House Dust Mite allergy
Do you have a house dust mite allergy?
Species and way of life
For centuries, house dust has been known to trigger allergy and asthma symptoms in people with particular sensitivity. The Flemish physician, John Baptista van Helmont, even described such symptoms from house dust mite allergy in one of his patients as early as 1662.
In the 1960s, researchers in both the Netherlands and Japan were independently able to prove that house dust mites from the family Pyroglyphidae, particularly Dermatophagoides pteronyssinus and Dermatophagoides farinae, were the main sources of allergens in house dust.
Allergens from house dust mites are known to cause sensitisation in susceptible people, and this sensitisation is associated with the development of allergic asthma in both children and adults. In addition, it is well documented that there is a dose-response relationship between exposure to house dust mite allergens and an increase in the severity and risk of worsening allergic asthma.
Asthma is a major health problem worldwide, and both its prevalence and severity appear to be increasing.
Source: ALK-Abelló
In the Western world, up to 30% of 13-year-old children are sensitised to house dust mites, and this sensitivity represents a significant independent risk factor for the development of asthma.
Two species of house dust mites in the family Pyroglyphidae, namely D. pteronyssinus and D. farinae, are the most significant in temperate regions – both in terms of numbers and health significance.
Clinically, they are important because most asthma symptoms in children and young adults are caused by both an immediate allergic reaction to their airborne allergens and a familial tendency to develop atopic diseases.
The house dust mite D. pteronyssinus produces at least seven major allergens, as well as a further seven minor allergens, which are officially recognised by the WHO/IUIS Allergen Nomenclature Subcommittee. The most important allergen, group 1, Der p 1, is the most widespread and therefore also the most researched in Europe.
Large quantities of Der p 1 are found in house dust mite faeces – reflecting their digestive processes, in which food and cell residues are enclosed by a peritrophic membrane (a non-cellular membrane that surrounds the food material in the mite’s midgut and performs selective filtration: it acts as an ultrafilter or semi-permeable membrane, allowing small, digested nutrient molecules, particularly from skin flakes, to pass through while retaining larger particles and potential pathogens).
House dust mites are approximately 1/3 mm long, colourless and rarely visible to the naked eye. They develop from eggs into adult house dust mites through several larval and nymph stages.
This development can take from a few weeks to many months depending on the surrounding environment. When house dust mites thrive best, they produce 20–40 faecal particles per day.
Their lifespan is 3–4 months, and the female lays 20–40 eggs once or twice during her lifetime.
House dust mites feed on epidermal material (hair and skin flakes) from humans and animals, and their ability to thrive also depends on suitable temperature and humidity. Development occurs fastest at 25°C and a relative humidity (RH) of 70%.
Allergens
The primary allergens are found in house dust mite excrement. Amino acid sequences show only a few differences between species.
For Dermatophagoides pteronyssinus, the structure of nine allergens is now known: Der p 1, Der p 2, Der p 3, etc.
The allergens are fairly stable under normal living conditions. The important Der p 1 allergen is therefore not affected by heating to 60°C, and allergen activity remains unchanged after four years of storage at 5–25°C.
Mite occurrence
Source: ALK-Abelló
In our bed environment, sweat and evaporation from the skin increase humidity and make house dust mite growth possible – even in cases where the moisture content of the air in the middle of the room is significantly below the level required by house dust mites. This is why the highest concentration of house dust mites in the home is most often found in the bed environment.
We shed several grams of skin flakes every night in bed, enough to feed a clinically significant house dust mite population.
The environment, particularly mite remains and faeces, from house dust mites in upholstered furniture resembles that of the bed, while carpets, in addition to harbouring their own house dust mite population, can act as a reservoir for allergens from beds or furniture.
House dust mites are found in smaller quantities in our clothing and can therefore be transported from one home to another.
If suitable growth conditions exist, significant house dust mite growth can occur in a new home after only 1–2 years, and if you simply replace the mattress without taking other measures, the mites can cause symptoms again after only four months.
The same applies to clothing that is washed infrequently and stored in places where there are house dust mites, for example in damp cupboards, basements or holiday homes.
Soft toys used for sleeping can also harbour many house dust mites.
In practice, humidity determines whether a house dust mite population can become established in a home. Homes where relative humidity is below 45% during the winter months have a very low occurrence of house dust mites.
Indoor humidity is highest in late summer and autumn, and correspondingly, the concentration of house dust mites and house dust mite allergens is highest during the autumn months, although there are very large variations from home to home.
In Denmark, significant house dust mite growth (more than 100 house dust mites/g of mattress dust) was found in 24% of randomly selected bedrooms in 1977, while the frequency in bedrooms in a non-allergic group was as high as 48% in 1992.
Source: ALK-Abelló
We now see significant exposure to house dust mite allergens (more than 2 mg/g of mattress dust) in two-thirds of children predisposed to atopy (an inherited tendency to develop certain forms of allergy) as early as six months of age, and in 52% of all adults.
House dust mite allergens are widespread throughout the home and are found particularly in carpets, furniture, mattresses, pillows, duvets, rugs, curtains, soft toys, clothing and furry animals – and can even, to a lesser extent, be detected in scalp hair, on the skin and in umbilical cord blood.
A large study from New Zealand showed that in dust samples from floors, beds and seats in hotels, hospitals, nursing homes, primary schools and nurseries, cinemas, churches, ski lodges, banks and aircraft, the level of Der p 1 was significantly lower in these public places compared with the home environment.
Although the level was significantly lower, it was found that carpets in public spaces still represent an important source of house dust mite allergens, and that lower Der p 1 levels are related to how frequently they are cleaned.
Allergen-proof covers such as AllerGuard significantly reduce house dust mite allergens on all types of bedding.
