Tuberculosis is an infectious bacterial disease, one of the most prevalent infections of humans and higher animals. It is the leading infectious killer of adults worldwide, and it is coming back in the wake of HIV-related illness. Not long ago, it seemed that TB was nearly eradicated. Today, the World Health Organization (WHO) estimates that about one-third of the world's population is infected, with seven to eight million new cases of tuberculosis occurring each year. Annual tuberculosis mortality is between two and three million people, making this disease the most common infectious cause of death in the world and the leading cause of death for people infected with HIV. Poverty, antibiotic resistance, HIV infection and global travel have all led to increases in both TB incidence and mortality. People with lowered resistance—due to lack of health care, poor health or immune disorders—are at increased risk of infection. Also at risk are those in close contact with an infected person.
Tuberculosis is caused by the infectious Mycobacterium tuberculosis or Koch bacillus which occurs in human, bovine and avian forms. It is transmitted mainly by airborne droplets of sputum and saliva containing the bacteria and gains access to the body by three possible channels: inoculation, inhalation and ingestion. In humans, tuberculosis mainly affects the lungs (pleurisy or pulmonary tuberculosis) but may invade almost any organ, particularly the lymph nodes, kidneys and bladder, and skin. Other affected structures include the mediastinum, nose, pharynx, sinus and trachea, adrenal glands, liver, breast, mouth, pericardium stomach, spleen and thyroid gland.
The severity of the disease varies considerably according to the organ attacked; tuberculosis affecting the membranes of the brain and causing meningitis is a particularly dangerous disease, particularly in infants and young children.
The bovine variety of tuberculosis bacteria is pathogenetic for all mammals and to a lesser extent for birds. The human variety causes tuberculosis in horses, dogs, swine, cats, sheep, birds, and cattle. The avian variety affects birds, swine, horses, dogs, and sometimes cattle.
Ancient Egyptian and Indian manuscripts mention tuberculosis symptoms, and traces of tuberculosis can be found in Egyptian mummies. It did not become endemic, however, until the rise of urban centres where the disease could spread easily. In London in the 17th and 18th centuries, the annual mortality rate from tuberculosis was over 800 per 100,000 population. Elsewhere in Europe it accounted for 20% of all deaths. The causative bacillus was first isolated in 1882 by R Koch.
Despite its great incidence, tuberculosis attracts only $16 million in funds for eradication of the disease. 90% of tuberculosis cases can be cured with 2 to 4 drugs for less than $11.
Tuberculosis, the single largest cause of death in adults from infectious diseases, was responsible for three million deaths in 1996, 95 per cent of which occurred in the developing world. One European in 7 used to die of it. Now most tuberculosis cases and deaths occur in developing countries, notably in Asia and Africa, but in developed countries the impact of tuberculosis has worsened in recent years. TB is a growing public health concern worldwide and a major problem in sub-Saharan Africa, Southeast Asia, India and the former Soviet Union countries. Young people and the elderly are more susceptible to infection than adults because of their weaker immune systems. With adults, any prevailing condition which weakens the immune system predisposes the person to infection: malnutrition, AIDS, drug use, alcoholism, crowded living conditions, unrelated lung damage and infections etc. According to (WHO) estimates, at least three million people die each year of tuberculosis. Every year four to five million new infectious cases are added to the statistics, and the same number of noninfectious cases develop, many of which will later become infectious. It is expected that 300 people will contract TB and some 30 to 40 million will die from it between 1995 and 2005. Tuberculosis holds third or fourth place among the main causes of death in many developing countries, and eight or ninth place in developed countries. The tuberculosis mortality per 100,000 population in 1970 was 5.4 in Germany, 8.2 in France, 15 in Japan, 36 in Hong Kong, and 82 in the Philippines. In the Philippines it is the second leading cause of death. 1000 people die each day from tuberculosis in the Western Pacific region. There are 2 million new cases each year. In France, the mortality from tuberculosis is three to five times higher among miners, sailors and fishermen than among persons engaged in the professions. In the USA, the incidence and mortality rates of tuberculosis among African Americans, Indians, Puerto Ricans, and other nonwhite groups are three to four times higher than among whites. Incidence and mortality are high among New Zealand aborigines, and among Australian aborigines relocated to regions where living conditions are unfavourable. In India approximately 10 million adults have active cases.
Worldwide, TB is the leading cause of death among people infected with HIV. An estimated 10-15 million Americans are infected with TB bacteria, with the potential to develop active TB in the future. About 10 percent of these infected individuals will develop TB at some point in their lives. However, the risk of developing TB disease is much greater for those infected with HIV and living with AIDS. Because HIV infection so severely weakens the immune system, people dually infected with HIV and TB have a 100 times greater risk of developing active TB disease and becoming infectious compared to people not infected with HIV. The US Centre for Disease Control estimates that 10 to 15 percent of all TB cases and nearly 30 percent of cases among people ages 25 to 44 occur in HIV-infected individuals. The revival of tuberculosis in the USA is clearly associated with the AIDS epidemic, particularly in inner cities. The number of new tuberculosis cases in the USA rose by 3% in 1989 over 1988; by 10% in New York City and by 35% in Newark, New Jersey.
Tuberculosis of animals is widespread, especially in western Europe where it causes substantial economic losses. More than 55 species of domestic and wild mammals and about 25 species of birds are susceptible to tuberculosis.