Development of allergic sensitivity to house dust mites and worsening of morbidity due to asthma, eczema and allergic rhinitis

Mites are insects related to ticks, chiggers and spiders. These eight-legged animals are too small to be seen with the naked eye. The term 'house-dust mites' (HDM) is used to designate about 10 species, two of which are dominant: Dermatophagoies pteronyssins (DP) and D. farine (DF). The optimal growth of HDMs require high humidity, moderate temperature (70-80°F, 21-27°C) and adequate food sources such as human skin flakes. The largest numbers of mites are usually found in dust samples taken from uncovered mattress surfaces, bedding, upholstered furniture and floor carpeting. However, stuffed toys, clothing and drapes can also be important mite habitats. Seasonal variations in mite levels occur mainly in carpets and other places where drying occurs relatively rapidly.

Sensitization and exposure to house-dust mites
Upon exposure to allergens, a certain proportion of the population (10-20%) will become 'sensitized' and develop a specific immune response that includes the production of the IgE antibody. Whether or not a given individual develops sensitization to a specific allergen depends on several factors, including the individual's genetic make-up, the immunogenic properties of the allergen and the timing and degree of exposure. The importance of genetics in determining the ability of an individual to respond to allergens is well supported by family and population studies. HDM allergens seem to be particularly immunogenic. Indeed, in areas of high exposure, it appears that almost all the potentially atopic children become sensitized to mite antigens over the first 14 years of life. Sensitization seems to be dependent both on the quantity of exposure and to some degree on the timing. Although this can occur at any age, predisposed individuals are more susceptible to sensitization during infancy. Evidence from a number of studies have shown that there is a dose-response relationship between exposure and sensitization to indoor allergens. In areas where mites do not flourish due to low humidity, sensitization to mite allergens is unusual. Once sensitization to inhalant allergens develop, it usually persists in adult life.


House-dust mites as a cause of asthma

It has been over 60 years since a German researcher proposed that exposure to dust mites was a cause of asthma in Germany. Inhalation of dust mite allergens in sensitized subjects can cause immediate and late narrowing of the bronchi and wheezing.

Many studies have shown the association between sensitization to dust mite allergens and asthma. In a prospective study of a cohort of children in New Zealand, there was a highly significant association between dust mite sensitization and the development of asthma. Case-control studies from around the world have confirmed the association in mite-allergic subjects. A prospective study reported from the UK indicated that exposure to greater than 10 micrograms of mite allergen per gram of dust in early childhood was an important predictor of the development of asthma by age 11. Additional controlled studies of patients presenting to emergency rooms with asthma have demonstrated significant associations with dust mite sensitization and exposure. It has been shown that dust mite sensitization is the strongest independent risk factor for asthma among school children in Virginia. Overall, it is clear that the association between mite exposure and the development of asthma in sensitized individual is very strong and that this association is consistent among different populations.

A causal role of mite exposure in asthma is further supported by evidence that the prevalence of the disease is lower in areas of low mite exposure. Interestingly, following contact with western civilization, an increase in prevalence of asthma form 0.7% to 7% was noted in some villages of Papua New Guinea. However this increase was the consequence of the introduction of blankets with subsequent mite infestation, resulting in sensitization and development of asthma in a previously non-exposed population. In other studies, decreased exposure to mite allergens have been shown to result in an improvement of symptoms and decrease in bronchi twitchiness. Thus, HDM allergens appear to be both a primary stimulus in the development of the inflammation of asthma, as well as an ongoing stimulus maintaining the bronchial reactivity that underlies symptoms. While inhalation of allergen cannot explain all cases of asthma, it is of primary importance in the development of the disease in most children and many young adults. The obvious therapeutic implication is that allergen avoidance should be a primary treatment for asthma in allergic patients.



House-dust mites as a cause of allergic rhinitis

Chronic symptoms of a blocked runny nose and sneezing can be very troublesome. Allergy to HDM is the most important cause of chronic perennial (all year-round) Allergic Rhinitis (AR). Although chronic rhinitis is sometimes thought to be a trivial illness, the morbidity and economic burdens associated with it are immense. Rhinitis is the most common of the allergic diseases, even more common than asthma. Traditionally, allergic rhinitis has been managed with the use of topical nasal steroids or the use of systemic antihistamines and specific immunotherapy (allergy shots). Allergen avoidance has always occupied a central role in the management of allergic rhinitis. Attempts at HDM reduction in individuals with perennial AR are logical and when implemented correctly and systemically usually result in good clinical results.



House-dust mites as a cause of atopic dermatitis

Atopic Dermatitis (AD) affects 10% of children and its prevalence is increasing steadily. Onset is usually in the first weeks and months of life and the dermatitis may persist into adult life. There are many factors including allergies, infections, emotional, climatic and other environmental influences that contribute to the causation of AD in genetically predisposed individuals.

There is some evidence that HDM and their allergens are of pathogenic significance in the provocation or maintenance of AD. A clinical trial was performed of a regimen of house dust mite eradication measures, including encasing of mattress and bedding, vacuum cleaning and use of an anti-mite spray. The active treatment resulted in major reductions in the quantities of mite allergens in the bed and carpets and this was associated with a highly significant clinical benefit in both children and adults with moderate to severe AD.