Prevalence and Epidemiology

Human TB is caused by infection with Mycobacterium tuberculosis, Mycobacterium bovis, or Mycobacterium africanum. It is a notifiable disease under legislation specific to individual countries; for example, in the United Kingdom, this comes under the Public Health (Control of Disease) Act of 1984. In 1993, the WHO declared TB to be a global emergency, with an estimated 7-8 million new cases and 3 million deaths occurring each year, the majority of which were in Asia and Africa. However, these statistics are likely to be an underestimate because they depend on the accuracy of reporting, and in poorer countries, the surveillance systems are often inadequate because of lack of funds.

Even in the United Kingdom, there has been an inconsistency of reporting particularly where an individual has concomitant infection with HIV. Some physicians found themselves caught in a dilemma of confidentiality until 1997, when the codes of practice were updated to encourage reporting with patient consent (68).

With the advent of rapid identification tests and treatment and the use of Bacillus Calmette-Guerin (BCG) vaccination for prevention, TB declined during the first half of the 20th century in the United Kingdom. However, since the early 1990s, numbers have slowly increased, with some 6800 cases reported in 2002 (69). In 1998, 56% of reported cases were from people born outside the United Kingdom and 3% were associated with HIV infection (70,71).

London has been identified as an area with a significant problem. This has been attributed to its highly mobile population, the variety of ethnic groups, a high prevalence of HIV, and the emergence of drug-resistant strains (1.3% in 1998 ) (PHLS, unpublished data—Mycobnet).

A similar picture was initially found in the United States, when there was a reversal of a long-standing downward trend in 1985. However, between 1986 and 1992, the number of cases increased from 22,201 to 26,673 (72). There were also serious outbreaks of multidrug-resistant TB (MDR-TB) in hospitals

Table 6 Symptoms of Tuberculosis

Anorexia and weight loss • Fever and night sweats

• Mild hemoptysis (rusty colored) • Cough with phlegm

• Swollen lymph glands_

in New York City and Miami (73). Factors pertinent to the overall upswing included the emergence of HIV, the increasing numbers of immigrants from countries with a high prevalence of TB, and perhaps more significantly, stopping categorical federal funding for control activities in 1972. The latter led to a failure of the public health infrastructure for TB control. Since 1992, the trend has reversed as the CDC transferred most of its funds to TB surveillance and treatment program in states and large cities. From 1992 to 2001, the annual decline averaged by 7.3% (74), but the following year this was reduced to 2%, indicating that there was no room for complacency. The WHO has been proactive and is redirecting funding to those countries most in need. In October 1998, a global partnership called Stop TB was launched to coordinate every aspect of TB control, and by 2002, the partnership had more than 150 member states. A target was set to detect at least 70% of infectious cases by 2005.

The acquisition of TB infection is not necessarily followed by disease because the infection may heal spontaneously. It may take weeks or months before disease becomes apparent, or infection may remain dormant for years before reactivation in later life especially if the person becomes debilitated or immunocompromised. Contrary to popular belief, the majority of cases of TB in people who are immunocompetent pass unnoticed. Of the reported cases, 75% involve the lung, whereas nonrespiratory (e.g., bone, heart, kidney, and brain) or dissemination (miliary TB) are more common in immigrant ethnic groups and individuals who are immunocompromised (75). They are also more likely to develop resistant strains. In the general population, there is an estimated 10% lifetime risk of TB infection progressing to disease (76).

There has been an increase in the number of cases of TB associated with HIV owing to either new infection or reactivation. TB infection is more likely to progress to active TB in HIV-positive individuals, with a greater than50% lifetime risk (77). TB can also lead to a worsening of HIV with an increase in viral load (78). Therefore, the need for early diagnosis is paramount, but it can be more difficult because pulmonary TB may present with nonspecific features (e.g., bilateral, unilateral, or lower lobe shadowing) (79).

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