The house dust mite (sometimes abbreviated by allergists to HDM), is a cosmopolitan guest in human habitation. Dust mites feed on organic detritus such as flakes of shed human skin and flourish in the stable environment of dwellings. In nature they are killed by micro-predators and by exposure to direct sun rays. House dust mites are a common cause of asthma and allergic symptoms worldwide. The enzymes they produce can be smelled most strongly in full vacuum cleaner bags. The European house dust mite (Dermatophagoides pteronyssinus) and the American house dust mite (Dermatophagoides farinae) are two different species, but are not necessarily confined to Europe or North America.

Size

The body of a house dust mite is visible against a dark background in a normal light. A typical house dust mite measures 420 µm in length (almost 0.5 millimetres) and 250 to 320 µm in width. Both male and female adult house dust mites are creamy white and have a globular shape. The body of the house dust mite also contains a striated cuticle. As a member of the class Arachnida, larval and post-larval stages of house dust mites have eight legs. Dust mites can be transported airborne by minor air currents generated from normal household activities.

Life cycle

The average life cycle for a male house dust mite is 20 to 30 days, while a mated female house dust mite can live for 70 days, laying 60 to 100 eggs in the last 5 weeks of her life. In a 10 week life span, a house dust mite will produce approximately 2000 fecal particles and an even larger number of partially digested enzyme-infested dust particles.

A simple washing will remove most of the waste matter. Both being exposed to temperatures of over 60 degrees Celsius (140 degrees Fahrenheit) for a period of one hour, or freezing, will typically prove fatal to house dust mites; a relative humidity less than 50 may also be fatal. Ten minutes in a household clothes dryer at lethal temperatures has been shown to be sufficient to kill all the dust mites in bedding. House dust mites reproduce quickly enough that their effect on human health can be significant.

Habitat and food

The house dust mite survives in all climates, except at high altitudes where reproduction is halted. If trying to control house dust mites and other indoor pathogens for which water is a gating factor, it is relative humidity that is important. This is because water condenses out of air onto a surface only when the air at that surface contains more water than it can hold at that temperature. When warm, moist air contacts a cool surface, the air touching that surface may cool and give up some of its moisture to condense on the surface. When humidity is less than optimal, house dust mites function more slowly, eventually become dormant and die. House dust mites thrive in the environment provided by beds, kitchens and homes in general, where the sun's rays do not reach them. Mites remain in mattresses, carpets, furniture and bedding, since they can climb lower down through the fabric to avoid sun, vacuum cleaners, and other hazards, and climb higher up to the surface if necessary to get another skin cell to feed on, when humidity is high. Even in dry climates, house dust mites survive and reproduce easily in bedding (especially in pillows) because of the humidity generated by the human body during several hours of breathing and perspiring.

House dust mites consume minute particles of organic matter. House dust mites have a rudimentary alimentary system (no stomach) and require most digestion to occur outside their body. For this reason they secrete enzymes and deposit the fungus Aspergillus repens on dust particles, to enable the fungus to pre-digest the organic matter with its enzymes. House dust mites eat the same particle several times, only partially digesting it each time. Between feedings house dust mites leave particles to decompose further. Dust Mite fecal matter consists of these particles at the point where they are fully digested. A person sheds about 1.5 grams of skin cells and flakes every day (approximately 0.3-0.45 kg per year), which is enough to feed roughly a million house dust mites under ideal conditions.. House dust mites in bedding derive moisture from human breathing, perspiration, and saliva.

Asthma and allergies

House dust mites are one of the most common allergens that trigger asthma. A safety and tolerability clinical trial (Phase IIa) has been completed with positive results by Cytos Biotechnology using an immunotherapeutic (CYT003-QbG10) for treatment of house dust mite-triggered allergies.

Some main signs of house dust mite allergies are itchiness, sneezing, inflamed/infected eczema, watering eyes, runny nose, (if asthma), lungs clogging up and hay fever.

Myths and Misconceptions

It is commonly believed that the accumulated detritus from dust mites can add significantly to the weight of mattresses and pillows. While it is true that the fecal matter of dust mites will increase over time, there is no scientific evidence for these claims.

Allergy and asthma sufferers are also often advised to avoid feather pillows due to the presumed increased presence of the house dust mite allergen (Der p I). The reverse, however, is true. A 1996 study from the British Medical Journal has shown that polyester fibre pillows contained more than 8 times the total weight of Der p I and 3.57 times more micrograms of Der p I per gram of fine dust than feather pillows.

References

External links

• Dust Mite Control - from Clemson Extension Home & Garden Information Center
• Dust Mite Information - Allergies, biology, control measures and locating dust mites
• house dust mites on the UF / IFAS Featured Creatures Web site