TUBERCULOSIS

(TB), infectious disease that is caused by several species of Mycobacterium, collectively called the tubercle bacillus. Tuberculosis in humans is usually caused by the human variety of the bacillus, M. tuberculosis, and in fewer cases by the bovine variety of the bacillus, M. bovis. Besides cattle, which are infected by the bovine type of bacillus, other domestic animals susceptible to tuberculosis include swine and fowl, the latter being infected by the avian type of bacillus. The tubercle bacillus is a small, rod-shaped bacterium that is extremely hardy; it can survive for months in a state of dryness and can also resist the action of mild disinfectants. The tubercle bacillus was discovered and identified as the cause of tuberculosis in 1882 by the German physician Robert Koch. Tuberculosis occurs in humans worldwide, and in many developing countries it is still a chief cause of death. Historically, tuberculosis rates have been notably higher in urban than in rural communities because of cities' greater crowding (hence greater opportunity for transmission of the bacillus) and poorer sanitation and hygiene. Accounts of tuberculosis can be found in the writings of the ancient Egyptians, in those of the Greek physician Hippocrates, and in the medical writings of most urbanized societies since then. The disease reached near-epidemic proportions in the rapidly urbanizing and industrializing societies of Europe and North America in the 18th and 19th centuries. Indeed, tuberculosis was the leading cause of death for all age groups in the Western world from that period until the early 20th century, at which time improved health and hygiene caused a gradual but continuing decline in its mortality rates. In the mid-1980s the number of deaths caused by tuberculosis began to rise again in developed nations. The disease's resurgence was attributed to deteriorating health-care systems, increased poverty and homelessness, and the spread of HIV (human immunodeficiency virus), which greatly increases the chance that a TB-infected individual will develop active tuberculosis. Tuberculosis remains a significant cause of death in the developing countries of Africa, Asia, and Latin America. It is generally estimated that the prevalence (or occurrence) of the disease is approximately 20 times the mortality from it in any given period. The death rate is especially high in densely populated countries with poor hygienic standards. Infection with tuberculosis spreads in two ways, by the respiratory route directly from another infected person or by the gastrointestinal route by drinking milk infected with the tubercle bacillus. In the former case the organism is M. tuberculosis, in the latter M. bovis. Most human tuberculosis is acquired by the inhalation of bacilli from the sputum or other fluid discharges of infected persons. The minute droplets discharged by sneezing, coughing, and even talking can contain hundreds of tubercle bacilli that may be inhaled by a healthy person. In infections with M. tuberculosis, the tubercle bacilli commonly affect the lungs, in which case the disease is known as pulmonary tuberculosis. By contrast, infections with M. bovis often affect the bones and joints. About 90 percent of all clinically recognized tuberculosis in humans is pulmonary. The anatomic unit of tuberculosis is a highly characteristic pathological lesion known as the tubercle. It usually consists of a centre of dead cells and tissues, cheeselike in appearance, in which can be found many tubercle bacilli. This centre is surrounded by radially arranged phagocytic (scavenger) cells and a periphery containing connective tissue cells. The tubercle forms as a result of the body's defensive reaction to the bacilli that lodge in bodily tissues. Tubercles are microscopic in size, but most of the visible manifestations of tuberculosis, from barely visible nodules to large tuberculous masses, are conglomerations of such tubercles. Primary pulmonary tuberculosis occurs mostly in childhood. Often the infection is without symptoms and immunity results. When the primary infection heals, a small scar in the lung is visible on X ray. Sometimes the infection does not heal but spreads into the bloodstream and through the body, causing miliary tuberculosis, a highly fatal form if not adequately treated. Another complication is tuberculous meningitis; before the advent of specific drugs, this was always fatal, but now most cases recover. Bone and joint tuberculosis in childhood has virtually disappeared in areas of the world where all milk is pasteurized; elsewhere it remains a serious disease. Postprimary pulmonary tuberculosis occurs mainly in young adults but can occur at any age. The onset of the disease is usually insidious, with lack of energy, weight loss, and persistent cough. These symptoms do not subside, and the general health of the patient deteriorates. Eventually, the cough increases, there is much sweating, the patient may have chest pain from pleurisy, and there may be blood in the sputum, an alarming symptom. The lesion in the lung consists of a collection of dead cells in which tubercle bacilli may be seen. This lesion spreads and may erode a neighbouring bronchus or blood vessel, causing the patient to cough up blood (hemoptysis). Tubercular lesions may spread extensively in the lung, causing large areas of destruction, cavities, and scarring. The amount of lung tissue available for the exchange of gases in respiration decreases, and if untreated the patient dies from failure of ventilation and general toxemia and exhaustion. Sometimes the infection extends into other systems, and almost any organ of the body may be affected: lymph nodes, bones and joints, skin, intestines, genital organs, kidneys, and bladder. The diagnosis of pulmonary tuberculosis depends on finding the tubercle bacillus in the sputum, in the urine, in gastric washings, or in the cerebrospinal fluid. An X ray of the lungs may show typical shadows caused by tubercular nodules or lesions. A skin test (see tuberculin test) shows whether a person is susceptible to tuberculosis or whether he has become immune after infection. The prevention of tuberculosis depends on good hygienic and nutritional conditions and on the recognition of infected patients and their early treatment. A vaccine, known as BCG vaccine (q.v.), is composed of specially weakened tubercle bacilli. Injected into the skin, it causes a local reaction and, later, immunity to infection. It has been widely used in some countries with success. Its use in young children has helped to control infection in the developing world, but the main hope of ultimate control lies in preventing exposure to infection; and this means treating infectious patients quickly, possibly in isolation until they are noninfectious. The treatment of tuberculosis now consists of drug therapy and good general care. In the 1940s and '50s several antimicrobial drugs were discovered that revolutionized the treatment of patients with tuberculosis. Isoniazid, rifampicin, and streptomycin are the three main drugs used to treat tuberculosis; all three are capable of virtually eradicating the tubercle bacillus from the human body. Other such drugs are ethambutol, para-aminosalicylic acid, thiacetazone, and pyrazinamide. Before these drugs were available, treatment consisted of long periods, often years, of bed rest and often surgical removal of useless lung tissue. With early drug treatment, surgery is now rarely needed. One problem with drug therapies, however, is that the bacilli may become resistant to some of the drugs; this is avoided mainly by giving combinations of the drugs. The patient is usually made noninfectious quite quickly, but complete cure requires treatment for several months at least. If the patient does not continue treatment for the required time or is treated with only one drug, the resistant bacilli multiply and the patient becomes sick again. If subsequent treatment is also incomplete, the surviving bacilli may become resistant to several drugs. These multidrug-resistant (MDR) strains of bacilli cause an acute form of the disease that is extremely difficult to cure and in most cases proves fatal.