House and Dust Mites

Mites are the most prevalent invertebrate inhabitants of our homes. Houses contain a diversity of mites that has not fully been explored. Representatives of all orders of the Acari are expected given that human habitation offers so many small niches and habitats that practically all lifestyle requirements can be accommodated for as in strong contrast to insects. The huge difference in size between mites and insects makes this possible.

Humans have been storing food and have been building homes for a much shorter period than animals such as birds and other mammals. The large number of acarine taxa that inhabit our homes suggests that mites are taking advantage of a habitat similar to that in which they have evolved. House dust mites of the genus Dermatophagoides (Pyroglyphidae, Astigmata) have been associated with bird's nests; their natural habitats are likely the nest and lair of birds and mammals

(Kniest 1994; Walter and Proctor 1999). Stored product mites of the genus Tyrophagus (Acaridae, Astigmata) are also inhabitants of dead leaf litter (Binotti et al. 2001; Walter and Proctor 1999). The short generation time of many mite species allows a fast evolution towards a synanthropic lifestyle.

Historically, the house dust mites or allergen mites have received notorious attention because of their medical importance as allergen producers and their involvement in chronic respiratory diseases or disorders. The population dynamics of some cosmopolitan and synanthropic species have been studied to such an extant that their behaviour on our cloth and in our mattresses can be precisely predicted in most climatic regions.

Based on the large amount of information available on the house dust mites (the allergen mites), the most abundant species indoors seem to belong to the family of Pyroglyphidae (Astigmata), which feed on the skin flakes and other dander found in our homes. In a global survey, Dermatophagoides pteronyssinus, D. farinae and Euroglyphus maynei can account for up to 90% of the house dust fauna (Blythe et al. 1974; Crowther et al. 2000). However, we feel that this represents the result of a repeated sampling bias toward skin feeding allergen mites. The majority of the investigators have used extraction and sampling methods for house mites associated with beds, mattresses, carpets of bedrooms bathrooms and living rooms where skin flakes are normally accumulated. On the other hand, the cabinets of the kitchen and/or the carpet of the dinning room could reflect a completely different scenario. Early on Hughes has catalogued 25 species of Astigmata, 9 of Prostigmata and 20 of Mesostigmata that are found in storage premises containing human food, such as kitchens or pantries (Hughes1976). For example, Carpoglyphus lactis (Carpoglyphidae, Astigmata), Melichares agilis (Ascidae, Mesostrigmata) and Blattisocius mali (Ascidae, Mesostigmata) might be common inhabitants of the Christmas pudding, while several species of Tyrophagus and Acarus (Acaridae, Astigmata) are well known as the major cheese mites. Lardoglyphus spp. (Lardoglyphidae, Astigmata) will engage in serious competition for dried meat, bones, and hides with the beetles of the genus Dermestes. And every detritivorous mite will attract its own mite predator species, which in part explains the high diversity of the indoor mite fauna.

Colonisation of new homes could be by movement of infested furniture or soft furnishings, through the use of another animal for transport (phoresy), airborne, or simply brought in by humans and their pets (Bischoff and Kniest 1998; Warner et al. 1999). Clothing can carry a large number of mites and could be a potential source (Bischoff et al. 1998). House dust mites do not like to establish populations in new houses until humans are present (Warner et al. 1999).

Larger households are associated with higher allergen levels than smaller households (van der Hoeven et al. 1992). The average level of mite infestation is proportional to the number of members in the household (Arlian 1989). Bigger buildings are likely to produce more food for house and dust mites and offer more constant conditions of temperature and humidity.

Astigmatid mites, which include the house dust mites and stored product mites, have no respiratory system, so gas exchange and water intake occurs through the cuticle. This means that in a dry habitat they are subject to desiccation. The relative humidity

(RH) is usually required to be above 55-60% for mites to uptake water, though stored product mites require higher humidity levels than house dust mites. Humidity sufficient for water uptake is only required for about 1-3 h a day, during which the mites can take up enough water to survive for the rest of the day (Schei et al. 2002).

The concentrations of dust mites in beds and on floors peak in the summer and decreases in winter, with a corresponding peak of allergens in the autumn and a decrease in spring (Chew et al., 1999a, b). The lag between peaks and troughs of allergens and mite concentrations is explained with the persistence of allergens after the mites' death.

Because of their small size, mites can take advantage of small clines in environmental factors. These clines are common in houses due to their fragmented nature offering varied microhabitats that mites can colonise. Each room may have a slightly different microcosm, causing variation of mite occurrences and densities between rooms. The richness of the mite fauna in dust collected from different rooms secures a high level of specificity.

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