The Biology of House Dust Mites and Dust Mite Allergies
By Máire Fox MSc
House Dust Mites are a major source of allergen and are found in temperate and humid areas. House Dust Mite avoidance can reduce exposure to allergens. This article examines the properties of House Dust Mite and makes some recommendations for reducing their numbers in the home.
Biology of the house dust mite
House Dust Mites are arachnids, not insects, and are related to ticks and spiders such as daddy long legs. Thirteen species of House Dust Mite have been found, however the two species that are the most common and are the main source of House Dust Mite allergen are Dermatophogoides farinae and Dermatophadoides pteronyssinus from the family Pyroglyphidae. The life cycle of Dermatophagoides pteronyssinus consists of 5 stages:
The adult mated female can lay 40 – 80 eggs in her lifetime. When the egg hatches a six legged larva emerges. There are two nymphal stages that feed and moult before an eight legged adult is developed. The adult House Dust Mite has a mouth-like appendage but no eyes or antennae.The duration of the cycle is usually one month but is dependent on the climate, ideally 25°C and 75% relative humidity. An adult House Dust Mite can live for one to three months under favourable conditions.
The translucent body of a house dust mite is 300 µm – 400 µm in length and only visible under a microscope. House Dust Mite faecal pellets range from 20 µm – 50 µm. It is estimated that the House Dust Mite can produce 20 faecal pellets per day. House Dust Mites are ~75% water by weight and therefore need to absorb water from the water vapour in the air making relative humidity a critical factor for survival.
Dust mite habitat
House Dust Mites primarily feed on organic detritus such as flakes of shed skin. Other nutrients are provided by animal dander, pollen, bacteria and mould. House Dust Mites survive and reproduce the best in soft furnishings (such as carpets with long pile, bedding and plush toys) that contain a large supply of their food source. This stable environmental dwelling is best provided inside homes. Indoor humidity is very important. When the humidity is less than 50%, house dust mites are unable to maintain their water balance and are more susceptible to desiccation.
The House Dust Mite selects food that has been pre-decomposed by fungi that reduce the fat content of skin cells. The fungi in turn use the House Dust Mite faeces and skin cells as a source of nitrogen (Denning et al. 2006), which forms a minute ecosystem in their environment.
Dust mite allergies
Allergies are a response of the human immune system to a foreign protein substance (allergen). A House Dust Mite allergy is the result of a hypersensitive immune system response to the mite faces. Faecal pellets easily become airborne and when suspended in the air they can be inhaled. The resulting allergic reaction will cause sneezing, wheezing, headaches, coughing, eye irritation, fatigue or dizziness. This is a major trigger for acute asthma attacks in sensitive individuals. Roughly 1 - 2% of the world's population (65 - 130 million people) are allergic to house dust mites (Colloff, 2009).
Dermatophagoides farinae (commonly found in drier regions) and Dermatophagoides pteronyssinus (commonly found in temperate to tropical regions), produce at least nineteen different allergens each. These allergens are further classified depending on their biochemical characteristics by group numbers:
- Group 1 allergens (Der p 1, Der f 1), glycoproteins originating from the digestive tract of the mites.
- Group 2 allergens (Der p 2, Der f 2), non-glycosylated proteins with a high IgE binding capacity and associated with the mite bodies.
- Group 3 allergens are associated with digestive enzymes
- Group 4 allergens are sugar associated, and so on.
Around 80% of mite allergic patients are sensitive to Group 1 and Group 2 allergens from House Dust Mite (Thomas, WR et al. 2004). Examples of other allergens include pollen, pet dander and mould. Allergy is the 5th leading chronic disease in the U.S. among all ages, and the 3rd most common chronic disease among children under 18 years old as reported in “Chronic Conditions: A Challenge for the 21st Century,” (pdf) National Academy on an Aging Society, 2000.
How to control dust mite allergen levels in the home
There is no cure for allergies to House Dust Mites. The only way to avoid allergy symptoms when it comes to dust mites is to prevent exposure. The use of a multifactorial approach to allergen control has been recommended by, the Report on the Third International Workshop on Indoor Allergens and Asthma (1997) as well as the National Heart, Lung and Blood Institute's (NHLBI) National Asthma Education and Prevention Program (2007).
Maintaining relative humidity below 50% by using dehumidifiers in homes is effective in reducing mite populations. Regular vacuuming and steam cleaning carpets can kill mites and remove surface allergens. Encasing mattresses and pillows with products that have verified allergen barrier properties is effective in reducing exposure to House Dust Mite and their allergens.
Many products claim to reduce allergens without independent evaluation of their effectiveness, making it difficult for the consumer to make the right choice. Allergy Standards Limited (ASL) is an international standards and certification body that helps consumers and retailers identify toys, bedding, vacuum cleaners, dehumidifiers etc. for people concerned about allergies, asthma and indoor air quality. ASL’s independent certification standards have been written in consultation with industry, retailer and health care professionals and are the only standards that are accepted by the Asthma and Allergy Foundation of America. Reputable retailers should ensure that all allergen claims avoid falling foul of labelling guidelines. Concerned consumers should also check that labelling where allergies are concerned has been validated by a third party independent testing authority.
The independent testing and research for the Asthma and Allergy Foundation of America certification program is conducted by airmid healthgroup. The research spans all disciplines relevant to indoor particulate contaminants, ranging from virology, microbiology, mycology, allergen, dust mite and bed bug research. The allergen laboratory of airmid healthgroup offers a complete range of advanced Allergen testing services specialising in Allergen Barrier Services, Dust Mite related testing, Claim verification services and Allergen detection.
Máire Fox MSc is the Laboratory Manager and Head of the Allergen Laboratory at airmid healthgroup. Maire graduated from NUIM with a BSc in Biology followed by laboratory analyst positions in the Biotechnology Unit in NUIM and the Vitamin Research Laboratory in TCD. Máire completed a research MSc. in Prion Disease while working as a Laboratory Analyst in the Prion Research Laboratory in UCD. She subsequently worked in the Department of Agriculture investigating the molecular characteristics of BSE strains in Ireland.
airmid healthgroup is an organisation involved in research, development and testing of products/services that seek to improve the indoor environment. We provide companies with independent data to support their product and service development and claims. Our facilities include an allergen laboratory ISO 17025 accredited for Der p 1 (house dust mite allergen) and Fel d 1 (cat allergen) analysis. In addition, our site in Dublin houses virology and microbiology labs. We also have several state-of-the-art test chambers where we carry out tests under tightly controlled environmental conditions. But our greatest resource is our highly skilled multidisciplinary team of scientists and clinicians. We play a flexible and dynamic role for the companies we partner with, ranging from testing finished products/services for claim verification purposes to consultation on product/service research and development.