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Tuberculosis (TB)
 

What is tuberculosis?

Tuberculosis (TB) is an infectious disease caused by bacteria whose scientific name is Mycobacterium tuberculosis. TB most commonly affects the lungs but also can involve most any organ of the body. Many years ago, this disease used to be called "Consumption" because without effective treatment, these patients often would waste away. Today, of course, tuberculosis usually can be treated successfully with antibiotics.

How does a person get TB?

A person can become infected with tuberculosis bacteria when he or she inhales minute particles of infected sputum from the air. The bacteria get into the air when someone who has a tuberculosis lung infection coughs, sneezes, shouts, or spits (which is common in some cultures). People who are nearby can then possibly breathe the bacteria into their lungs. You don't get TB by just touching the clothes or shaking the hands of someone who is infected. Tuberculosis is spread (transmitted) primarily from person to person during close contact by breathing infected air.

There is a form of tuberculosis, however, that is transmitted by drinking unpasteurized milk. Related bacteria, called Mycobacterium bovis, cause this form of TB. It previously was a major cause of TB in children, but rarely causes TB now since most milk is pasteurized (a heating process that kills the bacteria).

What happens to the body when a person gets TB?

When the inhaled tuberculosis bacteria enter the lungs, they can multiply, causing a local lung infection (pneumonia). The local Lymph node associated with the lungs may also become involved. In addition, TB can spread to other parts of the body. The body's immune (defense) system, however, can fight off the infection and stop the bacteria from spreading. The immune system does so ultimately by forming scar tissue around the TB bacteria and isolating it from the rest of the body.

If the body is able to form scar tissue (fibrosis) around the TB bacteria, then the infection is contained in an inactive state. Such an individual typically has no symptoms and cannot spread TB to other people. The scar tissue and lymph nodes may eventually harden, like stone. (That is, these scars and nodes can calcify.)

Sometimes, however, the body's immune system becomes weakened and the TB bacteria break through the scar tissue. For example, the immune system can be weakened by old age, the development of another infection or a cancer , or certain medications such as CORTISONE or anti-cancer drugs. The break through of bacteria can result in a recurrence of the pneumonia and a spread of TB to elsewhere in the body. The kidneys, bone, and lining of the brain and spinal cord are the most common sites affected by the spread of TB beyond the lungs.

How common is TB and who gets it?

Over 8 million new cases of TB occur each year worldwide. In the United States, it is estimated that 10-15 million people are infected with the TB bacteria and 22,000 new cases of TB occur each year.

Anyone can get TB, but certain people are at higher risk including:

• Those who live with individuals who have active TB infection
• Poor or homeless people
• Foreign-born persons from countries that have a high occurrence of TB
• Nursing home residents and prison inmates
• Alcoholics and Intravenous drug users
• Those with diabetes, certain cancers, and HIV infection (the AIDS virus)
• Health care workers

There is no strong evidence for a genetically determined (inherited) susceptibility for TB.

How do patients with tuberculosis feel?

As previously mentioned, TB infection usually occurs initially in the upper part (lobe) of the lungs. The body's immune system, however, can stop the bacteria from continuing to reproduce. Thus, the immune system can make the lung infection inactive (dormant). On the other hand, if the body's immune system cannot contain the TB bacteria, the bacteria will reproduce (become active or reactivate) in the lungs and spread elsewhere in the body.

It may take many months from the time the infection initially gets into the lungs until symptoms develop. The usual symptoms that occur with an active TB infection are a generalized tiredness or weakness, weight loss, fever , and night sweats. If the infection in the lung worsens, then further symptoms can include coughing, chest pain, coughing up of sputum (material from the lungs) and/or blood, and shortness of breath. If the infection spreads beyond the lungs, the symptoms will depend upon the organs involved.

How does a doctor diagnose tuberculosis?

TB can be diagnosed in several different ways, including chest X-rays, analysis of sputum, and skin tests. Sometimes, the chest x-rays can reveal evidence of active tuberculosis pneumonia. Other times, the x-rays may show scarring (fibrosis) or hardening (calcification) in the lungs, suggesting that the TB is contained and inactive. Examination of the sputum on a slide (smear) under the microscope can show the presence of the

tuberculosis bacteria. A sample of the sputum can also be grown (cultured) in special incubators so that the tuberculosis bacteria can subsequently be identified.

Several types of skin tests are used to screen for TB. These so-called tuberculin skin tests include the Mantoux test, the Tine test, and the PPD. In each of these tests, a small amount of dead tuberculosis bacteria is injected under the skin. If a person is not infected with TB, then no reaction will occur at the site of the injection (a negative skin test). If a person is infected with tuberculosis, however, a raised and reddened area will occur around the site of the test injection. This reaction, a positive skin test, occurs in about 48 to 72 hours after the injection.

If the infection with tuberculosis has occurred recently, however, the skin test can be falsely negative. The reason for a false negative test with a recent infection is that it usually takes two to ten weeks after the time of infection with tuberculosis before the skin test becomes positive. The skin test can also be falsely negative if a person's immune system is weakened or deficient due to another illness such as AIDS or cancer, or while taking medications that can suppress the immune response, such as cortisone or anti-cancer drugs.

Remember, however, that the TB skin test cannot determine whether the disease is active or not. This determination requires the chest x-rays and/or sputum analysis (smear and culture) in the laboratory. A special test to diagnose TB, called the PCR (polymerase chain reaction)  for TB, is now available. This test is extremely sensitive (detects minute amounts of the bacteria) and specific (detects only the TB bacteria) but, of course, it also is very expensive.

How is tuberculosis treated?

A person with a positive skin test, a normal chest X-ray, and no symptoms most likely has only a few TB germs in an inactive state and is not contagious. Nevertheless, treatment with an antibiotic may be recommended for this person to prevent the TB from turning into an active infection. The antibiotic used for this purpose is called isoniazid (INH). If taken for 6 to 12 months, it will prevent the TB from becoming active in the future. In fact, if a person with a positive skin test does not take INH, there is a 5 to 10% lifelong risk that the TB will become active.

Taking isoniazid can be inadvisable (contraindicated) with pregnancy, alcoholism, and liver disease. Also, isoniazid can have side effects. The side effects occur infrequently, but a rash can develop and the patient can feel tired or irritable. Liver damage from isoniazid is a rare occurrence and typically reverses once the drug is stopped. Very rarely, however, especially in older people, the liver damage (INH hepatitis) can even be fatal. It is important, therefore, for the doctor to monitor a patient's liver by periodically ordering blood tests called "liver function tests" during the course of INH therapy.

A person with a positive skin test along with an abnormal chest x-ray and sputum evidencing TB bacteria has active TB and is contagious. As already mentioned, active TB usually is accompanied by symptoms, such as a cough, fever, weight loss, and fatigue.

Active TB is treated with a combination of medications along with isoniazid. Rifampin (Rifadin), ethambutol (Myambutol), and pyrazinamide are the drugs commonly used to treat active TB in conjunction with isoniazid (INH). Streptomycin, a drug that is given by injection, may be used as well, particularly when the disease is extensive and/or the patients do not take their oral medications reliably (poor compliance). Treatment usually lasts for many months and sometimes, for years. Successful treatment of TB is dependent largely on the compliance of the patient. Indeed, the failure of a patient to take the medications is the most important cause of failure to cure the TB infection.

Drug-resistant TB (TB that does not respond to drug treatment) has become a very serious problem in recent years in certain populations. For example, INH resistant TB is seen among patients from Southeast Asia. The reason for this INH resistance is not quite clear. However, the presence of INH-like substances in the cough syrups in that part of the world may play a role in causing the INH resistance. An even more serious problem is the multi-drug resistant TB that has been seen in prison populations. Poor compliance by the inmates is thought to be the main reason for this multi-drug resistance.

Surgery on the lungs may be indicated to help cure TB when medication has failed, but in this day and age, surgery for TB is unusual. Treatment with appropriate antibiotics will usually cure the TB. Without treatment, however, tuberculosis can be a lethal infection. Early diagnosis, therefore, is important. Those individuals who have been exposed to a person with TB, or suspect that they have been, should be examined by a doctor for signs of TB and screened with a TB skin test.

What's in the future for TB?

Conceivably, TB could have been eliminated by effective treatment, vaccinations, and public health measures by the year 2000. However, the emergence of HIV changed the whole picture. Because of HIV, a tremendous increase in the frequency (incidence) of TB occurred in the 80s and throughout the 90s. This increase in TB happened because suppression of the body's immune (defense) system by HIV allowed TB to occur as a so-called opportunistic infection. Hopefully, control of HIV in the future will check this resurgence of tuberculosis.

Tuberculosis At A Glance

• Tuberculosis (TB) is an infection, primarily in the lungs (a pneumonia), caused by bacteria called Mycobacterium tuberculosis. It is spread usually from person to person during close contact by breathing infected air.
• TB can remain in an inactive (dormant) state for years without causing symptoms or spreading to other people.
• When the immune system of a patient with dormant TB is weakened, the TB can become active (reactivate) and cause infection in the lungs or other parts of the body.
• The risk factors for acquiring TB include close contact situations, alcohol and IV drug abuse, and certain diseases (e.g., diabetes, cancer, and HIV) and occupations (e.g., health care workers).
• The most common symptoms of TB are fatigue, fever, weight loss, coughing, and night sweets.
• The diagnosis of TB involves skin tests, chest x-rays, and sputum analysis (smear and culture).
• Inactive tuberculosis may be treated with an antibiotic, isoniazid (INH), to prevent the TB infection from becoming active.
• Active TB is treated, usually successfully, with INH in combination with one or more of several drugs, including rifampin, ethambutol, pyrazinamide, and streptomycin.
• Drug-resistant TB is a serious, as yet unsolved public health problem, especially in Southeast Asia and in prison populations.
• The occurrence of HIV has been responsible for an increased frequency of tuberculosis. Control of HIV in the future, however, should substantially decrease the frequency of TB.