TUBERCULOSIS

Tuberculosis (TB) is a disease that played a major role in mortality decline in developed countries, and it remains a major cause of morbidity (illness) and mortality in developing countries. Tuberculosis is caused by infection with the bacterium Mycobacterium tuberculosis, discovered in 1882 by the German scientist Robert Koch (1843–1910). The related germ Mycobacterium africanum also causes TB in sub-Saharan Africa, but even in this region Mycobacterium tuberculosis predominates. The bovine form of TB, caused by Mycobacterium bovis, can also be transmitted to humans, but pasteurization of milk has reduced the chances of this. In historical texts, TB is sometimes referred to as consumption or pthisis. A notable aspect of TB is that it is an infectious disease of adults, not primarily of youth or the elderly.

Both historically and at present, tuberculosis of the lungs, called pulmonary tuberculosis, accounts for the major portion of TB morbidity and mortality. But tuberculosis can assume many forms, including tuberculosis meningitis, Pott's disease (TB of the vertebral column), and infection of any internal organ. Tuberculosis is a notable example of the difference between infection and disease. Those infected with TB (i.e., harboring the bacilli in their bodies) may develop clinical signs of the disease either immediately or sometimes many years after the first infection. Person-to-person spread occurs when someone with active tuberculosis coughs, producing droplets containing bacilli, which can infect someone nearby. Not all active cases are contagious (i.e., produce bacilli when coughing), however.

A tuberculosis vaccine, BCG (bacille Calmette-Guerin), exists, but its protective efficacy against TB in adults is variable; it is more effective in protecting children. In some populations, vaccination with BCG also protects against leprosy, which is caused by Mycobacterium leprae, a cousin of the TB-causing bacterium. The first antibiotic against TB was streptomycin, first used widely in 1947. Before the introduction of antibiotics, TB patients were treated in specialized sanatoriums, reflecting the belief that fresh air was curative. While this is not true, the sanatoriums did diminish the chances of TB transmission by removing the infected from the general population.

Figure 1 shows age-specific death rates, by sex, for TB (all forms), for the United States. Death rates are shown for 1900, when TB as a cause of death was of overwhelming importance; for 1939, by which time TB mortality had declined markedly but was still fairly high; and for 1998, after a half-century of antibiotic use. The figure depicts four key facets of the demography of TB. First, the data highlight the radical decline of tuberculosis mortality during the twentieth century, with most of the drop occurring before the introduction of antibiotics. Second, they show that TB is as much a disease of middle age as it is of the young and the elderly. This is in contrast to most other infectious diseases of similar importance: Diseases such as measles or pertussis are concentrated in childhood; other diseases such as influenza cause morbidity at all ages, but mortality is typically concentrated at the youngest and oldest ages. Third, death rates for males and females exhibit distinctly different patterns, with males having on average significantly higher TB mortality rates, yet at some ages female mortality is higher. Finally, when TB death rates decline, they do not fall evenly at all ages; both the shape and the level of the age-mortality curves change. The figure is broadly representative of the decline of TB in other developed countries.

Worldwide, there is great variability across nations in TB death rates. In 2002, the following nations had the highest TB death rates in their respective world regions (deaths per 100,000 population): Zambia, 290; Djibouti, 164; Haiti, 137; Cambodia, 90; Indonesia, 68; Russia, 17. Estimates for other nations include: South Africa, 166; India, 46; China, 21; Mexico, 6; Portugal, 5; United Kingdom, 2; United States, 1. These are crude death rates, not adjusted for population age composition, but they do correctly

FIGURE 1

reflect overall tuberculosis disease burden in terms of deaths per capita. Compare these data to the following sample of historical estimates of pulmonary TB death rates (listed by country, year, and death rate per 100,000 population), in chronological order: England and Wales in 1861, 258; Japan in 1899, 127; United States in 1900, 168; Chile in 1904, 270; Australia in 1911, 67; Portugal in 1920, 125; England and Wales in 1921, 89; France in 1926, 143; and South Africa (nonwhite population) in 1951, 300.

Tuberculosis is central to the story of what was one of the great debates in population studies. The British epidemiologist Thomas McKeown (1911–1988) maintained that the standard of living in general, and improved nutrition in particular, played a more important role than did medicine and public-health measures in the historical decline of mortality. He used the case of TB in support of the thesis, noting, among other things, that much of the decline in TB mortality occurred before the introduction of antibiotics. While improved nutrition was certainly a factor in the decline of TB, a number of objections have been raised against McKeown's general argument. His analysis rested mainly on data from England and Wales, in which the decline of TB in the nineteenth century played an idiosyncratically large role in the decline of mortality overall, thus exaggerating the importance of the decline of TB. And the use of sanitariums to reduce TB transmission is best viewed as a public-health measure, not a factor reflecting improved living standards.

In the 1990s TB strains resistant to multiple antibiotics became a significant problem in several countries, most notably in the former Soviet Union–a worrisome development. In any region where such strains become prevalent, reversal of progress against TB cannot be ruled out. This is of particular concern in developing countries, where TB prevalence, transmission, and mortality are still relatively high and where historical declines have been much less rapid than in the advanced industrialized countries. Infection with human immunodeficiency virus (HIV) is a major risk factor for developing active TB disease, with initial HIV-positive status followed by TB exposure being more severe in most cases than the reverse order of infection. In regions where prevalence of TB and HIV/AIDS are both high, most notably in sub-Saharan Africa, HIVTB coinfection is a strong contributing factor to the lack of progress in reducing tuberculosis morbidity and mortality.

TUBERCULOSIS

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