Tuberculosis, or TB, is an infectious disease caused by rod-shaped bacteria called Mycobacterium tuberculosis. It mostly attacks the lungs but can also infect other organs. TB has been known for centuries and was first
recognized as a contagious disease in the eleventh century by Ibn Sina (980–1037), a Persian doctor. TB was not recognized as a single illness until the 1820s, however. Robert Koch (1843–1910), the German doctor considered the father of microbiology, identified M. tuberculosis as the cause of tuberculosis in 1882. He received the Nobel Prize in physiology or medicine for this discovery in 1905.
Although it is no longer the leading cause of death in the United States, TB is a leading cause of infection-related death worldwide. The World Health Organization (WHO) estimates that 3 million people die of TB each year around the world.
TB is spread by droplets coughed or sneezed into the air. Other people can become infected if they breathe in these droplets. TB is not known to affect any species other than humans. After the tuberculosis bacteria enter the body, they pass down through the airway to the lungs, with one of three outcomes:
- The bacteria multiply and cause primary tuberculosis. This condition is also called active TB.
- The bacteria may become dormant (inactive) and the patient will not feel sick or be able to spread the disease. This condition is called latent TB, because the patient will test positive for the bacterium if they have a skin test. About 90 percent of people infected with TB have the latent form of the disease.
- The bacteria are dormant for a while, but then begin to multiply again and the patient feels sick. This condition is called reactivation TB. About 10 percent of people with latent TB will eventually develop reactivation TB during their lifetimes, half of these cases occur in the first two years.
A person with active TB may first notice a cough lasting three weeks or longer with bloody sputum (mucus or phlegm). The coughing or even breathing may cause pain. Night sweats and a low-grade fever usually appear. Another common symptom is loss of appetite and unintended weight loss; in fact, TB used to be called consumption because the patient's body looked as if it were being consumed, or eaten up, from within.
In addition to the lungs, tuberculosis can spread through the bloodstream and affect the spine, bones and bone marrow, joints, kidneys, muscles, and central nervous system, which might create symptoms such
as severe back pain, blood in the urine, abdominal pain, swollen lymph nodes, or skin ulcers. These manifestations of TB are usually part of reactivation TB and can occur without any evidence of lung TB.
The frequency of active TB dropped in the United States even before the introduction of antibiotics in the 1950s, a decrease related to better living conditions and nutrition. It began to rise again in the 1980s among ethnic minorities and persons with HIV infection. In 1993 there were 25,200 cases of TB reported in the United States; that number declined again to 13,300 cases in 2007. There are about 4.4 cases of active TB in the United States for every 100,000 people. Over half these cases involve immigrants from four countries: Mexico, the Philippines, India, and Vietnam. The Centers for Disease Control and Prevention (CDC) estimates that 10–15 million people in the United States have latent TB as of 2008.
In general, it is difficult to become infected with the TB bacillus unless one lives in close long-term contact with a person with active TB. There is increased risk of infection for these groups:
- The elderly
- Those who have immune systems weakened by such diseases as AIDS, diabetes, rheumatoid arthritis, kidney disease, or Crohn disease
- Those who are malnourished or severely underweight
- Patients who have developed silicosis (a lung disease caused by breathing in rock dust produced by blasting and drilling)
- Homeless people
- People living in refugee camps or other crowded conditions
- Drug or alcohol abusers
- Patients being treated for cancer with chemotherapy, or who have received organ transplants
- Those who work or live in hospitals, nursing homes, prisons, and other institutions for long-term care
- People who come from countries with high rates of active TB
TB is caused by a bacterium, Mycobacterium tuberculosis. The bacteria enter a person's respiratory tract through the nose or throat, then travel to the lungs, where they multiply within the tiny air sacs known as alveoli. In the alveoli, the bacteria are picked up by cells that carry them to nearby lymph nodes in the chest cavity. The TB bacteria can then spread into the bloodstream and other organs or tissues.
Drug-Resistant Tuberculosis Bacteria
Beginning in the 1980s, doctors began to notice that people who failed to take medications for active TB were developing strains of the TB bacillus that are resistant to treatment. The two drugs that are considered most effective in treating TB are isoniazid (INH) and rifampin. A patient is considered to have multiple drug-resistant tuberculosis (MDR-TB) if a laboratory test shows that the bacteria in the sample are resistant to those two specific drugs. Patients diagnosed with MDR-TB must be treated for twice as long as those with ordinary TB. The drugs used to treat MDR-TB are less effective, often much more expensive, some have to be injected rather than taken by mouth, and they can produce severe side effects. Public health doctors insist that patients with MDR-TB be treated by the DOT method described in the main entry.
An even more severe form of drug-resistant TB is known as extremely drug-resistant TB or XDR-TB. It is diagnosed when a sputum test shows resistance to an injectable drug and a drug called levofloxacin as well as rifampin and isoniazid. XDR-TB is much more difficult to treat with the drugs available and has a very
The most common symptoms of active TB (that is still limited to the lungs) include:
- A cough that lasts longer than three weeks
- Pain when coughing or even breathing
- Loss of appetite and weight loss
- Pale complexion
- Low-grade fever and night sweats
- Clubbing of the fingers or toes
The diagnosis of TB can be difficult because the disease does not produce symptoms immediately even with an active infection. People exposed to a person with active TB and are at high risk to become infected should check with their doctor even if they do not feel sick. The doctor will perform a physical examination. Some people with active TB develop noises in the lungs known as crackles that can be heard through a stethoscope. The doctor will also feel the lymph nodes in the patient's neck and look for such other signs as clubbing of the finger tips.
A skin test known as the Mantoux test (also called the PPD test) is used to screen patients for infection with TB. The doctor injects a fluid derived from the TB bacteria under the top layer of the patient's skin. If the person has been infected with TB, a raised hard flat area will develop within forty-eight to seventy-two hours at the site of the injection. The patient's risk factors are used to determine how large the bump must be to be considered a positive reaction. A newer test is a blood test approved by the Food and Drug Administration (FDA). Called the QuantiFERON-TB Gold (QFT) test, it detects the presence of TB bacteria in the patient's blood and gives results in a day. As of 2008, it was not yet available in all parts of the United States, however.
A patient who tests positive on either the Mantoux or the QFT test will then be given a chest x ray and a sputum test. A sample of sputum is sent to a laboratory where it is cultured, to see whether the person has active TB and to see whether the TB bacteria respond to standard antibiotics. If not, the patient has multidrug-resistant TB (MDR-TB) and will need special treatment.
Patients who are found to have latent TB may be treated with a drug called isoniazid or INH. This drug is given to destroy dormant bacteria so that they cannot reactivate in the future. The patient usually takes INH for six to nine months.
Patients with active TB are treated with INH plus a combination of three other drugs to make sure that all the TB bacteria are destroyed. The doctor may change one of the drugs during therapy if it turns out that the patient has a form of the TB bacillus resistant to that particular drug. Drug treatment for TB is a long-term process that can take anywhere from six months to a year for non-drug resistant TB; MDR-TB may require two full years of drug therapy. The patient may be admitted to the hospital for the first two weeks of treatment or until tests show that they are no longer contagious.
It is very important for patients with active TB to take all their drugs exactly as directed for the full course of therapy even though they will usually start to feel better in a few weeks. If they do not, TB bacteria may survive and develop resistance to the drugs. MDR-TB is much more difficult to treat and is very dangerous. To make sure that patients take their medications correctly, some doctors or clinics use an approach called directly observed therapy or DOT. In DOT, a nurse or other health care worker gives the patient their drugs in the clinic so that they do not have to remember to take their medications at home.
Some patients with severe MDR-TB may need surgery as well as medications. Removing part of an infected lung reduces the number of active bacteria in the patient's body; it may also increase the effectiveness of drug therapy.
The prognosis of active tuberculosis depends on the patient'sage, overall health, and whether he or she has a drug-resistant strain of TB. About 4 percent of patients in the United States with active TB die from the disease; this is much lower than the 50 percent death rate in the 1920s. In general, people with MDR-TB have a worse prognoses than those who do not; patients with HIV infection as well as MDR-TB have poor prognoses.
Tuberculosis is a preventable disease. In addition to diagnosing and treating people with latent TB before they develop active infection, people can lower their risk of getting TB in several ways:
- Keep the immune system healthy.
- Get an annual skin test if one is frequently exposed to TB or if one has a weakened immune system.
- Get treatment for latent TB if infected.
- Patients diagnosed with active TB should stay home, avoid close contact with others, and cover their mouths when they sneeze or cough.
- A vaccine against TB, known as BCG, is widely used in Europe. It is not very effective in adults, however, and is not widely used in the United States because it can cause a false-positive result on the Mantoux test.
Before the 1980s, it was hoped that tuberculosis could be wiped out completely. The emergence of MDR-TB in the 1980s, however, destroyed that hope. In 1993 WHO declared the resurgence of TB as a global health emergency. Researchers are trying to develop a more effective vaccine than BCG and find new antibiotics to treat MDR-TB and XDR-TB. They are also testing fixed-dose combination tablets of anti-TB drugs to simplify patients’ medications.
SEE ALSO AIDS; Pneumonia
WORDS TO KNOW
Alveoli (singular, alveolus): Tiny air sacs in the lungs where the exchange of oxygen for carbon dioxide in the blood takes place.
Clubbing: Thickening of the tips of the fingers or toes.
Directly observed therapy (DOT): Treatment in which nurses or health care workers administer medications to patients in a clinic or doctor's office to make sure that the patients take the drugs correctly.
Latent: Referring to a disease that is inactive.
Sputum: Mucus that comes up when a person coughs.
Reichman, Lee B., and Janice Hopkins Tanne. Timebomb: The Global Epidemic of Multi-Drug-Resistant Tuberculosis. New York: McGraw-Hill, 2002.
American Lung Association (ALA). Tuberculosis (TB). Available online at http://www.lungusa.org/site/apps/nlnet/content3.aspx?c=dvLUK9O0E&b=4294229&ct=3052615 (updated May 2007; accessed September 14, 2008). The ALA was founded in 1904 as the National Tuberculosis Association.
Centers for Disease Control and Prevention (CDC). Questions and Answers about TB, 2007. Available online at http://www.cdc.gov/tb/faqs/default.htm (updated June 26, 2008; accessed September 14, 2008).
Mayo Clinic. Tuberculosis. Available online at http://www.mayoclinic.com/health/tuberculosis/DS00372 (updated December 21, 2006; accessed September 14, 2008).
National Institute of Allergy and Infectious Diseases (NIAID). Drug-Resistant TB—A Visual Tour. Available online at http://www3.niaid.nih.gov/topics/tuberculosis/Understanding/WhatIsTB/VisualTour/firstLine.htm (updated May 18, 2007; accessed September 14, 2008). This is a slide show illustrating the various treatment options for drug-resistant TB.
National Institute of Allergy and Infectious Diseases (NIAID). Tuberculosis (TB). Available online at http://www3.niaid.nih.gov/topics/tuberculosis/default.htm (updated June 25, 2008; accessed September 14, 2008).
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