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TB
Tuberculosis

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Many people think tuberculosis (TB) is a disease of the past. But, TB is still a leading killer of young adults worldwide. Some 2 billion people one-third of the world's population are infected with the TB bacterium, M. tuberculosis. TB is a chronic bacterial infection. It is spread through the air and usually infects the lungs, although other organs are sometimes involved. Most persons that are infected with M. tuberculosis harbor the bacterium without symptoms but many develop active TB disease. Each year, 8 million people worldwide develop active TB and 3 million die.

Is TB a problem in the United States?

In the United States, TB has re-emerged as a serious public health problem. In 2001, based on provisional data reported to the U.S. Centers for Disease Control and Prevention, the number of cases has decreased for the ninth straight year to 15,991 cases of active TB (infection with full-blown disease symptoms). This all-time low is due largely to improved public health control measures. In addition to those with active TB, however, an estimated 10 to 15 million people in the United States are infected with M. tuberculosis without displaying symptoms (latent TB) and about one in ten of these individuals will develop active TB at some time in their lives.

Minorities are affected disproportionately by TB: 54 percent of active TB cases in 1999 were among African-American and Hispanic people, with an additional 20 percent found in Asians.

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What caused TB to return?

Cases of TB dropped rapidly in the 1940s and 1950s when the first effective antibiotic therapies 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 forces, often interrelated, were behind TB's resurgence:

  • The HIV/AIDS epidemic. People with HIV are particularly vulnerable to turn infection with M. tuberculosis into active TB and are also more sensitive to developing active TB when they are first infected with the TB germ.

  • Increased numbers of foreign-born nationals from countries where many cases of TB occur, such as Africa, Asia, and Latin America. TB cases among those persons now living in the US account for nearly half of the national 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 their prescribed antibiotics against TB as directed.

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

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How do people catch TB?

TB is primarily an airborne disease. The disease is spread from person to person in tiny microscopic droplets when a TB sufferer coughs, sneezes, speaks, sings, or laughs. Only people with active disease are contagious.

It usually takes lengthy contact with someone with active TB before a person can become infected. On average, people have a 50 percent chance of becoming infected with M. tuberculosis if they spend eight hours a day for six months or 24 hours a day for two months working or living with someone with active TB. However, people with TB who have been treated with appropriate drugs for at least two weeks are no longer contagious and do not spread the germ to others.

Adequate ventilation is the most important measure to prevent the transmission of TB.

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What happens when someone gets infected with M. tuberculosis?

Between two to eight 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.

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What is "active" disease?

One in ten people that 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.

Early symptoms of active TB can include weight loss, fever, night sweats, and loss of appetite, or they may be vague and go unnoticed by the affected individual. One in three patients with TB will die within weeks to months if the disease is not treated. For the rest, their 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 affected.

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How is TB diagnosed?

Doctors can identify most people infected with M. tuberculosis with a skin test. They will inject a substance 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. Most people with previous exposure to M. tuberculosis will test positive on the tuberculin test, as will some people exposed to bacteria that are related to the TB germ.

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

The doctor 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 four weeks to confirm the diagnosis. An additional two to three weeks usually are n<eeded to determine which antibiotics the bacteria are susceptible to.

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Can TB be cured?

With appropriate antibiotic treatment, TB can be cured in more than nine out of ten patients.

Successful treatment of TB depends on close cooperation between the patient and doctor and other health care workers. Treatment usually combines several different antibiotic drugs which are given for at least six months, sometimes for as long as 12 months.

Patients must take their medicine on time every day for the 6 to 12 months. Some TB patients stop taking their prescribed medicines because they may feel better after only a couple of weeks of treatment. Another reason they may stop taking their medicine is because TB drugs can have unpleasant side effects.

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Why is it so important to finish all of the TB medicine?

If patients don't take all their medicine the way their doctor tells them, they can become sick again and spread TB to their friends and family. Additionally, when patients do not take all the drugs the doctor has prescribed or skip times when they are supposed to take them, the TB bacteria learn to outwit the TB antibiotics, and soon those medications no longer work against the disease. If this happens, the person now has resistant TB infection. Some patients have disease that is resistant to two or more drugs. This is called multidrug-resistant TB or MDR-TB because the TB germ, M. tuberculosis resists eradication with more than drug. This form of TB is much more difficult to cure.

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Can MDR-TB be treated?

Treatment for MDR-TB often requires the use of special TB drugs, all of which can produce serious side effects. To cure MDR-TB, patients may have to take several antibiotics, at least three to which the bacteria still respond, every day for up to two years. However, even with this treatment, between four and six out of ten patients with MDR-TB will die, which is the same as for patients with normal TB who do not receive treatment.

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How is TB prevented?

TB is largely a preventable disease. In the United States, doctors try to identify persons infected with M. tuberculosis as early as possible, before they have developed active TB. They will give a drug called isoniazid (INH) to prevent the active disease. This drug is given every day for 6 to 12 months. INH can cause hepatitis in a small percentage of patients, especially those older than 35 years. A nurse may watch the patients take their medicine to make sure all pills are taken.

Hospitals and clinics can take precautions to prevent the spread of TB. Precautions include using ultraviolet light to sterilize the air, special filters, and special respirators and masks. Until they can no longer spread the TB germs, TB patients in hospitals should be isolated in special rooms with controlled ventilation and airflow.

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Is there a vaccine for TB?

In those parts of the world where the disease is common, the World Health Organization (WHO) recommends that infants receive a vaccine called BCG made from a live weakened bacterium related to M. tuberculosis. BCG vaccine prevents M. tuberculosis from spreading within the body, thus preventing TB from developing.

However, the vaccine has its drawbacks. It does not protect adults very well against TB. In addition, BCG interferes with the TB skin test, showing a positive skin test reaction in people who have received BCG vaccine. In countries where BCG vaccine is used, the ability of the skin test to identify persons that are infected with M. tuberculosis is limited. Because of these limitations, more effective vaccines are needed and BCG is not recommended for general use in the United States.

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How is M. tuberculosis infection different in people with HIV infection?

The World Health Organization (WHO) estimates that 10 million people worldwide are infected with the M. tuberculosis bacterium and HIV virus at the same time. The primary cause of death in these patients is from TB, not AIDS. In the United States, it is estimated that about 2 out of ten persons who have TB are also infected with HIV.

One of the first indications that a person is infected with HIV may be that he or she suddenly develops TB. This form of TB often occurs in areas outside the lungs, particularly when the patient is in the later stages of AIDS.

In the United States, it is much more likely for persons infected with M. tuberculosis and HIV to develop active TB than it is for someone that is only infected with M. tuberculosis. However, TB disease can be prevented and cured, even in people with HIV infection.

People with MDR-TB that are also infected with HIV appear to have a more rapid and deadly disease course than do those patients with MDR-TB who are otherwise healthy. If no medicines are available for these patients as many as eight out of ten may die, often within months of diagnosis.

Diagnosing TB in HIV-infected people is often difficult. HIV infected patients frequently have disease symptoms similar to those of TB, and may not react to the standard TB skin test because their immune system does not work properly. X-rays, sputum tests, and physical exams may also fail to provide evidence of infection with M. tuberculosis in HIV-infected individuals.

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How is research helping the fight against TB?

The National Institute of Allergy and Infectious Diseases (NIAID) leads TB research at the National Institutes of Health. NIAID supports not only studies to better understand how M. tuberculosis infects and causes disease in humans but also how the human immune system responds to it. This research will help to develop new tools to diagnose TB, find better vaccines, and new medicines against TB. Below are some important advances that have been made in TB research:

Diagnosis: Potential new tests to speed the diagnosis of TB from four weeks to two days; differences found in the DNA of M. tuberculosis and the bacterium used in the BCG vaccine may lead to a test to tell the difference between people who really have TB and those who only react to previous BCG vaccination.

Treatment: Discovery of the molecules responsible for drug resistance, knowledge that will help doctors quickly select the best treatments for their patients; a new drug under study can be taken less often to help patients comply with their treatment regimen.

Vaccines: More than 90 vaccine candidates have been developed and tested in animals.

Training: An innovative TB telemedicine program where NIAID physicians share their expertise with doctors in Texas; an urban program in Washington, DC where NIAID TB clinical trials are made more accessible to inner city patients; international collaborations with investigators to help them build research capabilities, and carry out research that will benefit populations in countries where TB disease is most common.

Recognizing that disease knows no borders, NIAID has developed a global TB research agenda. A concerted global effort will require collaborations with sister agencies and other organizations with similar goals such as the Global Alliance for TB Drug Development and the STOP TB initiative, as well as partnerships with governments and scientists from countries where the burden of tuberculosis is greatest.

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Tuberculosis