Dust mites Studies examining thresholds of exposure relevant to disease

Public Health Significance of Urban Pests Allergic asthma 23 22 not be representative of total exposure. Although the same could be said for mouse aller- gen, it appears that a significant exposure to mouse allergen does arise from homes. Nevertheless, high levels of mouse allergen ≥ 2 µgg were found in 78 of New York City school classrooms during at least one sampling season Chew, Correa Perzanowski, 2005 and ranged from 0.3 µgg to 118µgg in 12 urban Baltimore schools Amr et al., 2003. 1.5. Dust mites and asthma 1.5.1.Sensitization and asthma The evidence for a causal role of exposure to allergens in the pathogenesis of perennial asthma has been better established for dust mite allergens than any other inhaled aller- gen Platts-Mills et al., 1992. In communities where exposure to dust mites is prevalent, dust mite allergens are often among the most common allergens to which atopic indivi- duals are sensitized Platts-Mills et al., 1997. A birth cohort study by Sporik and collea- gues 1990 found that children exposed to high levels of dust mite allergens in their homes in the first year of life were more likely to develop sensitization to dust mite aller- gens and that those exposed to the highest levels were the most likely to develop asthma. This established a dose–response relationship between exposure to dust mite allergens and sensitization. This relationship was also observed in a German study Lau et al., 2000; Illi et al. 2006. Sensitization to dust mites has been observed to be strongly associated with asthma in many areas of the world, including Australia, the United Kingdom, the mid-Atlantic and southern United States and especially New Zealand, which reports some of the highest residential dust mite allergen concentrations in the world Sporik et al., 1990; Peat et al., 1994; Squillace et al., 1997; Wickens et al., 1997. Sensitization to the storage mites of the genera Tyrophagus, Glycyphagus, Lepidoglyphus and Blomia has also been associated with allergic disease van Hage-Hamsten Johansson, 1998. Further evidence for the causality of exposure to dust mite allergens exacerbating asthma comes from studies that have shown successful improvement in asthma patient symptoms when exposure to dust mite allergens were reduced. The efficacy of avoiding allergens is dis- cussed in detail in section 1.7.

1.5.2. Factors associated with exposure to dust mite allergens

Dust mites are found in the bedding, pillows, mattresses, carpets and upholstered furni- ture of homes, where they feed on scales from human skin Tovey et al., 1981. They absorb water from the air, and therefore require a relative humidity above 50 to survive Arlian, 1992. T his restricts the environments inhabitable by mites and is a major controlling factor in the geographical distribution of dust mites. Reducing indoor air humidity has recently been used with some success as a mite eradication strategy Arlian, Neal Vyszenski-Moher, 1999; Arlian et al., 2001. In major cities of the north-eastern United States, multifamily buildings are often overheated during the winter, which leads to low humidity air indoors and precludes the proliferation of dust mites Chew et al., 1999a. Mite genera vary geographically, with Dermataphagoidesbeing more common in temperate climates and Blomia being found only in tropical climates Arlian, Morgan Neal, 2002.

1.6. Proposed thresholds of exposure to allergens associated with allergic asthma

1.6.1. Studies examining thresholds of exposure relevant to disease

Prior exposure to an allergen is necessary for an immediate hypersensitivity immune response; however, the quantity of allergen necessary for sensitization appears to depend on the allergen in question, concurrent exposure to other substances that stimulate the immune system and the genetic predisposition of the individual being exposed. In exa- mining thresholds of exposure to pests or more specifically allergens from those pests that lead to allergic disease, it is first necessary to consider allergic sensitization and symp- toms of allergic diseases as separate outcomes. The level of allergens in homes necessary for allergic sensitization – that is, development IgE antibodies – is probably less than is necessary for precipitating asthma symptoms Platts-Mills et al., 1992. For example, for dust mite allergens, the proposed threshold of exposure for sensitization is fivefold lower than that for asthma symptoms Sporik et al., 1990. The strongest scientific evidence for a threshold level of exposure to an allergen exists for dust mite allergens; however, the levels proposed by Sporik and colleagues 1990 have been debated by subsequent studies Warner et al., 1996. Due to the design of studies conducted to date, mostly cross-sectional and lacking controls non-asthmatics, the thres- holds for developing sensitization to cockroach and mouse allergens have not been well established. Recent studies suggest that the differences observed between studies that eva- luated the risk of exposure to mite allergens may likely be due to the influence of concur- rent exposures that alter the immunological response and genetic factors that affect an individual’s susceptibility to produce IgE antibodies in response to an allergen see sub- section 1.1.1.1 Kleeberger Peeden, 2005.

1.6.1.1. Dust mites

Based on findings from a prospective birth cohort in 1990, Sporik and colleagues propo- sed a threshold level of the dust mite allergen Der p 1 from the house dust mite Dermatophagoides pteronyssinus sufficient for sensitization 2 µgg and a threshold for an increased risk of asthma 10 µgg Sporik et al., 1990. However, subsequent studies have proposed that exposure to much lower concentrations is sufficient for sensitization in a sizeable fraction of the population Munir et al., 1995; Warner et al., 1996. Exposure to dust mite allergens in the home does appear to have a dose–response relationship, with those individuals exposed to more allergen being more likely to become sensitized to mite allergens, especially among those with atopic mothers Platts-Mills et al., 2001; Cole Johnson et al., 2004; Brussee et al., 2005; Kerkhof et al., 2005. In communities where dust mite allergen in most homes is greater than 10 µgg, the population response appears to plateau, leading to the majority of allergic individuals being sensitized to mite, likely indicating saturation Marks, 1998. As discussed in subsection 1.6.1, the response to exposure to dust mite allergens, as well as to other allergens, is likely influenced by environmental exposures and the genetic pre- Public Health Significance of Urban Pests Allergic asthma 25 24 disposition of the individual, and this can explain differences in thresholds reported from different studies. A recent study in a birth cohort in Manchester, England, found that increasing exposure to dust mite allergens was associated with an increased risk of sen- sitization, but that effect was dampened if the children were also exposed to higher levels of endotoxin. However, the modifying effect of endotoxin was only observed in children with a specific genetic polymorphism in a gene for an endotoxin binding receptor Simpson et al., 2006. From this example, it is clear that defining allergen exposure thres- holds is complicated by other environmental exposures and by individual genetic sus- ceptibilities.

1.6.1.2. Cockroaches