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Thursday, August 28, 2008

Pulmonary tuberculosis (TB)

Pulmonary tuberculosis (TB) is a contagious bacterial infection caused by Mycobacterium tuberculosis (M. tuberculosis) . The lungs are primarily involved, but the infection can spread to other organs.

Alternative Names:TB; Tuberculosis - pulmonary; Consumption
Causes, incidence, and risk factors:
Tuberculosis can develop after inhaling droplets sprayed into the air from a cough or sneeze by someone infected with M. tuberculosis . The disease is characterized by the development of granulomas (granular tumors) in the infected tissues.

The usual site of the disease is the lungs, but other organs may be involved. The primary stage of the infection is usually asymptomatic (without symptoms). In the United States, the majority of people will recover from primary TB infection without further evidence of the disease.

Primary pulmonary TB develops in the minority of people whose immune systems do not successfully contain the primary infection. In this case, the disease may occur within weeks after the primary infection. TB may also lie dormant for years and reappear after the initial infection is contained.

Infants, the elderly, and individuals who are immunocompromised -- for example, those with AIDS , those undergoing chemotherapy , or transplant recipients taking antirejection medications -- are at higher risk for progression to disease or reactivation of dormant disease. In pulmonary TB, the extent of the disease can vary from minimal to massive involvement. Without effective therapy, the disease becomes progressively worse.

The risk of contracting TB increases with the frequency of contact with people who have the disease, with crowded or unsanitary living conditions and with poor nutrition. Recently, there has been an increase in cases of TB in the U.S. Factors that may contribute to the increase in tuberculous infection in a population are:

  • Increase in HIV infection
    Increase in number of homeless individuals (poor environment and poor nutrition)
    The appearance of drug-resistant strains of TB
    Incomplete treatment of TB infections (such as failure to take medications for the prescribed length of time) can contribute to the emergence of drug-resistant strains of bacteria.
    Individuals with immune systems damaged by AIDS have a higher risk of developing active tuberculosis -- either from new exposure to TB or reactivation of dormant mycobacteria. In addition, without the aid of an active immune system, treatment is more difficult and the disease is more resistant to therapy.

In the U.S., there are 10 cases of TB per 100,000 people, but it varies dramatically by area of residence and socio-economic class. Also see:
Disseminated tuberculosis (affects the whole body)
Atypical mycobacterial infection

Ferrara G, Losi M, Meacci M, et al. Routine Hospital Use of a New Commercial Whole Blood Interferon-(gamma) Assay for the Diagnosis of Tuberculosis Infection. Am J Respir Crit Care Med . 2005 Sep 1;172(5):631-5. Epub 2005 Jun 16.
US Centers for Disease Control. Treatment of Tuberculosis. MMWR 2003; 52.
Diagnostic Standards: Classification of TB in Adults and Children. Am J Respir Crit Care Med 2000; 161.

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