes, or otherwise spreads the bacilli through generation of airborne droplet nuclei to individuals in close proximity sharing the same ventilatory space. Of note, M. bovis belonging to the MTB complex is transmitted by ingestion of contaminated, unpasteurized dairy products from infected cattle. M. tuberculosis is a rod shaped, non-spore-forming, nonmotile, thin aerobic bacterium measuring 0.5 μm × 3 μm. Mycobacteria, including M. tuberculosis, are often neutral on Gram stain; however, once stained, the bacilli are not easily decolorized by acid alcohol and hence the classification acid-fast bacilli (AFB) (Fig. 36.1; see also Fig. 34.15).

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