Endemicity in Southwest and South-Central Asia

TB is a global disease. An estimated 33% of the world’s population is infected with M. tuberculosis, although the incidence of infection has wide geographic variation (WHO 2006b; Wilson 1991).

In southwest and south-central Asia, TB is highly endemic. WHO estimated the regional incidence in 2004 to be 206 cases per 100,000 population (WHO 2006b). In that year, the estimated incidence in Iraq was 200 cases per 100,000, and in Afghanistan, 661 cases per 100,000 (WHO. 2006a). The burden of TB is particularly severe in Afghanistan, which in 2004 had the 12th highest per capita rate of TB cases in the world (Table 5.5) (WHO 2006a). The United States, in contrast, had only 3.6 cases per 100,000 in 2004 (WHO 2006a).

Thus, for US military personnel, the risk of exposure to TB is much greater in south-central and southwest Asia than domestically. Shipboard personnel and people who have extensive close contact with local populations—in prisons or hospitals, for instance—would be at higher risk than other troops for acquisition of TB during military service.

TABLE 5.5 The 12 Countries with the Highest Rate of TB, 2004

Country

Incidence of TB (All Forms), No. Cases per 100,000 Population

Rank

Djibouti

1,137

1

Swaziland

1,120

2

Kenya

888

3

Sierra Leone

847

4

Togo

718

5

Cambodia

709

6

Zambia

707

7

Timor-Leste

692

8

Somalia

673

9

Zimbabwe

673

10

South Africa

670

11

Afghanistan

661

12

SOURCE: Adapted with permission from WHO 2006a.

Risk of Progression from Latent Tuberculosis Infection to Active Tuberculosis

Persons with LTBI face a 5-10% lifetime risk of developing active TB. The risk is greatest during the first 2 years after infection (Figure 5.1). In general, the likelihood that TB infection will produce active disease varies with the intensity and duration of exposure, size of induration, and age (Figure 5.2) (Comstock et al. 1974a; Mandell et al. 2005; Vynnycky and Fine 1997). Infants, 15- to 25-year-olds, and the elderly are at greatest risk for progression from LTBI to active TB (Comstock et al. 1974b; Stead and Dutt 1991; Stead and Lofgren 1983; Stead and To 1987).



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