House Dust Mite and Cockroach Exposure: Risk Factors for Asthma

THE INCIDENCE AND PREVALENCE of asthma have risen sharply in the past 20–30 years. This rise in asthma has been particularly prominent in children and even more so in those living in urban environments. This is not unique to the United States but has in fact occurred in developed countries around the world. Current estimates in the United States indicate that the number of children with asthma has increased by at least 100% in the past 20 years. In fact, between 1980 and 1995, the number of children with asthma in the United States rose from 2.3 to 5.5 million.1–3 The increase has occurred in both sexes and all ethnic groups, with the sharpest increases occurring in children under the age of 5 years and in urban, predominantly minority populations.4 The reasons for this dramatic increase in asthma are not known and there are likely to be multiple contributing factors. Atopy, defined as the tendency to become allergic, is the most important predictor for the development of asthma, so anything that contributes to the development of atopy is also likely to be important to the development of asthma. While atopy is to a large extent genetically determined, genetic changes in a population occur slowly and therefore cannot explain the increases in asthma and atopy. There is evidence that exposure to both allergens and irritants, especially environmental tobacco smoke (ETS), early in life may play a pivotal role in the development of allergy and asthma. In addition, there is also substantial evidence that other exposures, particularly to infectious agents, may stimulate the immune system away from the development of allergy and asthma.5–7 This critical role of exposure to infectious agents in early life, or more importantly the lack thereof, has been termed the “hygiene hypothesis.” Simply stated, this hypothesis proposes that the less the immune system is exposed to infectious agents in early life (the first 6–12 months appear to be most important), the more it will (can) focus on allergic responses. The immunologic basis for this hypothesis is that the developing immune system can be directed toward either TH1 lymphocyte predominance, which focuses on response to infections, or a TH2 lymphocyte predominance, which focuses primarily on allergic responses, and that early exposure to infectious agents largely dictates the direction that the immune system will take. This is now widely accepted as a possible explanation for the increases in asthma and allergy in developed countries, where improved hygiene and smaller family size appear to predispose to the development of both allergy and asthma. Whether this is sufficient by itself or whether the immune system is simultaneously driven in the opposite direction (i.e., towards atopy) by exposure to allergens and other factors, will take many years to sort out. In this delicate balance of genetic predisposition and environmental exposures, several specific allergens appear to be of particular importance. This review will focus on the potential contribution of two of these allergens, dust mites and cockroach, to the development and clinical course of asthma.