TB
Tuberculosis (TB)
Tuberculosis is a contagious infection caused by an airborne bacterium, Mycobacterium tuberculosis.
Tuberculosis usually affects the lungs, although it can attack almost any organ in the body. Other mycobacteria (such as Mycobacterium bovis or Mycobacterium africanum) occasionally can cause a similar disease.
Tuberculosis has been a serious public health problem for a long time. In the 1800s, the disease was responsible for more than 30% of all deaths in Europe. With the advent of antituberculosis antibiotics in the 1940s, the battle against tuberculosis seemed to be won. Unfortunately—because of factors such as inadequate public health resources, reduced immune response due to AIDS, the development of drug resistance, and extreme poverty in many parts of the world—tuberculosis continues to be a deadly disease.
Worldwide, there are 8 million new cases of symptomatic tuberculosis and 3 million deaths from the disease every year. It is believed that one third of all the people in the world have a dormant (latent) tuberculosis infection, although only about 5 to 10% progress to active tuberculosis disease.
How Infection Develops
With most infectious diseases (such as strep throat or pneumonia), a person becomes sick right after the microorganism enters the body and is noticeably ill within 1 or 2 weeks. Tuberculosis does not follow this pattern.
Stages of Infection: Except for very young children, few people become sick immediately after tuberculosis bacteria enter their body (primary infection). Many tuberculosis bacteria that enter the lungs are immediately killed by the body's defenses. Those that survive are captured inside white blood cells called macrophages. The captured bacteria can remain alive inside these cells in a dormant state for many years, walled off inside tiny scars (latent infection). In 90 to 95% of cases, the bacteria never cause any further problem, but in about 5 to 10% of infected people they start to multiply (active disease). It is in this active phase that an infected person actually becomes sick and can spread the disease.
More than half the time, activation of dormant bacteria happens within the first 2 years, but it may not occur for a very long time.
Doctors do not always know why the dormant bacteria become active, but it often occurs when the person's immune system becomes impaired—for example, from very advanced age, the use of corticosteroids, or AIDS. Like many infectious diseases, tuberculosis spreads more quickly and is much more dangerous in people who have a weakened immune system. For such people (including the very young, the very old, and those who are also infected with HIV), tuberculosis can be life threatening.
Transmission of Infection: Mycobacterium tuberculosis can live only in people; it cannot be carried by animals, insects, soil, or other nonliving objects. A person can be infected with tuberculosis only from another person who has active disease. Touching someone who has the disease does not spread it, because the bacteria are transmitted only through the air. Mycobacterium bovis, a bacterium that can live in animals, is an exception. In developing countries, children become infected with it by drinking unpasteurized milk from infected cattle.
People with active tuberculosis in their lungs contaminate the air with bacteria when they cough, sneeze, or even speak. These bacteria can stay in the air for several hours. If another person breathes them in, that person may become infected. People who have latent disease or tuberculosis that is not in their lungs do not spread bacteria into the air and cannot transmit the infection.
Progression and Spread of Infection: The progression of tuberculosis from latent infection to active disease varies greatly. For example, tuberculosis often progresses more rapidly in blacks and Native Americans than in whites because of inherited differences in resistance. Impaired immunity also plays a role. Progression to an active disease is far more likely and much faster in people with AIDS. A person with AIDS who becomes infected with Mycobacterium tuberculosis has a 50% chance of developing active tuberculosis within 2 months and a 5 to 10% chance of developing active disease each year thereafter.
In people with a fully functioning immune system, active tuberculosis is usually limited to the lungs (pulmonary tuberculosis). Tuberculosis that affects other parts of the body (extrapulmonary tuberculosis) comes from pulmonary tuberculosis that has spread through the blood. As in the lungs, the infection may not cause disease, but the bacteria may remain dormant in a very small scar. Latent organisms in these scars can reactivate later in life, leading to symptoms in the organs involved. In pregnant women, the tuberculosis bacteria may spread to the fetus and cause disease; however, such congenital tuberculosis is uncommon.
Symptoms and Complications
Cough is the most common symptom of tuberculosis. Because the disease comes on slowly, an infected person at first may blame the cough on smoking, a recent episode of flu, or asthma. The cough may produce a small amount of green or yellow sputum in the morning. Eventually, the sputum may be streaked with blood, although large amounts of blood are rare.
Another symptom is awakening in the night drenched with a cold sweat. Sometimes there is so much sweat that the person has to change nightclothes or even the bed sheets. However, these night sweats are not specific to tuberculosis. Along with the cough and night sweats, the person feels generally unwell, with decreased energy and appetite. Weight loss often occurs after the illness has been present for a while.
Rapidly developing shortness of breath along with chest pain may signal the presence of air in the space between the lungs and the chest wall. About one third of tuberculosis infections first show up as a pleural effusion. Eventually, many people with untreated tuberculosis develop shortness of breath as the infection spreads in the lungs.
Tuberculosis that infects the tissues covering the brain (tuberculous meningitis) is life threatening. In the United States and other developed countries, tuberculous meningitis most commonly occurs among older people. In developing countries, tuberculous meningitis is most common among children from birth to age 5. Symptoms include fever, constant headache, neck stiffness, nausea, and drowsiness that can lead to coma. Tuberculosis also may infect the brain itself, forming a mass called a tuberculoma.
The tuberculoma may cause symptoms such as headaches, seizures, or muscle weakness.
Tuberculous pericarditis is tuberculosis affecting the pericardium. This infection causes the pericardium to thicken and sometimes leak fluid into the space between the pericardium and the heart. This limits the heart's ability to pump and causes swollen neck veins and difficulty breathing.
Intestinal tuberculosis occurs mainly in developing countries. This infection may not produce any symptoms but can produce abnormal growth of tissue, which may be mistaken for cancer, at the infected area.
Diagnosis
Sometimes the first indication of tuberculosis is an abnormal chest x-ray or positive tuberculin skin test (also known as a Mantoux test or PPD for purified protein derivative), because these tests are often done as routine screening tests. When a person has symptoms that suggest tuberculosis, a chest x-ray is taken, a tuberculin skin test is performed, and a sputum sample is sent to the laboratory.
The sputum sample is examined under a microscope to look for tuberculosis bacteria and used to grow the bacteria in a culture. The microscopic examination is much faster than a culture but is less accurate. Cultures do not provide results for many weeks because tuberculosis bacteria grow slowly.
When symptoms indicate the possibility of tuberculous meningitis, a doctor may need to perform a spinal tap to obtain a sample of spinal fluid for analysis. Because tuberculosis bacteria are hard to find in spinal fluid, and cultures usually take weeks, the sample is often sent for a test called polymerase chain reaction (PCR), which can detect tiny amounts of the bacteria's DNA. A doctor generally begins antibiotic therapy on the mere suspicion of tuberculous meningitis to prevent death and minimize brain damage.
Treatment
A number of antibiotics are effective against tuberculosis. But because tuberculosis bacteria are very slow-growing, the antibiotics must be taken for a long time—usually for 6 months or longer. Treatment must be continued long after the person feels completely well; otherwise, the disease tends to relapse because it was not fully eliminated.
To treat tuberculosis, two or more antibiotics with different mechanisms of action are always given, because treatment with only one drug can leave behind a few bacteria resistant to that drug. With most other bacteria, this would not be enough to cause a relapse, but people treated with only one drug develop tuberculosis resistant to that drug. A third and fourth drug are usually used during the initial, intensive phase of treatment to shorten the duration of treatment and to ensure success even if drug resistance exists at the outset.
Prevention
There are two aspects of prevention: stopping the spread of disease and treating early infection before it becomes active disease.
Because tuberculosis bacteria are airborne, good ventilation with fresh air lowers the concentration of bacteria and limits their spread. Since tuberculosis is transmitted only by people with active disease, early recognition and treatment of active disease is one of the best ways to stop it from spreading. People with active tuberculosis should cough into a tissue to reduce the spread of bacteria, and they should remain in isolation until they are no longer coughing. After only a few days of treatment with the correct antibiotics, a person is less likely to spread the disease and usually does not need to be isolated for longer than a week or two.
The second aspect of prevention consists of treating people with a positive tuberculin skin test who are not yet ill. Preventive therapy definitely benefits younger people who have a positive tuberculin skin test. A person with a positive tuberculin skin test who becomes infected with HIV is at very high risk of developing active infection; similarly, a person who takes corticosteroids has a greatly increased risk of activation of latent tuberculosis. Thus, such people usually need treatment of latent tuberculosis infection.
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