Public Health Significance of Urban Pests
House dust mites
87 86
associated with asthma worldwide Tovey, 1992. In temperate climates, D. pteronyssinus and D. farinae are the most abundant species Arlian et al., 1998. E. maynei is common,
but under-researched; some evidence, however, shows that it can be clinically important Mumcuoglu, 1977; van Hage-Hamsten Johansson, 1989.
HDMs belong to the family Pyroglyphidae. Mites belonging to this family are normally associated with birds, mammals and stored products van Bronswijk, 1981. It is likely
that HDMs were originally nest dwellers that moved to occupy peoples’ beds at the time of the first settlements Colloff, 1998. HDMs and mites in general are very difficult to
identify, largely due to their small size. A number of guides that specialize in mites found in dust exist – for example, by Fain 1990 and by Colloff Spieksma 1992.
3.1.1.2. Life cycle
The life-cycles of HDMs consist of five major stages: egg, larva, protonymph, tritonymph and adult. Development time from egg to adult under optimal conditions, 25ºC and 75
relative humidity, takes about 25 days. The speed of HDM development depends largely on temperature see subsection 3.1.3.3.
As the HDMs progress through each stage of life, they become larger and display more taxonomic features. Each active stage is separated by a quiescent phase that lasts for a
third to a half the time of the preceding active period Arlian, Rapp Ahmed, 1990. D. farinae
is able to survive adverse conditions, particularly low relative humidity, by for- ming a prolonged, so-called drought-resistant quiescent protonymph stage. The quies-
cent protonymph of D. farinae consumes 28.5 times less oxygen per hour than an active protonymph. It has a water exchange half-life of 160 days, compared with just 20 hours
for an active protonymph and 28 hours for an adult Arlian et al., 1983.
Current research indicates that D. pteronyssinusdoes not form a drought-resistant quies- cent protonymph stage similar to D. farinae. Arlian, Rapp Ahmed 1990 found that
quiescent D. pteronyssinus protonymphs were not formed even at low humidities, although they suggested that a combination of unfavourable conditions may trigger a
quiescent protonymph stage. De Boer Kuller 1997 observed that the quiescent phase of D. pteronyssinus was relatively brief and to their knowledge, not prolonged. De Boer,
Kuller Kahl 1998 reported that in the carpeted ground floor of Dutch houses, D. pteronyssinus
adults remained active throughout the winter. At 25°C and 75 relative humidity, the difference between the development time of
males and females is non-significant Arlian, Rapp Ahmed 1990. The lifespan of adult males, however, was found to be 77 days, compared with 45 days for unmated adult fema-
les and 31 days for mated adult females. During their adult lifespan, D. pteronyssinus females have been shown to produce between 40 and 80 eggs Colloff, 1987. D. farinae
females are able to live considerably longer than D. pteronyssinusfemales 100 and 31 days, respectively. The reproductive period and number of eggs produced were similar for
both species. After death, the bodies of HDMs dried thoroughly, allowing them to per- sist in the environment for a considerable period of time before breaking down.
3.1. Introduction
House dust mites HDMs are microscopic organisms that live where scales from human skin, the major component of their diet, accumulate. Although they can be found in
schools, offices and other workplace environments, they are more commonly found in dwellings, where a variety of habitats, particularly bedding and carpets, can provide
favourable conditions in terms of:
• material properties • relative security from disturbance, competition and predation
• food supply human skin scales or dander • temperature and relative humidity.
The physical spaces provided by modern mattresses whether of spring or foam cons- truction, duvets, pillows whether feather or synthetic, carpets and upholstered furni-
ture are well suited to support mite development. The interconnected air gaps and inter- stices provided are:
• the right size for them to crawl into • dark they are photophobic
• difficult to extract them from due to their ability to cling to fibres • close to sources of the food they require.
Given these conditions, their distribution within the built environment depends prima- rily on temperature and relative humidity. Since HDMs are unable to drink liquid water,
they absorb the water they need from the atmosphere. They thus need high levels of rela- tive humidity to survive.
H DMs do not attack, bite or transmit disease to people, nor are they a sign of poor hygiene. However, their faecal pellets contain a number of potent allergens known to
trigger and possibly cause allergic disease, such as perennial rhinitis, eczema and, most important, asthma. Although the HDM has not historically been considered to be an
urban pest, the increasing prevalence of these diseases, especially asthma, justifies its consideration in this context.
3.1.1. Biology and ecology
3.1.1.1. Origins and natural history
Mites are arthropods within the class Arachnida. HDM is primarily used to describe three species of mite, Dermatophagoides pteronyssinus, Dermatophagoides farinae and Euroglyphus
maynei .
1
The allergens these mites produce are probably the most important allergens
1
Unlike with many other urban pests, a sound system of common names does not exist for HDMs. For example, the names “American mite” and “European mite” are rarely used by acarologists, because they are misleading – neither species is exclusive to the region
named and both have a worldwide distribution. Therefore the system of nomenclature based on binomial Latin names is used for HDM species throughout this chapter.
Public Health Significance of Urban Pests
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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
