4. Tuberculosis

Tuberculosis is a contagious, potentially fatal infec­tion caused by the airborne bacterium Mycobacterium tuberculosis, M. bovis, or M.africanum.

Tuberculosis refers to disease most commonly caused by Mycobacterium tuberculosis, but occa­sionally caused by M. bovis or M. africanum. Al­though other mycobacteria cause diseases that mimic tuberculosis, those infections are not con­tagious, and most respond poorly to drugs that are very effective against tuberculosis.

People have been contracting tuberculosis since ancient times. Tuberculosis became a major scourge in Europe during the Industrial Revolu­tion, when overcrowding in cities was common, accounting for more than 30 percent of all deaths. With the development of the antibiotics strepto­mycin in the 1940s, isoniazid in the 1950s, etham-butol in the 1960s, and rifampin in the 1970s, the battle against tuberculosis seemed to be won. However, in the mid-1980s, the number of cases in the United States began to rise again. AIDS, combined with overcrowding and unsanitary con­ditions in many urban areas, homeless shelters, and prisons, has again made tuberculosis a seri­ous public health problem. The problem is espe­cially worrisome because some strains of tuber­culosis bacteria have become resistant to the antibiotics used to treat the disease. Nonetheless, in the United States, tuberculosis is starting again to decline as an epidemic.

Tuberculosis is more common among the el­derly. Of almost 23,000 cases reported in the United States in 1995, about 28 percent involved persons over age 65. There are three basic rea­sons why there are more cases in the elderly: (1) Many older persons became infected when tuberculosis was more common; (2) aging may reduce the effectiveness of the body's immune system, which can allow dormant bacteria to become re­activated; and (3) elderly people in chronic care facilities are likely to be in close contact with other elderly people at risk of contracting tuber­culosis.

The disease is also more common in blacks than in whites, partly because blacks are more likely to live in poverty and partly because of how tuberculosis has evolved. For thousands of years, tuberculosis exacted a heavy toll in Europe, which was populated primarily by whites; those who happened to be more resistant to the disease were favored for survival and reproduction. These people thus passed on their tuberculosis-resistance genes to subsequent generations. In contrast, ancestors of American blacks first en-countered tuberculosis after arrival in America, allowing much less time for them to develop tuberculosis-resistance genes to pass on to their progeny.

Symptoms and Complications

At first, an infected person may simply feel un­well or have a cough that is blamed on smoking or a recent episode of flu. The cough may produce a small amount of green or yellow sputum in the morning. The amount of sputum usually in­creases as the disease progresses. Eventually, the sputum may be streaked with blood, although large amounts of blood are rare.

One of the most common symptoms is awak­ening in the night drenched with a cold sweat so profuse that the person has to change nightclothes or even the bedsheets. Such a sweat is caused by the subsiding of a low-grade fever that is not apparent to the person.

Shortness of breath may signal the presence of air (pneumothorax) or fluid (pleural effusion) in the pleural space. About a third of infections that become manifest do so as a pleural effusion. About 95 percent of pleural effusions in young adults are caused by a recent infection with Mycobacterium tuberculosis. The diagnosis is usually difficult to make, but experienced doctors know the condition must be treated as tuberculosis or else about half of the infections will progress to full-blown tuberculosis in the lung or another organ.

In a new tuberculosis infection, the bacteria travel from the lesion in the lung to the lymph nodes that drain the lung. If the body's natural defenses can control the infection, it goes no fur­ther and the bacteria become dormant. However, in children the lymph nodes may become large and compress the bronchial tubes, causing a brassy cough and possibly even resulting in a col­lapsed lung. Occasionally, bacteria spread up the lymph channels to form a cluster of lymph nodes In the neck. An infection in these lymph nodes may break through the skin and discharge pus.

Tuberculosis can affect organs of the body other than the lung, a condition called extrapulmonary tuberculosis. The kidney and bones are probably the most common sites for extrapulmonary tuberculosis. Tuberculosis in the kidneys may produce few symptoms, but the infection may destroy part of the kidney. Tuberculosis can then spread to the bladder, but unlike other blad­der infections, it may cause few symptoms.

In men, the infection may also spread to the prostate, seminal vesicles, and epididymis, pro­ducing a lump in the scrotum. In women, tuber­culosis can scar the ovaries and fallopian tubes, causing sterility. From the ovaries, the infection may spread to the peritoneum (the membrane lining the abdominal cavity). Symptoms of this condition, called tuberculous peritonitis, may vary from fatigue and vague stomach pain with slight tenderness to excruciating pain that resem­bles that of appendicitis.

The infection may spread to a joint, causing tuberculous arthritis. The joint becomes inflamed and painful. The most commonly affected joints are the weight-bearing ones (the hips and knees), but the bones of the wrist, hand, and elbow may also be affected.

Tuberculosis can infect the skin, bowel, and adrenal glands. The infection has even been re­ported in the wall of the aorta (the main artery of the body), causing it to rupture. When tubercu­losis spreads to the pericardium (the membra­nous sac around the heart), the pericardium becomes distended with fluid, a condition called tuberculous pericarditis. The fluid may impair the heart's ability to pump blood. Symptoms include fever, enlargement of the neck veins, and short­ness of breath.

A tuberculosis infection that breaks out in the base of the brain (tuberculous meningitis) is ex­tremely dangerous. In the United States and other developed countries, tuberculous meningitis is now most common among the elderly. In devel­oping countries, it is most common among chil­dren from birth to age 5. Symptoms of tubercu­lous meningitis include fever, constant headache, nausea, and drowsiness that can lead to coma. The neck is often so stiff that the chin can't touch the chest. The longer treatment is delayed, the more likely irreparable brain damage will occur. Sometimes, as a person with tuberculous meningitis gets better, a tumor-like mass called a tuberculoma remains in the brain. The tuberculoma may cause symptoms such as muscle weakness, much as a stroke does, and may need to be surgically removed.

In children, the bacteria may infect the spine (vertebrae) and the ends of the long bones of the terns and legs. Pain occurs if the vertebrae are infected. Because x-rays of the person's spine may appear normal, other diagnostic techniques, such as computed tomography (CT) or magnetic resonance imaging (MRI), may be required. If the condition isn't treated, one or two vertebrae may collapse and lead to leg paralysis.

In developing countries, tuberculosis bacteria may be transmitted through contaminated milk and settle in the lymph nodes of the neck or in the small intestine. Because the mucous membrane of the digestive tract is resistant to the bac­teria, infection results only if a large number of bacteria remain in the small intestine for a long time or if the immune system is impaired. Intes­tinal tuberculosis may not produce any symp­toms but may result in an abnormal growth of tissue at the infected area, which may be mistaken for cancer.

Diagnosis

Often, the first indication of tuberculosis is an abnormal chest x-ray, taken as part of a diagnostic evaluation for a vague illness. On the x-ray, the disease shows up as irregular white areas against the normally dark background, although other in­fections and cancer can produce the same x-ray results. The x-ray may reveal pleural effusion or even an enlarged heart (pericarditis).

The diagnosis depends on results of the tuber­culin skin test and examination of sputum for Mycobacterium tuberculosis. Although a tuberculin skin test is one of the most useful tests for diag­nosing tuberculosis, it indicates only that an in­fection by the bacteria has occurred some time in the past. It doesn't reveal if the infection is currently active, only that live tuberculosis bac­teria are present somewhere in the body.

A tuberculin skin test is performed by injecting a small amount of protein derived from tubercu­losis bacteria between the layers of the skin, usually on the forearm. A control substance is sometimes injected at another site. The control substance is made from something that most peo­ple react to—yeast or fungi, for example. About 2 days later, the injection site is checked: Swelling and redness indicate a positive result. A person with no reaction to the control substance may have an immune system that isn't functioning properly. In this case, a negative tuberculin test result could be inaccurate. People with severe tuberculosis and a defective immune system also may have falsely negative test results.

To be sure of the diagnosis, a doctor has to obtain a sample of sputum, infected fluid, or tis­sue for laboratory analysis. A needle may be used to obtain a sample of fluid from the chest, the abdomen, a joint, or around the heart. A minor surgical procedure called a biopsy may be nec­essary to obtain a small piece of infected tissue. Sputum may provide an adequate sample from the lung; alternatively, a doctor may use an in­strument called a bronchoscope to inspect the bronchial tubes and obtain samples of mucus or lung tissue.

A spinal tap to obtain a sample of spinal cord fluid (cerebrospinal fluid) may be needed to look for evidence of tuberculous meningitis, an infec­tion of the membranes covering the brain and spinal cord. The fluid sample is sent to a labora­tory equipped to perform a test called polymerase chain reaction (PCR). Although test results are available quickly, a doctor generally begins antibiotic therapy on the mere suspicion of tuber­culous meningitis in order to prevent death and minimize brain damage.

Evaluating the kidneys for tuberculosis is con­siderably more difficult than evaluating the lungs. A sample of the person's urine can be used for a PCR test, but other tests may also be needed to determine what damage the disease has already caused. For example, a doctor may use an x-ray technique in which a dye is injected. The dye out­lines the kidneys on the x-rays and reveals any abnormal masses or cavities that may be caused by tuberculosis. Occasionally, the doctor will use a needle to obtain a tissue sample of a mass. The sample is examined under a microscope to help distinguish between cancer and tuberculosis.

To confirm tuberculosis of the female repro­ductive organs, a doctor may examine the pelvis through a tube with a light on the end (laparoscope). Sometimes the disease may be found by microscopic examination of scrapings taken from inside the uterus.

In some cases, a sample of tissue from the liver, a lymph node, or the bone marrow is required. Although such samples usually can be obtained using a needle, surgery may be necessary to ob­tain them.

Treatment

Antibiotics can usually cure even the most ad­vanced cases of tuberculosis. There are five anti­biotics that can be used, each of which can kill all but one in a million of the bacteria. Because an Infection of active pulmonary tuberculosis often contains a billion or more bacteria, any drug given alone would leave behind a thousand organisms totally resistant to it. Therefore, at least two drugs with different mechanisms of action are always given, which together can kill off virtually all the bacteria. Treatment must be continued long after the patient feels completely well, because it takes that long to kill all the slow-growing bacteria and reduce the chance of a relapse to near zero.

The most commonly used antibiotics are isoniazid, rifampin, pyrazinamide, streptomycin, and ethambutol. The first three drugs may be con­tained in the same capsule. This reduces the num­ber of pills the person has to take each day and also ensures that the person takes the appropri­ate drugs.

Isoniazid, rifampin, and pyrazinamide can cause nausea and vomiting, as a result of their effects on the liver. When nausea and vomiting occur, the drugs must be stopped until liver func­tion tests can be performed. If test results show a reaction to one of the drugs, a satisfactory sub­stitute can usually be found to complete treatment.

Ethambutol is started in a relatively large dose to help reduce the number of bacteria quickly. The dose is reduced after 2 months to avoid harmful side effects to the eyes. Streptomycin was the first drug found to be effective against tuberculosis but must be given by injection. Although, streptomycin is still a very effective drug for adsvanced infections, it can affect a person's balance and hearing if it is given in large doses or is co- tinued beyond 3 months.

Surgery to remove a portion of the lung is all most never needed today if the person faithfully follows the drug treatment plan. However, sometimes surgery is needed to drain pus from wherever it has accumulated and occasionally to correct a deformity of the spine caused by tuberculosis.

Prevention

There are several ways to prevent tuberculosis. For example, a germicidal ultraviolet light can be used in places where people with a variety of sicknesses may have to sit together for several hours, such as hospital and emergency room waiting areas. Such a light kills the bacteria in the air.

The drug isoniazid is very effective when given to people at greatest risk for developing tubeculosis. Such people include those who have been in close contact with someone with the disease, such as health care workers whose tuberculin skin test result has changed from negative to positive and whose chest x-ray shows no disease. The latter signifies a recent infection that is not fully developed; it can be cured by taking isoniazid daily for 6 to 9 months. Studies have shown that about 10 percent of people with recent infections develop tuberculosis if treatment is not given, regardlless of their age.

Notes:

pleural effusion плеврвльный выпот, плеврит

extrapulmonary tuberculosis внелегочный туберкулез

excruciating pain мучительная боль

tuberculoma творожистое скопление вещества при туберкулезе, туберкулема

PCR полимерная цепная реакция

exact требовать

be favored for благоприятствлвать ч-либо

pass on a progeny передавать по наследству

drench with заливать