Introduction
Dust mites are microscopic creatures that are not visible to the naked eye. Female dust mites are generally between 200 to 500µm in size, with males being smaller. They prefer warm, moist surroundings close to sources of food. Since one of their food sources is human skin scales, they are most prevalent in high-use areas such as living rooms, bedrooms, and areas with upholstered furnishings where shed skin scales can collect and serve as food. It has been estimated that the skin one human sheds each day (~1 gm) could feed several thousand mites for up to three months. Pillows and mattresses are also primary locations for dust mites. These are also key areas for exposure since in these locations a person’s face is in very close proximity to the allergens, which are present in the dust mite feces and dried body fragments.
Classification
Dust mites belong to the kingdom of animals; phylum, Arthropoda; class, Arachnida; and group, Astigmata; with three genera (Dermatophagoides, Euroglyphus, and Blomia) important for humans indoors. The mites most commonly found in house-dust in homes worldwide are D. farinae, D. pteronyssinus, E. maynei, and B. tropicalis. In the Unites States, all of these dust mites may be found indoors, but D. farinae and D. pteronyssinus are found most frequently and are most widely distributed geographically.
Ecology
Both food and water are critical for mite survival. Since adequate food (e.g., human skin flakes) is generally available, it is the relative humidity of a place that determines mite prevalence in a location. In humid regions, usually all homes and many other buildings contain breeding populations of dust mites. In dry climates, fewer homes contain dust mites and they are usually at low levels. However, by raising indoor humidity through the usage of evaporative coolers may alter the indoor environment that may lead to conditions that facilitate mite survival. Ambient relative humidity may influence the rate at which feeding mites produce allergens and its accumulation in dust. It has been shown that by lowering the relative humidity in a place may significantly drop the production of fecal allergen even though mites may continue to survive. However, lowering indoor humidity can reduce mite population density overtime because mites gradually dehydrate and die below 50% relative humidity.
Health Effects
Hypersensitivity diseases caused by allergens from mites that live indoors constitute a major health problem in the U.S. and elsewhere. House dust mites are primarily a concern in human dwellings, but dust mites and mite allergen have also been identified in office buildings in association with and without health complaints. Dust mite allergens are considered to be the major biological agent to have sufficient evidence for the casual relationship of their exposure to the development of asthma in susceptible children. They not only aggravate the problem in susceptible individuals but also may cause susceptible children to develop asthma. House dust mites are also known to play a major role in triggering asthmatic attacks in susceptible individuals. House dust mite allergen is the inhaled substance that actually triggers an attack by causing an allergic reaction. Dust mite allergens are proteins, which come from the digestive tract of mites and are found at high levels in mite feces. A dust mite fecal pellet, containing partially digested food and digestive enzymes, is ~10 to 35µm in diameter and contain allergens (protein) called Der p 1, Der f 1 and mite group 2. These allergens (proteins), when inhaled, attach to sensitized cells in the air passages causing hay fever and asthma, and aggravate atopic dermatitis in people who are susceptible to this problem. Approximately 85% of asthmatics are allergic to dust mite allergens.
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