tuberculosis

/too berr'kyeuh loh"sis, tyoo-/, n. Pathol.

1. an infectious disease that may affect almost any tissue of the body, esp. the lungs, caused by the organism Mycobacterium tuberculosis, and characterized by tubercles.

2. this disease when affecting the lungs; pulmonary phthisis; consumption.

3. any disease caused by a mycobacterium. Also called TB (for defs. 1, 2).

[1855-60; < NL tuberculosis; see TUBERCLE, -OSIS]

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▪ pathology

Introduction

  infectious disease that is caused by the tubercle bacillus, Mycobacterium tuberculosis (Mycobacterium). In most forms of the disease, the bacillus spreads slowly and widely in the lungs (lung), causing the formation of hard nodules (tubercles) or large, cheeselike masses that break down the respiratory tissues and form cavities in the lungs. Blood vessels also can be eroded by the advancing disease, causing the infected person to cough up bright red blood. During the 18th and 19th centuries, tuberculosis reached near-epidemic proportions in the rapidly urbanizing and industrializing societies of Europe and North America. Indeed, “consumption,” as it was then known, 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 brought about a steady decline in its mortality rates. Since the 1940s, antibiotic drugs have reduced the span of treatment to months instead of years, and drug therapy has done away with the old TB sanitariums where patients at one time were nursed for years while the defensive properties of their bodies dealt with the disease. Still, in less-developed countries where population is dense and hygienic standards are poor, tuberculosis remains a major fatal disease. In addition, the prevalence of the disease has increased in association with the HIV epidemic; tuberculosis is one of the main causes of death among AIDS patients. Finally, some new strains of the tubercle bacillus that are resistant to conventional antibiotics have appeared, requiring the use of combinations of drugs.

The course of tuberculosis

      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. Infection spreads primarily by the respiratory (respiratory disease) route directly from an infected person who discharges live bacilli into the air. Minute droplets ejected by sneezing, coughing, and even talking can contain hundreds of tubercle bacilli that may be inhaled by a healthy person. There the bacilli become trapped in the tissues of the body, are surrounded by immune cells, and finally are sealed up in hard, nodular tubercles. A tubercle usually consists of a centre of dead cells and tissues, cheeselike (caseous) in appearance, in which can be found many bacilli. This centre is surrounded by radially arranged phagocytic (scavenger) cells and a periphery containing connective tissue cells. The tubercle thus forms as a result of the body's defensive reaction to the bacilli. Individual tubercles are microscopic in size, but most of the visible manifestations of tuberculosis, from barely visible nodules to large tuberculous masses, are conglomerations of tubercles.

      In otherwise healthy children (childhood disease and disorder) and adults, the primary infection often heals without causing symptoms. The bacilli are quickly sequestered in the tissues, and the infected person acquires a lifelong immunity to the disease. A skin test taken at any later time may reveal the earlier infection and the immunity, and a small scar in the lung may be visible by X-ray. In this condition, sometimes called latent tuberculosis, the affected person is not contagious. In some cases, however, sometimes after periods of time that can reach 40 years or more, the original tubercles break down, releasing viable bacilli into the bloodstream. From the blood the bacilli create new tissue infections elsewhere in the body, most commonly in the upper portion of one or both lungs. This causes a condition known as pulmonary tuberculosis, a highly infectious stage of the disease. In some cases the infection may break into the pleural space between the lung and the chest wall, causing a pleural effusion, or collection of fluid outside the lung. Particularly among infants, the elderly, and immunocompromised adults (organ transplant recipients or AIDS patients, for example), the primary infection may spread through the body, causing miliary tuberculosis, a highly fatal form if not adequately treated. In fact, once the bacilli enter the bloodstream, they can travel to almost any organ of the body, including the lymph nodes, bones and joints, skin, intestines, genital organs, kidneys, and bladder. An infection of the meninges that cover the brain causes tuberculous meningitis; before the advent of specific drugs, this disease was always fatal, though most affected people now recover.

      The onset of pulmonary tuberculosis 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, the patient may have chest pain from pleurisy, and there may be blood in the sputum, an alarming symptom. Fever develops, usually with drenching night sweats. In the lung, the lesion consists of a collection of dead cells in which tubercle bacilli may be seen. This lesion 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 will die from failure of ventilation and general toxemia and exhaustion.

Diagnosis and treatment

      The diagnosis of pulmonary tuberculosis depends on finding tubercle bacilli in the sputum, in the urine, in gastric washings, or in the cerebrospinal fluid. The primary method used to confirm the presence of bacilli is a sputum smear, in which a sputum specimen is smeared onto a slide, stained with a compound that penetrates the organism's cell wall, and examined under a microscope. If bacilli are present, the sputum specimen is cultured on a special medium to determine whether the bacilli are M. tuberculosis. An X-ray of the lungs may show typical shadows caused by tubercular nodules or lesions. The prevention of tuberculosis depends on good hygienic and nutritional conditions and on the identification of infected patients and their early treatment. A vaccine, known as BCG vaccine, is composed of specially weakened tubercle bacilli. Injected into the skin, it causes a local reaction, which confers some immunity to infection by M. tuberculosis for several years. It has been widely used in some countries with success; its use in young children in particular has helped to control infection in the developing world. The main hope of ultimate control, however, lies in preventing exposure to infection, and this means treating infectious patients quickly, possibly in isolation until they are noninfectious. In many developed countries, individuals at risk for tuberculosis, such as health care workers, are regularly given a skin test (see tuberculin test) to show whether they have had a primary infection with the bacillus.

      Today, the treatment of tuberculosis consists of drug therapy and methods to prevent the spread of infectious bacilli. Historically, treatment of tuberculosis consisted of long periods, often years, of bed rest and surgical removal of useless lung tissue. In the 1940s and '50s several antimicrobial drugs (antibiotic) were discovered that revolutionized the treatment of patients with tuberculosis. As a result, with early drug treatment, surgery is rarely needed. The most commonly used antituberculosis drugs are isoniazid and rifampicin (rifampin). These drugs are often used in various combinations with other agents, such as ethambutol, pyrazinamide, or rifapentine, in order to avoid the development of drug-resistant bacilli. Patients with strongly suspected or confirmed tuberculosis undergo an initial treatment period that lasts two months and consists of combination therapy with isoniazid, rifampicin, ethambutol, and pyrazinamide. These drugs may be given daily or two times per week. The patient is usually made noninfectious quite quickly, but complete cure requires continuous treatment for another four to nine months. The length of the continuous treatment period depends on the results of chest X-rays and sputum smears taken at the end of the two-month period of initial therapy. Continuous treatment may consist of once daily or twice weekly doses of isoniazid and rifampicin or isoniazid and rifapentine.

      If a patient does not continue treatment for the required time or is treated with only one drug, bacilli will become resistant and multiply, making the patient sick again. If subsequent treatment is also incomplete, the surviving bacilli will become resistant to several drugs. Multidrug-resistant tuberculosis (MDR TB) is a form of the disease in which bacilli have become resistant to isoniazid and rifampicin. MDR TB is treatable but is extremely difficult to cure, typically requiring two years of treatment with agents known to have more severe side effects than isoniazid or rifampicin. Extensively drug-resistant tuberculosis (XDR TB) is a rare form of MDR TB. XDR TB is characterized by resistance to not only isoniazid and rifampin but also a group of bactericidal drugs known as fluoroquinolones and at least one aminoglycoside antibiotic, such as kanamycin, amikacin, or capreomycin. Aggressive treatment using five different drugs, which are selected based on the drug sensitivity of the specific strain of bacilli in a patient, has been shown to be effective in reducing mortality in roughly 50 percent of XDR TB patients. In addition, aggressive treatment can help prevent the spread of strains of XDR TB bacilli.

      In 1995, in part to prevent the development and spread of MDR TB, the World Health Organization began encouraging countries to implement a compliance program called directly observed therapy (DOT). Instead of taking daily medication on their own, patients are directly observed by a clinician or responsible family member while taking larger doses twice a week. Although some patients consider DOT invasive, it has proved successful in controlling tuberculosis.

Other mycobacterial infections

      The above discussion of tuberculosis relates to the disease caused by M. tuberculosis. Another species, M. bovis, is the cause of bovine tuberculosis. M. bovis is transmitted among cattle and some wild animals through the respiratory route, and it is also excreted in milk. If the milk is ingested raw, M. bovis readily infects humans. The bovine bacillus may be caught in the tonsils and may spread from there to the lymph nodes of the neck, where it causes caseation of the node tissue (a condition formerly known as scrofula). The node swells under the skin of the neck, finally eroding through the skin as a chronic discharging ulcer. From the gastrointestinal tract (alimentary canal), M. bovis may spread into the bloodstream and reach any part of the body. It shows, however, a great preference for bones and joints, where it causes destruction of tissue and eventually gross deformity. Tuberculosis of the spine, or Pott disease, is characterized by softening and collapse of the vertebrae, often resulting in a hunchback deformity. pasteurization of milk kills tubercle bacilli, and this, along with the systematic identification and destruction of infected cattle, has led to the disappearance of bovine tuberculosis in humans in many countries.

      The AIDS epidemic has given prominence to a group of infectious agents known variously as nontuberculosis mycobacteria, atypical mycobacteria, and mycobacteria other than tuberculosis (MOTT). This group includes such Mycobacterium species as M. avium (or M. avium-intracellulare), M. kansasii, M. marinum, and M. ulcerans. These bacilli have long been known to infect animals and humans, but they cause dangerous illnesses of the lungs, lymph nodes, and other organs only in people whose immune systems have been weakened. Among AIDS patients, atypical mycobacterial illnesses are common complications of HIV infection. Treatment is attempted with various drugs, but the prognosis is usually poor owing to the AIDS patient's overall condition.

Tuberculosis through history

      Evidence that M. tuberculosis and humans have long coexisted comes primarily from studies of bone samples collected from a Neolithic human settlement in the eastern Mediterranean. Genetic evidence gathered from these studies indicates that roughly 9,000 years ago there existed a strain of M. tuberculosis similar to strains present in the 21st century. Evidence of mycobacterial infection has also been found in the mummified remains of ancient Egyptians (Egypt, ancient), and references to phthisis, or “wasting,” occur in the writings of the Greek physician Hippocrates. In the medical writings of Europe through the Middle Ages and well into the industrial age, tuberculosis was referred to as phthisis, the “white plague,” or consumption—all in reference to the progressive wasting of the victim's health and vitality as the disease took its inexorable course. The cause was assumed to be mainly constitutional, either the result of an inherited disposition or of unhealthy or dissolute living. In the first edition of Encyclopædia Britannica (1768), it was reported that a tendency to develop “consumption of the lungs” could tragically be expected in people who were fine, delicate, and precocious:

This is known from a view of the tender and fine vessels, and of the slender make of the whole body, a long neck, a flat and narrow thorax, depressed scapulæ; the blood of a bright red, thin, sharp, and hot; the skin transparent, very white and fair, with a blooming red in the cheeks; the wit quick, subtle, and early ripe with regard to the age, and a merry chearful disposition.

      Based on the stage of the disease, treatments included regular bloodletting, administration of expectorants and purgatives, healthful diet, exercise such as vigorous horseback riding, and, in the grim final stages, opiates. The view that tuberculosis might be a contagious disease also had its adherents, but it was not until 1865 that Jean Antoine Villemin (Villemin, Jean Antoine), an army doctor in Paris, showed that it could be transmitted from tuberculous animals to healthy animals by inoculation. The actual infectious agent, the tubercle bacillus, was discovered and identified in 1882 by the German physician Robert Koch (Koch, Robert). By that time the cultural status of the disease was assured. As summarized by Dr. J.O. Affleck of the University of Edinburgh, Scotland, in the ninth edition of Encyclopædia Britannica (1885), “Few diseases possess such sad interest for humanity as consumption, both on account of its widespread prevalence and its destructive effects, particularly among the young.” Causing as much as one-quarter of all deaths in Europe, arising with particular frequency among young adults between the ages of 18 and 35, and bringing on a lingering, melancholy decline characterized by loss of body weight, skin pallor, and sunken yet luminous eyes, tuberculosis was enshrined in literature as the “captain of death,” the slow killer of youth, promise, and genius. Prominent artists who died of consumption in the 19th century included the English poet John Keats (Keats, John), the Polish composer Frédéric Chopin (Chopin, Frédéric), and all of the Brontë sisters (Charlotte (Brontë, Charlotte), Emily (Brontë, Emily), and Anne (Brontë, Anne)); in the early 20th century they were followed by the Russian playwright Anton Chekhov (Chekhov, Anton), the Italian painter Amedeo Modigliani (Modigliani, Amedeo), and the German writer Franz Kafka (Kafka, Franz). Without a clear understanding of the bacterium that caused the disease, little could be done for its victims except to isolate them in sanitariums, where cleanliness and fresh air were thought to help the body's natural defenses to stop or at least slow the progress of the disease.

      Preventive inoculation against tuberculosis, in which live but attenuated tubercle bacilli are used as a vaccine, was introduced in France in 1921 by bacteriologists Albert Calmette (Calmette, Albert) and Camille Guérin (Guérin, Camille). The strain designated BCG (Bacillus Calmette-Guérin), of bovine origin, became attenuated while growing on culture media containing bile. After its introduction by Calmette, large numbers of children were vaccinated in France, elsewhere in Europe, and in South America; after 1930 the vaccine was used on an extensive scale. In 1943–44 the Ukrainian-born microbiologist Selman A. Waksman (Waksman, Selman Abraham) and his associates, working at Rutgers University, New Jersey, U.S., discovered the potent antimicrobial agent streptomycin in the growth medium of the soil microorganism Streptomyces griseus. In 1944–45 veterinarian W.H. Feldman and physician H.C. Hinshaw, working at the Mayo Clinic in Minnesota, demonstrated its specific effect in inhibiting tuberculosis in both animals and people. Wide clinical use of streptomycin promptly followed, eventually in combination with other drugs to attack resistant bacilli.

 In 1952 a great advance was made with the successful testing of isoniazid in the United States and Germany. Isoniazid is the most important drug in the history of chemotherapy for tuberculosis; other drugs were brought out in following years, pyrazinamide in 1954, ethambutol in 1962, and rifampicin in 1963. By this time the industrialized countries were already seeing the health benefits of economic improvement, better sanitation, more widespread education, and particularly the establishment of public health practice, including specific measures for tuberculosis control. The rate of deaths from tuberculosis in England and Wales dropped from 190 per 100,000 population in 1900 to 7 per 100,000 in the early 1960s. In the United States during the same time period, it dropped from 194 per 100,000 to approximately 6 per 100,000. In the popular mind, tuberculosis was then a disease of the past, of the indigent, and of the Third World. In the mid-1980s, however, the number of deaths caused by tuberculosis began to rise again in developed nations. The disease's resurgence was attributed in part to complacent health-care systems, increased immigration of people from regions where tuberculosis was prevalent, and the spread of HIV. In addition, throughout the 1990s the number of cases of tuberculosis increased in Africa. Global programs such as the Stop TB Partnership, which was established in 2000, have worked to increase awareness of tuberculosis and to make new and existing treatments available to people living in developing countries most affected by the disease. In the early 2000s, as a result of the rapid implementation of global efforts to combat the disease, the epidemic in Africa slowed and incidence rates stabilized. Despite a leveling off of per capita incidence of tuberculosis, the global number of new cases continues to rise due to population growth, especially in regions of Africa, Southeast Asia, and the eastern Mediterranean. The mortality rate from tuberculosis remains between 1.6 million and 2 million deaths per year.

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