mites of the genus Dermatophagoides were a major source of house dust ''atopen" (Voorhorst et al., 1967). They also developed techniques for growing mites in culture, which made it possible to produce extracts commercially for skin testing. Most patients with positive skin tests to house dust have specific immunoglobulin E (IgE) antibodies to dust mite allergens (Johansson et al., 1971). Sensitization thus can be detected either by skin tests or measurement of serum IgE antibodies.

Dust mite sensitivity was found to be strongly associated with asthma by J. M. Smith and colleagues (1969) and Miyamoto and coworkers (1968). Indeed, in some countries (e.g., Brazil, Australia, New Zealand, Japan, the Netherlands, Denmark, and England), sensitivity to dust mites appears to be so common among young asthmatics that other sources of indoor allergens are relatively unimportant (see Arruda et al., 1991; Clarke and Aldons, 1979; Sears et al., 1989; and Sporik et al., 1990). In dry climates, however, such as in northern Sweden and central Canada, and in high-altitude areas (e.g., Colorado), mite growth is poor and domestic animals predominate as the major source of indoor allergens. Humidity enhances the growth of mites in carpets, mattresses, and other household items (Korsgaard, 1983a). In some inner-city areas, cockroach debris or rodent urine may be the dominant sources of allergens in house dust (Bernton et al., 1972; Hulett and Dockhorn, 1979; Kang et al., 1979; Twarog et al., 1976). Many different protein sources thus contribute to house dust allergenicity (see Table 3-1).

Heavy exposure to house dust can give rise to sneezing in anyone, and it has been suggested that endotoxins or other substances in dust can be directly toxic. The association between exposure to house dust and diseases such as asthma, chronic rhinitis, and atopic dermatitis, however, has been shown only in individuals who have developed hypersensitivity. The symptoms produced by house dust allergens in sensitized (i.e., allergic) individuals include asthma, perennial rhinitis, and atopic dermatitis. For each disease the symptoms range from severe to very mild; moreover, some individuals, despite their having IgE antibodies, suffer no discernible symptoms. In some cases, the correlation between exposure to a specific indoor allergen and symptoms is obvious; certainly, many individuals who are allergic to cats experience the rapid onset of symptoms on exposure to cat allergens. In contrast, most symptoms related to exposure to house dust are nonspecific and not temporally related to exposure. Thus, in general, it is not possible to distinguish the role of different specific indoor allergen sources solely on the basis of an individual's medical history. Indeed, many patients with asthma are not aware of any other symptoms that would be recognized as allergic. Because their histories are not specific and in many cases exposure to house dust allergens is perennial, understanding the relationship between exposure and disease has required both measurement of exposure and documentation of sensitization.

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