What is tuberculosis (TB)?

Definition

Tuberculosis (TB) is a contagious infectious disease with either active or latent forms. Although TB can affect many organ systems, it mostly affects the lungs.

Causes

Tuberculosis is caused by the bacterium Mycobacterium tuberculosis. Persons near someone with active TB of the lungs may inhale the bacteria if the infected person coughs or sneezes. TB is easily spread in crowded conditions. It is also easily spread among people who are ill or who have weakened immune systems.

Risk Factors

The factors that increase the chance of developing TB include a weakened immune system or chronic diseases (highest risk); human immunodeficiency virus (HIV) infection; malnutrition; intravenous drug use; alcoholism; tobacco use; leukemia, lymphoma, and other cancers; poorly controlled diabetes mellitus; severe kidney disease; corticosteroids; some medications used for treating rheumatoid arthritis and other diseases, including etanercept, infliximab, and adalimumab; and a suppressed immune system caused by medications, such as drugs to prevent rejection of a transplanted organ.

Other risks factors include silicosis (an occupational lung disease) and living in crowded, indoor conditions (such as homeless shelters, dormitories, and military barracks). Persons at higher risk for TB are infants, young children, the elderly, and those who are HIV-positive. Pregnant women may be screened because, if infected, they can transmit TB to the fetus.

Symptoms

Tuberculosis causes no symptoms in most persons. In others it is fatal. The bacteria lie dormant in the lungs and may remain there permanently without causing illness. During this time, the infected person cannot spread TB to others. The infection spreads only when the bacteria are active.

Other symptoms for TB are a severe cough that lasts more than two weeks, coughing up blood and sputum (mucus from deep in the lungs), pain in the chest, weakness or fatigue, unexplained weight loss, chills, fever, night sweats, and loss of appetite.

Screening and Diagnosis

A skin test is used to screen for TB. A small amount of tuberculin test fluid is injected into the skin of the lower part of the arm. The test is positive if, after two to three days, a raised, firm welt appears at the injection site. The welt is 10 mm or greater in diameter (5 mm or 15 mm under some situations). A positive test means a person was exposed to TB, even if he or she never became ill. People at high risk for TB should have a skin test regularly. Also, a blood test is available to screen for TB. The blood test is recommended for those who are unlikely to follow-up on a skin test, who have had the TB vaccine, or who have had a positive skin test in the past. If a person has symptoms or signs of active TB, a doctor may order a chest X ray and also get samples of sputum to test for the presence of the bacterium.

Although screening is typically only performed for those at risk of contracting TB, rather than the population at large, some employers and educational institutions require screening prior to enrollment. Higher-risk occupations include those in health care, education, the military, migrant labor, and corrections.

Treatment and Therapy

Medication can prevent TB from becoming active. It can also help cure active TB. One should take all medication as prescribed, even if the symptoms disappear. If a person does not finish the medication, does not take it as often as required, or has a recurrence of TB, he or she may develop drug-resistant TB, which is difficult to cure.

Persons who have a positive skin test but no signs of active TB may need to take medication to prevent active TB. The drug isoniazid is usually given for six months or longer. For persons with active TB, a doctor may prescribe a combination of isoniazid, rifampin, pyrazinamide, ethambutol, or streptomycin. Under special circumstances, other drugs may be prescribed. Multidrug-resistant TB fails to respond to the two primary anti-TB drugs, isoniazid and rifampin, while extensively drug-resistant TB cannot be treated with those or several other second-line medications. Drug-resistant TB remains rare but has been found around the world.

A person with active TB must be isolated from friends, family, and coworkers until a doctor determines the person is no longer contagious. This is usually after the first several weeks of the infected person taking medication. Isolation will help prevent the spread of TB. Infected persons can resume normal activities after getting a doctor’s approval and will need to continue taking the prescribed drugs until the doctor says it is okay to stop. Drugs may need to be taken for six months and, in some cases, up to two years.

In directly observed therapy, a health care professional, community health worker, or even a family member watches as a TB patient takes his or her medication either at home or in a clinic setting, with the goal of helping the patient complete his or her course of treatment, particularly if the patient struggles with self-administering medication.

Prevention and Outcomes

If the infected person has a positive skin test, he or she might be able to prevent active TB from developing by taking medication. A TB vaccine is available, but it is not routinely used in the United States because of the unreliable protection it provides. A person with active TB can prevent its spread by avoiding contact with people and by taking all medication as prescribed for the full course of treatment. HIV-positive individuals are at the highest risk of death from TB and therefore must take additional precautions, such as avoiding visiting locations where the disease is endemic.

Bibliography

Centers for Disease Control and Prevention. “The Difference Between Latent TB Infection and Active TB Disease.” Available at http://www.cdc.gov/tb/topic/basics.

Division of Tuberculosis Elimination. "Extensively Drug-Resistant Tuberculosis (XDR TB)." CDC. Centers for Disease Control and Prevention, 18 Jan. 2013. Web. 29 Dec. 2015.

Levitzky, Michael G. Pulmonary Physiology. 7th ed. New York: McGraw-Hill Medical, 2007.

Maartens, G., et al. “Tuberculosis.” The Lancet 370 (2007): 2030.

Rom, William N., and Stuart M. Garay, eds. Tuberculosis. 2d ed. Philadelphia: Lippincott Williams & Wilkins, 2004.

"TB Screening Tests." Lab Tests Online. Amer. Assn. for Clinical Chemistry, 29 Aug. 2014. Web. 29 Dec. 2015.

West, John B. Pulmonary Pathophysiology: The Essentials. 7th ed. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins, 2008.