because it fails to take into account the varying sizes of the so urce

Because it fails to take into account the varying

This preview shows page 9 - 14 out of 71 pages.

, because it fails to take into account the varying sizes of the so urce populations across the different age groups. There were over 85 million persons in the 25- to 44-year-old a ge group, as compared with under 67 million persons in the 4 5- to 64-year-old category, and under 36 million persons in the group aged 65 years or older. By calculating incidence rates, it is possible to compensate for disparities in sizes of the source populations. As illustrated in Figure 3, the incidence of tuberculosis rises with age, reaching a level of 8.8 cases per 100,000 persons among those 65 years of age or older. The incidence among persons in the oldest age group is over 40% higher than that for the 25- to 44-year-old a ge group.
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10 Fig. 3 Incidence rates for reported tuberculosis, grouped by age, in the United States, 2002.
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11 A number of factors contribute to the nonrandom relatio nship between incidence of tuberculosis and age. 1. The long latent period between infection and development of clinical symptoms means that the ages at detection of illness are expected to be skewed toward later life. 2. Because elderly individuals lived through time periods when t he disease was more common, they are more likely to have been infected than younger persons ( birth cohort effect ). 3. Older persons are more likely to have other illnesses (eg, canc er, diabetes mellitus) that may make them more susceptible to tu berculosis. 4. The decline in immune function associated with the normal ag ing process may increase susceptibility. 5. Elderly persons are more likely to live in closed communal set tings that are conducive to the spread of tuberculosis.
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12 An equally striking nonrandom pattern of occurrence is seen whe n incidence is examined as a function of race or ethnicity (Figure 4). The highest incidence rate of tuberculosis in the United States is found among Asians and Pacific Islanders; it is more than 18 tim es greater than the incidence rate for white non-Hispanics. Foreig n-born persons account for about 95% of tuberculosis cases amo ng Asians and Pacific Islanders in the United States. many of these individuals acquire the infection in the high-risk c ountry of origin, but do not develop symptomatic disease until th ey arrive in the United States. About three out of four tuberculosi s cases among Hispanics in the United States also occur in foreig n-born persons. Overall, foreign-born persons account for slightl y more than half of all tuberculosis cases in the United States.
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13 Fig. 4 Incidence rates for reported tuberculosis, grouped by race and ethnicity, in the United States, 2002. The high incidence rates of tuberculosis within certain minority groups in the United States reflect the influences of other risk factors. TB is a disease that is associated with socioeconomic disadvantage. The combination of crowded housing, poor nutrition, inadequate access to preventive and therapeutic medical services, alcoholism, and injecting drug use, as well as any predisposing medical conditions, contributes to the high risk of TB among the poor.
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