Tuberculosis, << too `bur` kyuh LOH sihs, >> is an infectious disease that mainly affects the lungs but can also involve other organs. Tuberculosis is often called TB and was called consumption in the past. This disease once ranked among the most common causes of death in the world. Today, improved methods of prevention, detection, diagnosis, and treatment have greatly reduced both the number of people who get the disease and the number of people who die from it. However, tuberculosis remains a major concern in developing countries where these improved methods are not widely available.
Tuberculosis strikes people of all ages and races. Those at greater risk are homeless or undernourished people, and those with immune systems weakened by diseases, such as cancer or AIDS, or by HIV, the virus that causes AIDS. The disease can also afflict animals, especially such livestock as cattle, hogs, and poultry. The disease is caused by rod-shaped bacteria called tubercle bacilli. The German physician Robert Koch discovered tubercle bacilli in 1882. Tubercle bacilli belong to a genus (group) of bacteria called Mycobacterium. They are aerobes—that is, they must have oxygen to live.
How tuberculosis affects the body
In most cases, a person becomes infected with tubercle bacilli by inhaling tiny droplets of moisture that contain Mycobacterium tuberculosis bacteria. These droplets form when a person with tuberculosis coughs or sneezes. Infection also can result from eating food contaminated with the bacteria or from drinking milk from cattle infected with Mycobacterium bovis bacteria. Such infection rarely occurs in developed countries, where milk is pasteurized and animals are routinely tested for diseases.
The body expels many inhaled tubercle bacilli before they can do harm. Some bacilli settle into the layer of mucus that lines most of the respiratory system, including the nasal passages and the tracheobronchial tree. The tracheobronchial tree is the branching system of tubes that brings air to and from tiny air sacs called alveoli in the lungs. It consists of the trachea (windpipe), two bronchi, and hundreds of thousands of smaller airways called bronchioles. It is lined with cells that can move the layer of mucus covering them upward. Bacilli trapped in the mucus layer are moved up the airways toward the throat, mouth, and nose. The bacilli may then be sneezed, spat, coughed, or blown out. They also may be swallowed and pass harmlessly through the digestive tract.
Primary infection
is likely to result from tubercle bacilli that penetrate beyond airways lined with mucus into the alveolar sacs deep in the lungs. Primary infection is a stage in the development of tuberculosis, but it does not always lead to the disease. Tubercle bacilli that enter an alveolar sac are usually engulfed by large, amebalike cells called alveolar macrophages. These cells are usually able to digest bacteria. However, tubercle bacilli resist digestion and most of them actually thrive and multiply inside the macrophages. Some of the macrophages carrying these bacteria may migrate to the mucus layer and be carried out of the body. Others may carry the bacteria to another part of the lungs or even into the blood.
Within several weeks of the initial infection, a small, hard swelling called a tubercle forms in the alveolar sac. The tubercle begins to form as macrophages containing tubercle bacilli clump together. These macrophages are joined by T cells and possibly other white blood cells. In time, these clumps of cells grow larger and destroy surrounding lung tissue.
As cells inside the tubercle die, they form caseous (soft, cheeselike) areas that support the growth of tubercle bacilli. At the same time, tough scar tissue begins to surround the tubercles. This scar tissue prevents further spreading of the bacilli, and it may decrease the amount of oxygen they receive. The bacilli walled off by the scar tissue remain alive but inactive.
In an otherwise normal, healthy adult, primary infection by the bacilli may produce no symptoms and may thus go undetected. In some cases, however, primary infection causes fever, swelling of the glands, and pneumonia.
The disease
known as tuberculosis develops if the tubercle bacilli again become active. It may occur immediately after the primary infection, especially in infants, children, and the elderly, and in people who have other illnesses. In most cases, however, tuberculosis develops many years after the primary infection has occurred. What causes this reactivation of the bacilli is not entirely clear. It may occur when the body’s defense mechanisms are impaired by another illness or by old age. Reactivation of the bacteria causes the tubercles to rupture and the bacilli to reproduce rapidly. Cells may carry the bacteria to other parts of the lung or into the lymph vessels. The bacteria also may enter blood vessels and be transported to other organs, including the bones, brain, joints, kidneys, and skin.
In tuberculosis of the lungs, called pulmonary tuberculosis, alveolar macrophages and white blood cells accumulate at the sites of the reactivated bacteria and form caseous material. The caseous material eventually liquefies and moves up the respiratory tract with the mucus layer. The patient coughs this mucus and caseous material up as sputum.
Coughing and sputum production are the most common early symptoms of pulmonary tuberculosis. The cough is not usually severe and the symptoms are often mistaken for a lingering cold. If blood vessels in the lungs are damaged, there may be blood in the sputum. In advanced stages of the disease, the patient may cough up large quantities of blood. Other symptoms of advanced tuberculosis include chest pain, fever, sweating at night, fatigue, weight loss, and loss of appetite. Although tuberculosis may lead to a rapid death, it occurs more commonly as a long-term, progressively worsening disease.
Diagnosis of tuberculosis
Physicians use several methods to detect tuberculosis. The chief methods are skin tests, chest X rays, and laboratory tests.
Skin tests
can determine if a person has been infected with tubercle bacilli in the past. However, further tests are needed to determine whether the patient has the active disease or an inactive infection. All types of skin tests are based on specific allergic reactions to the tubercle bacilli. The body develops the allergy to the bacilli within a few weeks after the primary infection.
Chest X rays
may reveal tubercles or other signs of tuberculosis in the lungs. Chest X rays are usually done after a skin test has indicated a previous infection. However, chest X rays done for other reasons sometimes reveal the presence of tubercles.
Laboratory tests
are normally the final step in the diagnosis of tuberculosis. A physician examines the patient’s sputum under a microscope to determine if bacilli are present. If bacilli are present, they are cultured (grown in laboratory dishes or test tubes). Culturing determines if they are M. tuberculosis or other bacilli. It also helps find out which drugs will be most effective against the bacteria.
Treatment of tuberculosis
The first effective treatment for tuberculosis was provided by health resorts called sanitariums. Sanitariums were developed during the later 1800’s by physicians in Europe and the United States. At a sanitarium patients received bed rest, fresh air, and mild exercise. They also were isolated from the general public and thus kept from infecting other people. Sanitarium treatment helped many tuberculosis patients overcome the disease. However, most of them had to spend months or even years in a sanitarium before they recovered.
Today, almost all tuberculosis patients can be treated successfully with drugs taken for a period of six months. Physicians prescribe combinations of drugs—for example, isoniazid (INH) in combination with rifampin, pyrazinamide, ethambutol, or streptomycin. These drugs stop the bacteria from multiplying in the body. Doctors typically prescribe multiple drugs because tubercle bacilli can become resistant to one or more medications. Drug-resistant TB (MDRTB) can also develop if a patient stops taking the prescribed medications before the disease is completely cured. Tuberculosis that is resistant to one or more drugs is difficult to treat and cure. If it spreads, it can be dangerous to other people.
Prevention of tuberculosis
The most effective way to limit the spread of TB is to identify people with the active disease and treat them adequately. A patient with untreated, active TB can infect an estimated 15 people in a year. The drug isoniazid prevents most detected tuberculosis infections from developing into the disease. Doctors often prescribe isoniazid for people who have a positive skin test. In hospitals and other places where patients come into contact with people who do not have the disease, the use of ultraviolet overhead light helps prevent the disease from spreading.
A vaccine called BCG (Bacillus Calmette-Guerin) has been used in many parts of the world in an effort to prevent tuberculosis. However, the vaccine is not always effective, especially among certain populations.
In spite of advances against the disease, tuberculosis remains a major cause of sickness and death in many less developed countries. During the 1980’s, outbreaks again began to increase in the United States and other developed nations where TB had been declining. The number of TB infections grew dramatically among people whose immune systems had been weakened by HIV, which is the virus that causes AIDS. TB infections also rose among the homeless and people who misuse drugs.
As the number of infections rose, a growing number of TB bacteria began to develop resistance to drugs used to treat the disease. This development led to renewed concern about TB as a worldwide public health problem. In 1998, researchers successfully identified the complete sequence of genes (units of heredity) in tuberculosis bacteria. These genes contain chemical instructions that control all the processes in the bacterial cells, including how they infect people and how they resist drugs. Scientists hope that knowledge about the bacterial genes will reveal more effective ways to prevent and treat TB.
