Sepkowitz KA. How contagious is tuberculosis? Clin Infect Dis 1996; 23:954-962.
This article’s title question remains surprisingly unanswerable to some degree because the determinants of transmissibility for tuberculosis still are only partially understood, the author reports.
Concern among both the public and health care workers regarding the potential transmissibility of tuberculosis arose in the late 1980s as the disease began its resurgence in the United States. However, people exposed to a source case found that their seemingly simple question "Am I going to catch TB?" had an extremely complicated and incomplete answer, he finds.
"Incredibly, despite decades of inquiry and hundreds (if not thousands) of reports, it is still not clear why one person with tuberculosis is more contagious than the next," the author states. "Much of our current understanding of the transmissibility of tuberculosis derives from inference and accident rather than from intentional scientific study."
Factors that influence transmission of tuberculosis include the source case; the environment, including ventilation; the duration and intensity of exposure; and the tubercle bacillus itself. Numerous community-based studies have demonstrated repeatedly that a single variable the acid-fast bacilli (AFB) smear status of the source case strongly predicts which patients are the most contagious. Beyond that, however, much has been left to speculation. Simply covering the mouth while coughing, however, has been shown to reduce transmission, he adds. That intervention often is overlooked in favor of the current elaborate strategies for decreasing transmission, such as sophisticated filters on masks, ultraviolet light, sterilization of air, and negative-pressure ventilation in rooms, he notes.
Understanding of tuberculosis transmission is further hampered by reliance on the tuberculin skin test to identify latently infected and newly infected individuals. Numerous investigations of outbreaks have been complicated by the relatively poor sensitivity and specificity of this old test, the author laments. Improved understanding of tuberculosis transmission may await the development of a more reliable test for diagnosing individuals who are infected. Use of molecular techniques may provide the additional technological advantage needed to better comprehend the principles of transmission.
"On the other hand, it may be that one more modern technique will fall short of completely unlocking the mysteries of the tubercle bacillus," the author concludes.
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