Human Diseases and Conditions

What Is Tuberculosis?

Tuberculosis (also known as "TB") is a disease caused by a type of bacteria called Mycobacterium tuberculosis. TB mainly infects the lungs, although it can also affect other organs.
When someone with untreated TB coughs or sneezes, the air is filled with droplets containing the bacteria. Inhaling these infected droplets is the usual way a person gets TB.
One of the worst diseases of the 19th century, TB was the eighth leading cause of death in children 1 to 4 years of age in the 1920s. As the general standard of living and medical care got better in the United States, the incidence of TB decreased. By the 1960s, it wasn't even in the top 10 causes of death among children of any age group.
But TB is making a comeback in the United States today — particularly among the homeless, those in prison, and people whose immune systems have been weakened because of HIV infection.

Sign & Symptoms

Although your body may harbor the bacteria that cause tuberculosis, your immune system often can prevent you from becoming sick. For this reason, doctors make a distinction between:

• Latent TB. In this condition, you have a TB infection, but the bacteria remain in your body in an inactive state and cause no symptoms. Latent TB, also called inactive TB or TB infection, isn't contagious.
• Active TB. This condition makes you sick and can spread to others.

Signs and symptoms of active TB include:

• Unexplained weight loss
• Fatigue
• Fever
• Night sweats
• Chills
• Loss of appetite

Tuberculosis usually attacks your lungs. Signs and symptoms of TB of the lungs include:

• Coughing that lasts three or more weeks
• Coughing up blood
• Chest pain, or pain with breathing or coughing

Tuberculosis can also affect other parts of your body, including your kidneys, spine or brain. When TB occurs outside your lungs, symptoms vary according to the organs involved. For example, tuberculosis of the spine may give you back pain, and tuberculosis in your kidneys might cause blood in your urine.

Causes 

Tuberculosis is caused by an organism called Mycobacterium tuberculosis. The bacteria spread from person to person through microscopic droplets released into the air. This can happen when someone with the untreated, active form of tuberculosis coughs, speaks, sneezes, spits, laughs or sings. Rarely, a pregnant woman with active TB may pass the bacteria to her unborn child.
Although tuberculosis is contagious, it's not especially easy to catch. You're much more likely to get tuberculosis from a family member or close co-worker than from a stranger. Most people with active TB who've had appropriate drug treatment for at least two weeks are no longer contagious.
TB infection vs. active TB
If you breathe TB bacteria into your lungs, one of four things might happen:

• You don't become infected with TB. Your immune system immediately destroys the germs and clears them from your body.
• You develop latent TB infection. The germs settle in your lungs and begin to multiply. Within several weeks, however, your immune system successfully "walls off" the bacteria in your lungs, much like a scab forming over a wound. The bacteria may remain within these walls for years — alive, but in a dormant state. In this case, you're considered to have TB infection and you'll test positive on a TB skin test. But you won't have symptoms and won't transmit the disease to others.
• You develop active TB. If your immune defenses fail, TB bacteria begin to exploit your immune system cells for their own survival. The bacteria move into the airways in your lungs, causing large air spaces (cavities) to form. Filled with oxygen — which the bacteria need to survive — the air spaces make an ideal breeding ground for the bacteria. The bacteria may then spread from the cavities to the rest of your lungs as well as to other parts of your body.
  If you have active TB, you're likely to feel sick. Even if you don't feel sick, you can still infect others. Without treatment, many people with active TB die. Those who survive may develop long-term symptoms, such as chest pain and a cough with bloody sputum, or they may recover and go into remission.
  
• You develop active TB years after the initial infection. After you've had latent TB for years, the walled-off bacteria may suddenly begin multiplying again, causing active TB, also known as reactivation TB. It's not always clear what triggers this reactivation, but it most commonly happens after your immune system becomes weakened. Your resistance may be lower because of aging, drug or alcohol abuse, malnutrition, chemotherapy, prolonged use of prescription medications such as corticosteroids or TNF inhibitors, and diseases such as HIV/AIDS.

Only about one in 10 people who have TB infection goes on to develop active TB. The risk is greatest in the first two years after infection and is much higher if you have HIV infection.
HIV and TB
Since the 1980s, the number of cases of tuberculosis has increased dramatically because of the spread of HIV, the virus that causes AIDS. Tuberculosis and HIV have a deadly relationship — each drives the progress of the other.
Infection with HIV suppresses the immune system, making it difficult for the body to control TB bacteria. As a result, people with HIV are many times more likely to get TB and to progress from latent to active disease than are people who aren't HIV-positive.
TB is one of the leading causes of death among people with AIDS, especially outside the United States.  One of the first indications of HIV infection may be the sudden onset of TB, often in a site outside the lungs.
Drug-resistant TB
Another reason TB remains a major killer is the increase in drug-resistant strains of the bacterium. Ever since the first antibiotics were used to fight TB 60 years ago, the germ has developed the ability to survive attack, and that ability gets passed on to its descendants. Drug-resistant strains of TB emerge when an antibiotic fails to kill all of the bacteria it targets. The surviving bacteria become resistant to that particular drug and frequently other antibiotics as well. Today, for each major TB medication, there's a TB strain that resists its treatment.
The major cause of TB drug resistance is inadequate treatment, either because the wrong drugs are prescribed or because people don't take their entire course of medication.
There are two types of drug-resistant TB:

• Multidrug-resistant TB (MDR TB). This form of TB can't be killed by the two most powerful antibiotics for TB, isoniazid and rifampin. Although MDR TB can be successfully treated, it's much harder to combat than is regular TB and requires long-term therapy — up to two years — with drugs that are very expensive and can cause serious side effects. People with untreated MDR TB can transmit this serious type of TB to others.
• Extensively drug-resistant TB (XDR TB). XDR TB is a less common form of MDR TB in which the bacteria resist isoniazid and rifampin as well as most of the alternative or second line drugs used to treat TB. XDR TB has shown up across the world, including 49 cases in the United States between 1993 and 2006. Treatment for XDR TB is challenging and lengthy and leads to serious side effects and a higher rate of failure. Recently, the first cases of completely drug-resistant TB were reported — the bacteria could not be killed by any available TB drug.

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-like bacteria. Bacteria of the Mycobacterium family, including atypical mycobacteria, stain positive with special dyes and are referred to as acid-fast bacteria (AFB). A sample of the sputum also is usually taken and grown (cultured) in special incubators so that the tuberculosis bacteria can subsequently be identified as tuberculosis or atypical tuberculosis.
Several types of skin tests are used to screen for TB infection. These so-called tuberculin skin tests include the Tine test and the Mantoux test, also known as the PPD (purified protein derivative) test. In each of these tests, a small amount of purified extract from 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 about 48-72 hours after the injection. When only the skin test is positive, or evidence of prior TB is present on chest X-rays, the disease is referred to as "latent tuberculosis." This contrasts with active TB as described above, under symptoms.
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 10 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 anticancer 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. The organism can take up to six weeks to grow in culture in the microbiology lab. A special test to diagnose TB called the PCR (polymerase chain reaction) detects the genetic material of the bacteria. This test is extremely sensitive (it detects minute amounts of the bacteria) and specific (it detects only the TB bacteria). One can usually get results from the PCR test within a few days.

Prevention

Preventing TB depends on:

• avoiding contact with those who have the active disease
• using medications as a preventive measure in high-risk cases
• maintaining good living standards

New cases and potentially contagious patients are identified through proper use and interpretation of the tuberculin skin test.
A vaccine called BCG (Bacille Calmette-Guérin) is considered controversial because it isn't very effective in countries with a low incidence of TB, such as the United States. However, it may be considered for people moving to countries where TB is common.

Contagiousness

Tuberculosis is contagious when it's airborne and can be inhaled by others. The incubation period (the time it takes for a person to become infected after being exposed) varies from weeks to years, depending on the individual and whether the infection is primary, progressive, or reactivation TB.

Treatment

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 six 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%-10% lifelong risk that the TB will become active.
Taking isoniazid can be inadvisable (contraindicated) during pregnancy or for those suffering from alcoholism or liver disease. Also, isoniazid can have side effects. The side effects occur infrequently, but a rash can develop, and the individual 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. Another side effect of INH is a decreased sensation in the extremities referred to as a peripheral neuropathy. This can be avoided by taking vitamin B6 (pyridoxine), and this is often prescribed along with INH.
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). Four drugs are often taken for the first two months of therapy to help kill any potentially resistant strains of bacteria. Then the number is usually reduced to two drugs for the remainder of the treatment based on drug sensitivity testing that is usually available by this time in the course. 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 (termed "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 as prescribed is the most important cause of failure to cure the TB infection. In some locations, the health department demands direct monitoring of patient compliance with therapy.
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. Therefore, early diagnosis 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.