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Tuberculosis

A Worldwide Epidemic

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Updated April 24, 2009

Tuberculosis, also known as “TB” or "consumption," is an airborne contagious disease that can be fatal if left untreated. It gets its name from the “tubercle,” which refers to a TB bacteria-containing lesion that is walled off by white blood cells following the immune response to TB infection. Still a leading killer worldwide, TB has been called a “disease of poverty”, because it is more prevalent in developing countries where poverty and malnutrition are a problem. In the U.S., TB is less prevalent than in the past, but the disease has been on the rise in recent years.

Species Name: Mycobacterium tuberculosis

Type of Microbe: Bacteria

How it causes disease: Mycobacterium tuberculosis is an intracellular bacterium, which means it grows and divides inside of cells. When it is inhaled, it gets taken up by white blood cells known as macrophages, whose normal function is to kill microbes and pathogens. The TB bacteria is able to manipulate the macrophage so that it avoids getting killed and actually lives within the macrophage.

How it spreads: TB is spread primarily via the airborne route after a contagious person coughs. Risk for infection is greatest for people who spend long periods of time in closed environments with other infected persons. It is not spread through touching or most foods, but consumption of unpasteurized dairy products from infected cows can lead to a TB-like infection by another species of mycobacteria (M. bovis).

Who’s at risk? TB is a serious health problem in many countries, especially those whose socioeconomic conditions lead to poverty, malnutrition, and crowded housing. It is less prevalent in the U.S., but remains a problem in hospitals, prisons, and homeless shelters, where crowded conditions prevail. Re-emergence of TB in the U.S. has been attributed in part to travel to and from countries where TB is endemic, as well as to association with the HIV epidemic. Studies of TB transmission on airplanes have shown that the risk for transmitting TB increases on long flights (8 or more hours), especially for individuals who are in close proximity to a contagious person.

Symptoms: Active TB begins with a bad cough (that produces blood-tinged phlegm) that lasts 3 or more weeks and is accompanied by chest pain, fatigue, loss of appetite, weight loss, fever, chills and night sweating.

Diagnosis: Exposure to TB can be determined with a tuberculin skin test, in which a bacterial extract is injected beneath the skin of the forearm. A positive skin test results when the immune response causes a localized swelling at the site of injection. This test has several limitations and can miss many exposures. Newer tests, such as the QuantiFERON TB-Gold test or T-Spot TB test, are blood tests that measure the immune response to TB using a small blood sample. In individuals who have been exposed to TB or have symptoms of TB, active disease is diagnosed using chest x-rays and bacterial cultures or microscopic examination of coughed up sputum (phlegm) are used to confirm diagnosis.

Prognosis: Of individuals exposed to TB, approximately 90% of infected people develop latent TB, in which the infection is neither symptomatic nor contagious. Many people with latent TB never develop active TB, however, active TB is serious and can be fatal as a result of tissue destruction that leads to rupture of blood vessels and hemorrhage (uncontrolled bleeding). TB is fatal in up to 50% of patients who do not receive proper treatment. In the HIV/AIDS population, who are at higher risk for infection, TB can kill within months, with fatality rates up to 80%.

Treatment: Latent TB (asymptomatic and non-contagious) is treated with 6 to 9 months of isoniazid (or other antibiotic). An alternative treatment is 4 months of rifampin.

Active TB is treated with a combination of 4 medications for 6 to 8 weeks, followed by two drugs for a total duration of 6 to 9 months. Duration of treatment depends on the number of doses given each week.

Antibiotic resistance is relatively common in some parts of the world, and leads to the development of “multiple drug-resistant” TB (MDR-TB) or “extensively drug-resistant” TB (XDR-TB). Antibiotic-resistant TB requires alternative medications for sometimes longer durations.

Prevention: When traveling to areas where TB is endemic, avoid exposure to crowded environments, such as hospitals, prisons, or homeless shelters. If traveling to areas with high incidence of TB, appropriate face masks can be used for protection.

Vaccine:A TB vaccine, known as the Bacille Calmette-Guerin (BCG) vaccine is given at birth in most developing countries. It has been shown to reduce the rate of TB meningitis and miliary TB in small children, whereas its effect in adults is limited. However, its effectiveness in preventing disease is variable, and it interferes with diagnostic testing, leading to false-positive results. Hence, it is not routinely recommended in the U.S.

Complications: TB can spread from the lungs through the lymphatic system and bloodstream to other parts of the body, including the kidneys, brain, heart and spine.

Sources

Centers for Disease Control. Division of Tuberculosis Elimination. http://www.cdc.gov/tb/default

Mims CA, et al. Medical Microbiology. ©1993. Mosby-Year Book Europe Limited. London.

Salyers AA and Whitt DD. Bacterial Pathogenesis: A Molecular Approach. ©1994. American Society for Microbiology. Washington, D.C.

World Health Organization. Tuberculosis. http://www.who.int/tb/en/

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