TABLE 2-1. Differences Between Latent Tuberculosis Infection and Active Pulmonary Tuberculosis

Latent Tuberculosis Infection

Active Tuberculosis (in the lungs)

Few tuberculosis bacteria in the body

Many tuberculosis bacteria in the body

Tuberculin skin test reaction usually but not always positive

Tuberculin skin test reaction usually but not always positive

Chest radiograph normal

Chest radiograph usually abnormal

Sputum smears and cultures negative

Sputum smears and cultures usually positive

No symptoms

Symptoms (e.g., cough) often present

Not infectious

Potentially infectious before treatment

SOURCE: Adapted from CDC (1999c, Module 1, p. 7).


Tuberculosis is primarily caused by Mycobacterium tuberculosis (also called the tubercle bacillus). Humans can also develop bovine tuberculosis, a less serious disease that is primarily caused by drinking milk from cattle infected with Mycobacterium bovis.3 Other mycobacteria can cause tuberculosis in various animals, but they rarely cause disease in humans.4 As discussed later in this chapter, bovine and other mycobacteria may produce positive reactions to the tests used to screen people for tuberculosis.

Although M. tuberculosis was first identified in 1882, its behavior remains poorly understood in some important respects. In particular, much remains to be learned about the mechanisms of latent tuberculosis infection and disease activation and reactivation.

Transmission of M. tuberculosis

M. tuberculosis typically spreads through the air when people who have infectious tuberculosis in their respiratory tract cough, sneeze, speak, sing, or otherwise expel tiny particles containing the bacteria. A fine mist (an aerosol) of infectious particles can also be created in other ways, for


In developed countries, bovine tuberculosis has largely been eliminated by programs of tuberculin testing in cattle followed by slaughter of infected animals. Infected cattle, deer, and other animals are still occasionally found in the United States, and human cases continue to be reported (Dankner and Davis, 2000; Hernandez and Baca, 1998; Palmer et al., 2000; Pillai et al., 2000; Schmitt et al., 1997). Aside from testing and destruction of infected animals, public health programs rely on compulsory pasteurization and certification of milk to prevent transmission of the disease to humans.


For patients with AIDS, Mycobacterium avium complex disease can be life-threatening, but information about its epidemiology and effective management is limited (Gangadharam and Jenkins, 1998).

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