Understanding Tuberculosis

by National Institute of Health

In developed countries, such as the United States, many people think tuberculosis (TB) is a disease of the past. TB, however, is still a leading killer of young adults worldwide. Some 2 billion people-one-third of the world's population-are thought to be infected with TB bacteria, Mycobacterium tuberculosis.

TB is a chronic bacterial infection. It is spread through the air and usually infects the lungs, although other organs and parts of the body can be involved as well. Most people who are infected with M. tuberculosis harbor the bacterium without symptoms (have latent TB), but some will develop active TB disease. According to World Health Organization (WHO) estimates, each year, 8 million people worldwide develop active TB and nearly 2 million die.

One in 10 people who are infected with M. tuberculosis may develop active TB at some time in their lives. The risk of developing active disease is greatest in the first year after infection, but active disease often does not occur until many years later.

TB in the United States

In 2005, the Centers for Disease Control and Prevention (CDC) reported 14,093 cases of active TB. While the overall rate of new TB cases continues to decline in the United States since national reporting began in 1953, the annual decrease in TB cases has slowed from an average of 7.1 percent (1993 to 2000) to the current average of 3.8 percent (2001-2005), according to CDC. In addition to those with active TB, an estimated 10 to 15 million people in the United States have latent TB.

Minorities are affected disproportionately by TB, which occurs among foreign-born individuals nearly nine times as frequently as among people born in the United States. This is partially because they were often exposed to M. tuberculosis in their country of origin before moving to the United States. In 2004, a very high percentage of Asians (95 percent) and Hispanics (75 percent) who were born outside the United States were reported to have TB.

What caused TB cases to increase the United States?

Cases of TB dropped rapidly in the 1940s and 1950s when the first effective antibiotic treatments for TB were introduced. In 1985, however, the decline ended and the number of active TB cases in the United States began to rise again. Several factors, often interrelated, were behind TB's resurgence.

The HIV/AIDS epidemic- People with HIV are particularly vulnerable to moving from infection with M. tuberculosis to active TB and are also more likely to develop active TB when they are first infected with TB bacteria.

People from many nationalities live in the United States- Increased numbers of foreign-born nationals come from places where many cases of TB occur, such as Africa, Asia, and Latin America. TB cases among foreign-born nationals now living in the United States account for more than half of the U.S. total.

Increased poverty, injection drug use, and homelessness-TB transmission is rampant in crowded shelters and prisons where people weakened by poor nutrition, drug addiction, and alcoholism are exposed to M. tuberculosis.

Failure of patients to take all prescribed antibiotics against TB-TB patients who do not complete TB drug treatment can stay infectious for longer periods of time and therefore can spread TB to more people. In addition, treatment failures may result in M. tuberculosis strains that are resistant to one or more of the standard medicines given to TB patients, making the disease much more difficult to treat.

Increased numbers of residents in long-term care facilities such as nursing homes-Many elderly people whose general health has declined develop active TB from TB infection they had much earlier in life. Other elderly people, especially those with weak immune systems, become newly infected with M. tuberculosis and can develop active TB rapidly.

Transmission

TB is primarily an airborne disease. The bacteria are spread from person to person in tiny microscopic droplets when a TB sufferer coughs, sneezes, speaks, sings, or laughs. Only people with active TB can spread the disease to others. People with TB who have been treated with the correct drugs for at least 2 weeks, however, are no longer contagious and do not spread the bacteria to others.

Diagnosis

To identify those who may have been exposed to M. tuberculosis, health care providers typically inject a substance called tuberculin under the skin of the forearm. If a red welt forms around the injection site within 72 hours, the person may have been infected. This doesn't necessarily mean he or she has active disease. People who may test positive on the tuberculin test include most people with previous exposure to M. tuberculosis, some people exposed to bacteria related to M. tuberculosis, some people born outside the United States who were vaccinated with the TB vaccine (see TB vaccine below) used in other countries.

If people have an obvious reaction to the skin test, other tests can help to show if they have active TB. In making a diagnosis, doctors rely on symptoms and other physical signs, the person's history of exposure to TB, and X-rays that may show evidence of M. tuberculosis infection.

The health care provider also will take sputum and other samples to see if the TB bacteria will grow in the lab. If bacteria are growing, this positive culture confirms the diagnosis of TB. Because M. tuberculosis grows very slowly, it can take 4 weeks to confirm the diagnosis. An additional 2 to 3 weeks usually are needed to determine which antibiotics to use to treat the disease.

What happens when someone gets infected with M. tuberculosis?

Between 2 to 8 weeks after being infected with M. tuberculosis, a person's immune system responds to the TB germ by walling off infected cells. From then on the body maintains a standoff with the infection, sometimes for years. Most people undergo complete healing of their initial infection, and the bacteria eventually die off. A positive TB skin test, and old scars on a chest X-ray, may provide the only evidence of the infection.

If, however, the body's resistance is low because of aging, infections such as HIV, malnutrition, or other reasons, the bacteria may break out of hiding and cause active TB.

Symptoms

Early symptoms of active TB can include weight loss, fever, night sweats, and loss of appetite. Symptoms may be vague, however, and go unnoticed by the affected person. For some, the disease either goes into remission (halts) or becomes chronic and more debilitating with cough, chest pain, and bloody sputum. Symptoms of TB involving areas other than the lungs vary, depending upon the organ or area affected.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.