Mites, an introduction

Mites and their allergenicity
Mites (Acari) are a subclass of the Arachnida, the spiders. They have eight legs in their nymph- and adult stages. The difference with other spiders is that they lack much of the segmentation.

There are probably millions of species, most of which are not yet known to us. Mites can survive in various habitats, in wood- or grasslands, in birds and mammalian nests. Some eat very small particles like decaying matter, or fungi, others are predatory on all kind of animals, from other mites to insects, birds and mammals.

The mites that can cause human allergy mainly eat small food-particles in our environment. Allergies arise by contact with mite material. Most mite allergies are directed against mites in house-dust, but allergies also develop against the mites in stored food (storage mite allergies).  Where mites are used in the biological control in greenhouses, workers can develop allergies against predatory mite species.

Taxonomy
Acarus siro, one of the storage mites, was discovered and named by Linnaeus in the 18th century (Acarus siro L. 1758). It took to the end of the 19th century and the introduction of better microcopes that a larger number of acarologists investigated the Acari. Important scientists from this period are Canestrini, Berlese and Oudemans.

Despite of over a hundred years of taxonomic research, there is still no unequivocality on the classification of Acari. However, new genetic techniques, like the sequencing of nuclear genes, may finally elucidate the biological relationships (Klimov and OConnor 2008).

House dust mites
In the 1960’s, the house dust mite (Dermatophagoides pteronyssinus) was discovered. House dust allergy was vigorously reseached in that time and it was concluded that the symptoms were caused by an allergy against this house dust mite (Spieksma 1967, Voorhorst R. 1967).

Other important species of house-dust mites originate from the same family: Dermatophagoides farinae, Dermatophagoides microceras and Euroglyphus maynei. These mites live in bedding, tapestry and upholstery, and their natural diet consists for a considerable amount of human skinscales. Their populations fluctuate seasonally, under the influence of temperature and humidity (Hart 1990, Arlian and Morgan 2003).

Allergens
Mite allergy is caused by certain mite proteins, the allergens. The major allergens of the Dermatophagoides species are  predominantly found in the faeces  that the mites drop in their environment. Although less, and in a different molecular ratio, mite bodies contain approximately the same allergens.

Two of the major allergens from these mites are the group 1 allergen, a cystein-protease, and the group 2 allergen, a protein with a hydrophobic tunnel. The biological function of Der p1 is not known. The group 2 allergen is thought to be a transport protein.

The classical allergy and mite research on house dust mites started in Europe and North America. However, both in Asia and in South and Middle-America research has grown. In Middle and South America, the highly allergenic species Blomia tropicalis often lives in house dust. Also a new species was thought to be found, Dermatophagoides siboney. This species might be a smaller subspecies of Dermatophagoides farinae.

Storage mites
Most storage mites eat stored food like grains and animal feed. However, they also may live in bird nests.

When food is available within their humidity and temperature range, most of these mites are fast-developing pioneer species. The allergy against these storage mites seems less wide-spread. There are many species living of stored food.

The most important species from an allergological point of view are Glycyphagus domesticus, Blomia tropicalis, Lepidoglyphus destructor, Acarus siro, Tyrophagus putrescentiae, and Chortoglyphus arcuatus (Fernandez-Caldas et al. 2007).

Allergy-treatments
Treatments to reduce allergy symptoms vary from avoidance measures to allergological treatment.

Mite avoidance measures range from extra cleaning to removing tapestry and upholstery, sealing mattresses in plastic, or killing the mites with specific chemicals.

For an allergological treatment, the specific allergy needs to be identified. Depending on the alllergen, treatment can be anti-histaminica or mite-specific immunotherapy.

Literature
Arlian, L. G., and M. S. Morgan. 2003. Biology, ecology, and prevalence of dust mites. Immunology and Allergy Clinics of North America 23: 443-+.

Voorhorst R., S. F. T. M., Varekamp H., Leupen M.J. 1967. The house-dust mite Dermatophagoides peronyssinus and the allergens it produces. Identity with the house-dust allergen. The Journal of Allergy 39: 325-339.