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Core Curriculum on Tuberculosis, 2000
Chapter 2
Transmission and Pathogenesis
Summary
TB is a communicable disease caused by Mycobacterium tuberculosis,
or the tubercle bacillus. It is spread primarily by tiny airborne
particles (droplet nuclei) expelled by a person who has infectious
TB. If another person inhales air containing these droplet nuclei,
transmission may occur. Some bacilli reach the alveoli, where they
are ingested by macrophages. Infection begins with the multiplication
of tubercle bacilli within these alveolar macrophages. Some of the
bacilli spread through the bloodstream when the macrophages die;
however, the immune system response usually contains the bacilli
and prevents the development of disease. Persons who are infected
but who do not have TB disease are asymptomatic and not infectious;
such persons usually have a positive reaction to the tuberculin
skin test. About 10% of infected persons will develop TB disease
at some time in life, but the risk is considerably higher for persons
who are immunosuppressed, especially those with HIV infection. Although
the majority of TB cases are pulmonary, TB can occur in almost any
anatomical site or as disseminated disease.
Objectives
After working through this chapter, you will be able to
- Describe how TB is spread;
- List at least 10 conditions that increase the risk that TB infection
will progress to TB disease;
- Define primary and secondary drug resistance;
- Describe the classification system for TB.
In the United States, the vast majority of TB cases are caused
by Mycobacterium tuberculosis, sometimes referred to as the
tubercle bacillus. M. tuberculosis and three very closely
related mycobacterial species (M. bovis, M. africanum,
and M. microti) can cause tuberculous disease, and they compose
what is known as the M. tuberculosis complex. M. bovis
and M. africanum are very rare causes of disease in the United
States; M. microti does not cause disease in humans. Mycobacteria
other than those comprising the M. tuberculosis complex are
called nontuberculous mycobacteria. Nontuberculous mycobacteria
may cause pulmonary disease resembling TB. 1
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Released October 2008
Centers for Disease Control and Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Division of Tuberculosis Elimination - http://www.cdc.gov/tb
Please send comments/suggestions/requests
to: hsttbwebteam@cdc.gov,
or to
CDC/Division of Tuberculosis Elimination
Communications, Education, and Behavioral Studies Branch
1600 Clifton Rd., NE - Mailstop E-10, Atlanta, GA 30333 |