Food Limiting factors Distribution in Europe and N orth America

Public Health Significance of Urban Pests 89

3.1.1.5. Habitats within the home

HDMs occupy a wide range of habitats in the home, including mattresses, bedding, car- pets, soft furnishings, gaps in laminate floors, clothes and toys. It is difficult to generalize about where HDMs are most abundant within houses, since population size may be influenced by a number of factors, including variations in microclimatic conditions. Mattresses and carpets, however, are generally considered to be the main living and bree- ding grounds for HDMs. The mattress forms a complex hygrothermal environment, containing areas that are more and less favourable to mites than room conditions Pretlove et al., 2001. Mattresses are not always the dominant HDM habitat. Arlian, Bernstein Gallagher 1982 found a living room carpet in Ohio, which supported a population of HDMs primarily D. farinae seven times larger than that found in the mat- tresses. Other soft furnishings may also support HDMs. Mitchell and colleagues 1969 found that mite density was greater in frequently used furniture than in pieces rarely used. This is likely to be due to the greater number of skin scales supplying food for HDMs and the higher moisture content of furniture used regularly. D. pteronyssinus and D. farinae show behavioural differences that enable them to occupy different niches within the same habitat. Van Bronswijk 1981 reported findings from a study by Wharton that noted that although D. pteronyssinusand D. farinae occupied the same niche, D. farinae tended to crawl on top of the substrate while D. pteronyssinus remained beneath it. Generally, HDMs are found closer to the surface of the mattress, although de Boer 1990a observed that HDMs are also present deep inside a mattress, not just near the surface.

3.1.2. Distribution in Europe and N orth America

H DMs inhabit a wide geographical range. T heir distribution is well understood in Europe and North America, although comprehensive studies in other parts of the world are limited. D. farinae is generally the most dominant species in North America and other continental regions with prolonged dry weather, while D. pteronyssinusis abundant in temperate areas with constantly higher humidity levels, such as the United Kingdom Platts-Mills Chapman, 1987. Within mattresses in the United Kingdom, D. pteronyssinusis usually the most abundant mite species, followed by E. maynei, although there is some variation in the proportions of these two species Rao et al., 1975; Blythe, Williams Smith, 1974. In general, studies have recorded no or very few D. farinae in samples and speculate that conditions within the United Kingdom may be too cool and damp for the survival of this species Blythe, Williams Smith, 1974; Abbott, Cameron Taylor, 1981; Hart Whitehead, 1990; Wanner et al., 1993. Walshaw Evans 1987 stated that there was a strong correlation between the density of E. maynei within a mattress and social class, increasing numbers of mites being found as social class declined. However they did not examine this relationship with D. ptero- nyssinus . Colloff 1991a also showed that E. maynei occurred more frequently in samples from homes that he assessed as being damp. House dust mites 88

3.1.1.3. Food

As well as skin scales, pollen, spores of microorganisms, fungal mycelia and bacteria have all been found in the gut of Dermatophagoidesspp. van Bronswijk, 1981. A person typi- cally produces 0.5–1.0 g of dead skin a day, although a large proportion of this will be removed via the cleaning of clothes and bed sheets and the vacuuming of carpets. Food is not normally considered to be a constraint on HDM population growth, although its scarcity after washing or vacuuming could conceivably impede HDM population growth and allergen production de Boer, 1998. It is thought that freshly shed skin scales may be dry and indigestible and that mites pre- fer older skin scales with a higher moisture content Maunder, 1990. Also, fungal growth on skin scales can affect their nutritional value. Van Bronswijk 1981 reported that D. pteronyssinus grew better on skin that had previously been partially digested by the fun- gus Aspergillus amstelodami than skin with no fungal treatment. However, Douglas Hart 1989 concluded that no experimental studies provided definite evidence of this, although they did show that small quantities of the fungus Aspergillus penicillioides may be of nutritional value. De Saint Georges-Gridelet 1987 reported that the fungus Aspergillus repens also increases the nutritional value of skin scales to the mites by redu- cing the fat content and adding vitamins B and D. Mites kept in the laboratory are normally fed yeast-based food, which is often supple- mented with different types of animal protein. Experiments indicate that the type of food used can affect the rate of HDM growth Hart et al., 2007. This can have a significant impact on the comparability of laboratory trials.

3.1.1.4. Limiting factors

In most situations, HDM populations are primarily limited by temperature and relative humidity van Bronswijk, 1981. HDM habitats, such as carpets and especially mattres- ses, provide vast amounts of space for these mites. HDMs and food are more commonly found near the top surfaces of spring and foam mattresses de Boer Kuller, 1994; Hay, 1995. However, HDMs may be present throughout solid foam mattresses, providing food and suitable hygrothermal conditions are available de Boer Kuller, 1994. Food quality is also likely to have an effect on mites. Arlian 1992 found that feeding rates are affected by the moisture content of food; this could have implications for nutritional uptake and the speed of reproduction. The carrying capacity of an artificial yeast- and liver-based mite food has been found to be about 12000 mites per gram of culture Wilkinson et al., 2002. Food is only likely to become a limiting factor when HDMs expe- rience prolonged favourable hygrothermal conditions. The food quality is also likely to vary considerably between habitats; carpet dust, for example, contains considerably more grit than mattress dust. Although fungi provide nutritional benefits to HDMs, at very high levels of relative humidity they also can kill mites, either by producing toxins or as a result of the physical effect that a large quantity of fungi has on the HDM habitat Arlian et al, 1998; van Asselt, 1999. The impact of predation is also thought to be mini- mal in the domestic environment see section 3.3.8. Public Health Significance of Urban Pests 91 Second, Arlian 1977 also concluded that H DMs are unlikely to obtain significant amounts from the oxidization of carbohydrates and fats and, third, passive absorption through the outer surface of the body does occur, but only slowly Arlian Veselica, 1982. Finally, the active uptake of water is facilitated by a pair of glands, the supracoxal glands. These glands secrete a hygroscopic salt solution that contains sodium chloride and potassium chloride Wharton, Duke Epstein, 1979; the secretion flows down a duct, eventually reaching the HDM mouth. Providing the relative humidity exceeds the CEH, the salt solution experiences a net gain in water. Wharton, Duke Epstein 1979