Tuberculosis

I

Introduction

Tuberculosis (TB), chronic or acute bacterial infection that primarily attacks the lungs, but which may also affect the kidneys, bones, lymph nodes, and brain. The disease is caused by Mycobacterium tuberculosis, a rod-shaped bacterium. Many people harbor the bacteria but have no symptoms of disease. When symptoms develop, they include coughing, chest pain, shortness of breath, loss of appetite, weight loss, fever, chills, and fatigue. Children and people with weakened immune systems are the most susceptible to TB.

In 1993 the World Health Organization (WHO) declared TB to be a global emergency, the first such designation ever made by that organization. According to WHO, someone becomes infected with the bacteria that cause TB every second. One-third of the world’s population is infected with the bacteria, and as many as one in ten of those infected will develop active symptoms of tuberculosis at some point in their lives. People living with HIV are at much greater risk than others.

II

Transmission and Infection

The TB-causing bacteria are transmitted from person to person, usually in infected air droplets. When someone who has symptomatic TB coughs, sneezes, or speaks, small particles that carry two to three bacteria surrounded by a layer of moisture are released in the air. When someone else inhales these particles, the bacteria may lodge in that person’s lungs.

A less common route of transmission is through the skin. Pathologists and laboratory technicians who handle TB specimens may contract the disease through skin wounds. TB has also been reported in people who have received tattoos and people who have been circumcised with unsterilized instruments.

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A person may become infected with TB bacteria and not develop the disease. His or her immune system may destroy the bacteria completely. In fact, only 5 to 10 percent of those infected with TB actually become sick. The bacteria may remain inactive for years until a weakening of the body’s resistance provides the bacteria with an opportunity to multiply and produce symptoms. Someone with an inactive infection cannot transmit the disease. The disease can take two paths: primary and secondary.

A

Primary TB

Primary TB does not produce noticeable symptoms in its early stages, when it is not contagious. Macrophages, immune cells that detect and destroy foreign matter, ingest the TB bacteria and transport them to the lymph nodes where they may be inhibited or destroyed. If the immune cells fail to control the infection, the bacteria can multiply.

If the TB bacteria multiply, active primary tuberculosis develops. Patients with active primary TB experience such symptoms as coughing, night sweats, weight loss, and fever. A chest X ray may show shadows in the lung or fluid collection between the lung and its lining. If the immune system destroys the bacteria, the patient may experience no more than mild symptoms, such as a cough. If the bacteria are inhibited, rather than destroyed, the body’s immune cells and the bacteria form a lump known as a granuloma or tubercle. In effect, the immune cells form a wall around inactive bacteria. As long as the immune system remains strong, the TB bacteria remain walled off and inactive. The tubercles may appear as shadows in a chest X ray. If the immune system later becomes weakened, the tubercle may open, releasing the bacteria, and the infection may develop into secondary TB.

B

Secondary TB

In secondary, or post-primary, TB, the formerly dormant bacteria multiply and destroy tissue in the lungs. They also may spread to the rest of the body via the bloodstream. Fluid or air may collect between the lungs and the lining of the lungs, while tubercles continue to develop in the lung, progressively destroying lung tissue. Coughing of blood or phlegm may occur.

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