including exposure to new allergens. The New Zealand experience (Sears et al., 1989) indicates that several of these factors are likely to contribute to increased mortality and that broad intervention can reduce these rates (Burrows and Lebowitz, 1992; Lofdahl and Svedmyr, 1991).
Genetic Factors Genetic factors are important in asthma and atopy (Gregg, 1983; Horwood et al., 1985; Lebowitz et al., 1984; Sibbald, 1980; Sibbald et al., 1980). The highest prevalence of asthma occurs among children whose parents have evidence of allergic disease (see Table 2-5; Barbee et al., 1985; Luoma, 1984; Sporik et al., 1990). In addition, the symptom of ''persistent wheeze" has been shown to have a familial component. Theoretically, familial aggregation of cases of asthma could be explained either by genetic factors or by common exposure to increased environmental risks. The low prevalence of asthma (as well as low rates of smoking-related lung disease) among Native Americans has been attributed to unspecified genetic factors.
Bronchial Hyperreactivity Bronchial hyperreactivity is both a feature of asthma and a risk factor for its development. The risk of bronchial hyperreactivity increases with increasing skin test reactivity (Burrows and Lebowitz, 1992; Lofdahl and Svedmyr, 1991).
Skin Test Reactivity The presence of skin test reactivity is clearly a strong risk factor for asthma. This relationship has been shown using a variety of definitions of asthma and different study designs. New asthma that develops before age 40 is likely to be associated with allergen skin test reactivity, high total serum IgE, a family history of atopy, and prior symptomatology. Total IgE is still strongly related to asthma prevalence and incidence above age 60 (Burrows et al., 1991). Overall, half of existing cases of asthma have been attributed to allergenic factors.
Recently, the risk of asthma has been related to skin test reactivity to specific allergens. Gergen and Turkeltaub (1992), using data from the second National Health and Nutrition Examination Survey (NHANES II), demonstrated an independent association of asthma with reactivity to an extract of crude house dust (odds ratio, 2.9) and to alternaria (odds ratio, 2.3). In a birth cohort of New Zealand children that was monitored up to age 13, sensitivities to house dust mite (odds ratio, 6.7) and to cat dander (odds ratio, 4.2) were highly significant independent risk factors associated with the development of asthma (Sears et al., 1989). Grass sensitivity, although common, was not an independent risk factor (odds ratio, 1.33). Aspergillus fumigatus was uncommon (skin test positivity in only 2.4 percent of the sample) but was significantly correlated with asthma (odds ratio, 13.8).