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Tuberculosis

Tuberculosis (TB) is a disease caused by a bacterium called Mycobacterium tuberculosis (Mtb). The bacteria usually attack the lungs, but TB bacteria can attack any part of the body such as the kidney, spine, and brain. If not treated properly, TB disease can be fatal. According to World Health Organization (WHO) estimates, each year, 8 million people worldwide develop active TB and nearly 2 million die.

Causes

When a person breathes in Mtb-contaminated air, the inhaled TB bacteria reach the lungs. This causes an Mtb infection. However, not everyone infected with TB bacteria becomes sick. The bacteria can remain dormant (asleep) for years and not cause any TB disease. This is called latent TB infection. People who have latent TB infection do not get sick and do not spread the bacteria to others. But, about one in ten people infected with TB bacteria develop active TB disease at some point in their lives. So it is important to get the appropriate treatment and get rid of the bacteria even in latent TB infection.

For someone to develop active TB disease, the following two events must take place:

  • The bacteria enter the body and cause an Mtb infection.
  • The immune system cannot stop the TB bacteria from growing and spreading after the initial infection.

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.

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 (saliva).

Symptoms of TB involving areas other than the lungs vary, depending upon the organ or area affected.

Diagnosis

To identify those who may have been exposed to Mycobacterium tuberculosis (Mtb), healthcare 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 Mtb
  • Some people exposed to bacteria related to Mtb
  • Some people born outside the United States who were vaccinated with the TB vaccine used in other countries (see TB vaccine below)

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 Mtb infection.

The healthcare 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 Mtb 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.

Treatment

With appropriate antibiotic treatment, TB can be cured in most people.

Successful treatment of TB depends on close cooperation between patient and healthcare provider. Treatment usually combines several different antibiotic drugs that are given for at least 6 months, sometimes for as long as 12 months.

Some people with TB do not get better with treatment because their disease is caused by a TB strain that is resistant to one or more of the standard TB drugs. If that happens, their healthcare providers will prescribe different drugs and increase the length of treatment.

The Importance of Finishing the TB Medicine

People who do not take all the required medicines can become sick again and spread TB to others. Additionally, when people do not take all the prescribed medicines or skip times when they are supposed to take them, the TB bacteria evolve to outwit the TB antibiotics. Soon those medicines no longer work against the disease. If this happens, the person now has drug-resistant TB.

Some people have disease that is resistant to two or more drugs. This is called multidrug-resistant TB or MDR TB. This form of TB is much more difficult to cure.

Treatment for MDR TB

Treatment for MDR TB often requires the use of special TB drugs, all of which can produce serious side effects. People with MDR TB may have to take several antibiotics, at least three to which the bacteria still respond, every day for up to 2 years. Even with this treatment, however, between four and six out of ten patients with MDR TB will die, which is the same rate seen with TB patients who are not treated.

Prevention

TB is largely a preventable disease, and adequate ventilation is the most important measure to prevent its transmission in the community.

In the United States, healthcare providers try to identify people infected with Mycobacterium tuberculosis (Mtb) as early as possible, before they have developed active TB. They will give infected people a medicine called isoniazid (INH) to prevent active disease. This medicine is given every day for 6 to 12 months. INH can cause hepatitis (inflammation of the liver) in a small percentage of people, especially those older than 35 years.

Hospitals and clinics take precautions to prevent the spread of TB, which include using ultraviolet light to sterilize the air, special filters, and special respirators and masks. In hospitals, people with TB are isolated in special rooms with controlled ventilation and airflow until they can no longer spread TB bacteria.

TB Vaccine

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

BCG has its drawbacks, however. It does not protect adults very well against TB. In addition, BCG may interfere with the TB skin test, showing a positive skin test reaction in people who have received the vaccine. In countries where BCG vaccine is used, the ability of the skin test to identify people infected with Mtb is limited. Because of these limitations, U.S. health experts do not recommend BCG for general use in this country.

TB and HIV Infection

The World Health Organization (WHO) estimates 11.4 million people worldwide are infected with both Mycobacterium tuberculosis (Mtb) and HIV. The primary cause of death in those infected with body microbes is from TB, not AIDS. In the United States, health experts estimate about two out of ten people who have TB are also infected with HIV.

One of the first signs 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 person is in the later stages of AIDS.

It is much more likely for people infected with Mtb and HIV to develop active TB than it is for someone that is only infected with Mtb. Fortunately, TB disease can be prevented and cured, even in people with HIV infection.

People infected with both MDR TB and HIV appear to have a more rapid and deadly disease course than do those with MDR TB only. If no medicines are available, as many as eight out of ten people with both infections may die, often within months of diagnosis.

Diagnosing TB in people with HIV infection is often difficult. They frequently have disease symptoms similar to those of TB and may not react to the standard TB skin test because their immune systems do not work properly. X-rays, sputum tests, and physical exams may also fail to show evidence of Mtb infection in people infected with HIV.

 

Sources

Centers for Disease Control and Prevention, USA.

National Institute of Allergy and Infectious Diseases, USA.