Tuberculosis

Tuberculosis has been one of history's more notorious global pandemics. Mycobacterium tuberculosis (TB) has been a leading infectious cause of morbidity and mortality throughout the world, with over one third of the world's population infected with the bacterium that causes tuberculosis. Despite some success in curbing TB infections since 1950, there has been a resurgence of TB infections at the end of the 20th century, particularly in resource-limited settings.

Evidence suggests that TB has affected humans for most of the history of humankind. Clear evidence of TB has been recovered from Egyptian remains dating back to 2400 BCE. In western Europe and the Americas, TB was associated with poverty and overcrowding in the 19th century and was commonly referred to as “consumption” and “the white plague.” Despite Dr. Robert Koch's discovery of the causative bacteria of TB in 1882, it was not until 1944 that the first antibiotic to treat TB was developed. The combination of treatment with antibiotics and isolation of infectious individuals significantly reduced the number of TB cases, particularly in North America and western Europe.

TB is spread from person to person. Infection often occurs when a patient is exposed to an infectious individual who is coughing and thereby spreading the bacterium. Following infection, most individuals are able to control the bacterium and will go on to develop “latent” (noninfectious) TB. Approximately 10% of people with latent TB will go on to develop active tuberculosis and become ill from the disease in their lifetime. However, individuals with reduced immune function (due to AIDS, use of immunosuppressive drugs, chemotherapy, or other reasons) have a dramatically increased risk of developing active TB. As a result, areas of the world most affected by the HIV pandemic have seen dramatic increases in tuberculosis. In addition, tuberculosis remains one of the most common causes of illness and death in HIV-infected individuals worldwide.

The diagnosis of TB disease requires microscope and culture methods not used for other bacterial infections. The most common test used to identify the bacterium is sputum microscopy. In this technique, sputum from an individual suspected of having active TB is stained using a special technique (acid-fast staining, or AFB staining) and examined under the microscope for TB bacteria. To detect TB by AFB staining, the patient must have a large number of bacteria in their lungs. As a result, this test can be falsely negative in some [Page 1687]patients with TB disease. Therefore, although the AFB stain is relatively inexpensive and commonly used, this test can miss a substantial number of individuals who have TB. A more sensitive test to detect TB is mycobacterial culture. However, culturing TB is slow and requires specialized laboratory facilities. As a result, mycobacterial culture is expensive and is often unavailable in areas of the developing world that are most affected by TB.

A highly effective vaccine to prevent the development of TB has not yet been developed. The Bacille Calmette-Guerin vaccine (also known as the BCG vaccine) was developed in France in 1908 and is often given with routine vaccinations to children in areas of the world with high rates of tuberculosis. The BCG vaccine contains weakened Mycobacterium bovis, a strain of TB found in cattle, which induces an immune response that offers protection against TB. BCG offers some protection against the spread of tuberculosis from the lungs to other organs, particularly in children. However, BCG offers little protection against pulmonary (lung) TB, the most common type of TB. Treatment is important to control the spread of TB as well as to effectively cure individual patients. Individuals with latent TB are treated to prevent the development of active TB in the future and to prevent subsequent infections. Latent TB can be treated with a single drug, isoniazid (INH), taken for 6 months. In patients with active TB, treatment usually involves four drugs for 2 months, followed by at least two drugs for at least another 4 months. Early in the course of treatment (typically for at least 2 weeks), the patient remains contagious, and efforts are made to limit the patient's exposure to vulnerable populations. After this initial period, the patient needs to continue his or her antibiotics to ensure cure, but the patient is typically no longer at risk of spreading TB to others.

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