Summary

House dust mites (HDMs) are the most important causes of allergic sensitization and allergic disease, including Dermatophagoides farinae as one of the principal species. Mites dominate in environments with temperate climates, damp, and humid dwellings. The HDM fecal pellets are reported to be the major source of allergens causing allergic reactions after inhalation. Mite allergens can trigger the symptoms in a sensitized individual through inhalation, ingestion and direct contact. Out of all the D. farinae allergens listed, Der f 1, Der f 2, Der f 11 and Der f 23 are the major allergens. A high degree of cross-reactivity is noted between D. pteronyssinus and D. farinae extracts, whereas the reactivity between Dermatophagoides and Blomia tropicalis is limited. Furthermore, a high degree of cross-reactivity is also observed between Der p 1 and Der f 1, Der p 2 and Der f 2, and between Der f 23 and Der p 23. Tropomyosins play an important role in the cross-reactivity. It has been reported that tropomyosin allergens from HDMs cross-react with crustaceans (shrimp, lobster, crab, crayfish) and mollusks (mussel, oyster, scallop, clams, abalone, snails, squid, octopus, cuttlefish). Furthermore, Der f 10 is found to be cross-reactive with Blo t 10, Lep d 10, Pen a 1, Per a 7 and Hom a 1.

Epidemiology

Worldwide distribution

Generally, high altitudes are not favorable place for dust mites to live in, and hence it is important to know that, even before the dust mites were discovered, sanitaria were frequently built for patients with respiratory diseases, e.g., in the Alps (Europe) and Denver, CO (USA). Low sensitization and growth of dust mites have been observed in the Alps than at sea level, due to the lower indoor humidity observed at high altitudes. However, contrasting results were obtained in a study conducted at high altitudes in a tropical developing country. According to this cross-sectional study, 87.9% of asthmatic children were found positive to at least one HDM type (n=61). Further, out of the sensitized children, 70.7% showed sensitivity to D. farinae.

According to a systematic analysis conducted on 163 articles, involving 114,302 allergic cases in China, the rate of D. farinae sensitization was reported to be 75.2% in patients with allergic rhinitis (AR), whereas it was 78.5% in patients with allergic asthma.

Route Of Exposure

Main

Airway inhalation is the primary route of exposure to HDM. It has been reported that post-inhalation of a dust mite allergen, reduction in mucociliary clearance is observed, which in turn is found to escalate the deposition of inhaled particles. Further, it has been found that exposure of HDM allergen can act as a trigger in exacerbating the existing condition of asthma.

Clinical Relevance

Mite allergens can trigger the allergic symptoms in a sensitized person through various routes, including inhalation (asthma, AR, eczema), ingestion (anaphylaxis, urticaria), and direct contact (conjunctivitis, eczema).

Mite-allergic patients with asthma might also have symptoms of AR. This supports the “unified airway” concept that asthma and AR may not be separate entities, but rather linked manifestations of allergic inflammation, occurring throughout the upper and lower airways.

According to a 13-month, multicenter survey, severe or very severe burden of AR and asthma, caused by HDM was noted in terms of irritability, tiredness, disturbed sleep, and troublesome professional life.

Allergic rhinitis

According to a study conducted in China (Qingdao region), D. farinae was responsible for causing AR in 66.4% of children (1887 out of 2841 children). It was reported that sensitization to HDM was more in older children with AR, particularly the males. Clinical manifestations of AR are likely to have an effect on studies, work productivity and efficiency, as well as the quality of life.

Asthma

Many patients are unaware that dust mites are a trigger for their asthma, yet report symptoms of sneezing, wheezing or eye irritation while performing their activities, such as house cleaning, or upon waking up.

Exposure to HDM during early childhood elevates immunoglobulin E (IgE) levels, which in turn predisposes to asthma. Mite allergy is a major risk factor for asthma, and sensitization to mites, early in life has a significant impact on pulmonary function.

A study was conducted in 2087 allergic patients, out of which, 82% of patients demonstrated a positive skin prick test (SPT) for HDM allergens. D. farinae was one of the predominant species, with a prevalence of 61%. The dust allergen load (>10 µg/g) was notably higher in Der f 1 (57.6%) than Der p 1 (20%), thus indicating as a risk factor for the development of asthma.

In a cross-sectional study, 31% of severely asthmatic children were found to have an obstruction (forced expiratory volume in one second/forced vital capacity [FEV1/FVC] <80%). A significant correlation (Rho: −0.34; CI 95%: −0.55 to −0.09; P=0.008) was observed between FEV1/FVC and positive SPT, indicating the presence of obstruction. Further, it has been observed that an elevated risk of progressive lung function loss could be noted in children with asthma, atopy, and reduced pulmonary function.

Atopic dermatitis

The prevalence of sensitization to mites can be very high in patients with atopic dermatitis (AD). The increase in the permeability of atopic skin and the ability of mite proteases to decrease skin barrier function may allow more effective sensitization with aeroallergens, initiating a vicious cycle of inflammation and further allergen exposure.

A retrospective, cohort study was conducted on 102 children with AD (median age at onset: 1.5 years). Children were majorly sensitized to HDMs, including D. farinae and D. pteronyssinus, across all the age groups (<2 years, between 2-7 years and >7 years), however, sensitization to HDMs was higher in children >7 years of age.

Other

Unintentional ingestion of dust mites in the form of food (contaminated with mites), can cause systemic allergic symptoms. In a case of a 48-year-old male, ingestion of fried pastry, prepared from flour contaminated with D. farinae, led to anaphylaxis.

Prevention And Therapy

Allergen immunotherapy

According to evidence, sublingual immunotherapy (SLIT) could be considered as a safe and effective route of administration. In a retrospective study conducted among 282 children with AR in China, SLIT with D. farinae drops was found to be safe and efficacious in pre-school and school-age children with AR, induced by HDMs.

Prevention strategies

Avoidance

The exposure of HDMs can be prevented or reduced by wrapping the mattresses, pillows, and box springs in coverings, that are allergen-proof, washing the bedding with hot water in a week, cleaning the house, wearing an appropriate mask while cleaning, changing furnace and air-conditioner filters, and using a dehumidifier, that help in decreasing the humidity in homes.

In a study conducted in Malaysia, use of commercial ionizer was found to be successful as well as effective in increasing the mortalities in mites (including D. farinae and D. pteronyssinus) by increasing the length of exposure time of ionizer. It has been reported that production of negative ions by the ionizer had the capability of killing the HDMs and further decreasing the population of natural mites on surfaces, such as clothes, mattresses, floors, curtains, etc.

In mite-sensitized asthmatic patients, bronchial hyperreactivity and bronchospasm was found to be aggravated, upon exposure to mites, however, the symptoms got improved in mite-free environment. Decreased exposure to mites in patients with mite allergy can be an effective tool in managing allergic patients, such as asthma.

Cross-Reactivity

The allergic sensitivity, following the ingestion of HDMs, show symptoms in two different forms - the ingestion of invertebrates demonstrating cross-reactivity with mite allergens, and the ingestion of foods that are contaminated with dust mites.A high degree of cross-reactivity is noted between D. pteronyssinus and D. farinae extracts, however, the reactivity between Dermatophagoides and B. tropicalis has been reported to be low.

According to a study, co-sensitization and cross-reactivity has been reported between B. tropicalis and two Dermatophagoides species, i.e., Der p and Der f. In this study, 70.14% of allergenic patients (1050 out of 1497 patients) were found to be co‐sensitized to B. tropicalis, Der p, and Der f. However, the cross-reactivity between B. tropicalis and Dermatophagoides was limited.

The structural similarities between Der p 1 and Der f 1 was demonstrated with an X-ray crystal structure analysis. The analysis showed a surface conservation of a crystal structure of natural Der f 1 with Der p 1, having 71% of amino acid similarity, along with an overlapping catalytic area. This high structural similarity observed between Der p 1 and Der f 1 are commonly believed to be the basis for their cross-reactivity.

A high degree of cross-reactivity has also been reported between Der p 2 and Der f 2 (from D. farinae).

Group 11 allergens (Der p 11, Der f 11) are considered as major allergen molecules in patients with AD, having sensitization to HDM, and thus should be included among allergen components for the routine testing in the clinical laboratory.

A high amino acid sequence similarity has been found between Der p 23 and Der f 23 (87%). Different structural studies of Der p 23 and consecutive modelling of Der f 23 on its structure, might imply on the occurrence of considerable cross-reactivity between the two proteins.

Der f 10 (tropomyosin from Der f) and Der p 10 of HDM, both are found be cross-reactive with Lep d 10 (tropomyosin from the storage mites), due to high level of homology.

Tropomyosins are a large family of heat-resistant, alpha-helical proteins. These proteins form a coiled-coil structure of two parallel helices, that includes two sets of seven alternating actin-binding sites. This feature plays a vital role in regulating the function of actin filaments.

One of the most important cause of cross-reactivity, among mites, shellfish, helminths, and cockroaches is the Tropomyosin, although glutathione transferase may also be included. In cases where genuine sensitization is unclear, specific allergen components can be useful to identify primary allergy.

Tropomyosin allergens from HDMs are reported to show cross-reactivity with tropomyosin allergens of invertebrates, such as crustaceans (shrimp, lobster, crab, crayfish), mollusks (mussel, oyster, scallop, clams, abalone, snails, squid, octopus, cuttlefish) and insects (cockroaches).

Der f 10 allergen has shown sequence similarity with shrimp tropomyosin Pen a 1, American cockroach tropomyosin Per a 7, and lobster tropomyosin Hom a 1.

Further, cross-reactivity has been reported between HDM and shrimp-reactive IgE antibodies, in patients with shrimp allergy. In patients with allergy to HDMs, reactivity to shrimp has also been revealed, especially in patients who were never been exposed to shrimps, because of religious dietary habits.