House Dust Mites
Dermatophagoides pteronyssinus (EU)
Dermatophagoides farinae ( USA )
Euroglyophus maynei (EU)
An adult female is 0.3 mm long; the adult male is slightly smaller and thinner.
The colour can vary slightly depending on what they feed on, or the background they are set against, but they are mainly translucent.
House dust mites are small translucent organisms belonging to the same group as spiders and scorpions making them one of the arachnids. Unlike these close relatives the HDM does not have a hard outer covering (exoskeleton) instead they are covered in a thin slightly permeable skin which makes them vulnerable to water loss. They are oval with stridulations across the cephalathorax (body), and belong to the sub-order Astigmata which means that they do not have spiracles.
The adult HDM have eight legs, no antennae, no eyes.
HDM are commonly found in mattresses or pillows of beds where they can reach a constant source of skin scales. Humans spend approximately 8 hours a day in bed and will shed roughly 1g of skin a day whilst sleeping. The HDM likes to live in places of high humidity - ideally 75 - 80% (RH). As humans produce approximately a pint of water in sweat every night whilst sleeping some areas of mattresses and pillows are an ideal environment for them to live and breed.
House dust mites can also be found in carpets, sofas, cushions, other upholstery, clothing, and soft toys. These are all sites that can accumulate skin scales if not cleaned often and thoroughly, and, like mattresses and pillows, they are changed infrequently, allowing dust and therefore mite populations to build up over many years.
The adult female HDM lays between 40 to 80 eggs in her lifetime. These eggs hatch into 6 legged larval nymphs, followed by two 8 legged nymphal stages called protonymphs and tritonymphs respectively. Each stage feeds and then moults onto the next stage, taking approximately one month to develop to adult. The newly emerged adults will live between one and three months. The speed of development and adult longevity depends on temperature and humidity.
House dust mites feed on human skin scales and the fungi associated with them. This fungus removes the fat from the skin scale and adds water making a much softer and easier to digest skin scale for the HDM.
HDM live in areas where they have a ready access to skin scales and are in a warm moist environment. They are commonly found in bedding, upholstery and other soft furnishings, therefore they can easily be transmitted through moving these items around the house, or to other homes. Pillows, mattresses and other soft furnishings are often kept for many years either without proper cleaning or washing, this causes and accumulation of skin scales, house dust mites and their faecal pellets.
Why HDM are a problem?
Throughout their lifetime (approximately 3 months), house dust mites produce around 2000 faecal pellets, each containing digestive enzymes to which some people are allergic.
These small pellets are breathed in by humans, becoming lodged in the bronchi, which are the two branches of the respiratory tract between the trachea and the bronchioles. In allergic people, these pellets cause a local inflammation in the bronchi where they have lodged. If a person is exposed to house dust mite faeces over a long period of time the inflammation becomes chronic i.e. persists for greater periods, and becomes spread over a wide area. Because the once wide, smooth airway are now narrowed and uneven, the airflow through them becomes turbulent, causing wheezing and shortness of breath.
In short, the house dust mite allergen plays a major part in the development of allergic asthma .
How can an asthmatic attack occur?
If, in a non-asthmatic person, something becomes lodged in one of the bronchi, the body's natural response is to send the bronchus into spasm until the blockage is removed. For example: if a piece of food gets stuck in the throat, it is instinctive to try to cough it up and out. If, in the asthmatic as described above, an irritant causes the already inflamed bronchi to become more so, the airway becomes so constricted that the body thinks there is a blockage and tries to cough it out. However, since there is no blockage, the coughing continues and it becomes very difficult to breathe in - they experience an asthmatic attack.
This, as all asthmatics know, is a very frightening experience both for the sufferer and for any onlooker. An asthmatic attack can be triggered by many different factors. Some are listed below:
- Pollen grains
- Pet dander
- Tobacco smoke
- Traffic fumes
- A cold or flu
- Vigorous exercise
- Cold air
- Strong emotion
85% of asthmatics are allergic to House Dust Mite allergens and it is thought that the HDM allergen is may be able to cause asthma.
What can be done?
Asthma is a very debilitating disease, causing many deaths each year. Millions of pounds are spent every year on medicines such as bronchodilators, which open the airways to stop asthmatic attacks, and corticosteroids, which reduce the bronchal inflammation. However, neither of these treatments will cure asthma - they just alleviate the symptoms. If you suffer from mite-induced asthma, reducing the number of mites and level of mite allergen in the house can make an immeasurable improvement to your quality of life. Several simple measures can do this:
To reduce the number of mites in your home and bed:
- wash or dry-clean pillows and bedclothes regularly, at a high temperature if possible
- if hot washing is not possible, 24 hours in the freezer will kill all the mites in a pillow prior to a cold wash
- leave bedclothes turned back during the day
- leave windows open where and when this is sensible to do so.
- Barrier fabrics, dehumidifiers, acaricide, and steam cleaners.
The last two measures will increase the ventilation and reduce the humidity around the bed and in the home generally. This makes the home less habitable for house dust mites. Also, increasing ventilation prevents a build-up of dust, etc. in the home, which may irritate the throat and provoke asthmatic attacks. Obviously, with the risk of burglary, many people are unable to leave windows open during the day, and unwilling to do so at night during the winter. An alternative is to invest in a ventilation unit with a heat exchanger. This will increase the air changes in a room without affecting the temperature, extract allergens from the room and prevent allergens from outside entering.
To reduce the level of mite faeces, and therefore allergen, in the home:
- wash pillows and bedclothes regularly
- vacuum mattresses, carpets, upholstery, etc. thoroughly
- dispose of, and replace, mattresses and pillows over 10 years old
- carpet washing
- anti allergen sprays
- air filters.
It is important that vacuuming be carried out with a good quality cleaner that does not pump the dust straight back out. This, as well as having little effect on the allergen levels in the home, can also provoke an asthmatic attack.
By reducing the allergen levels in the home (either directly or by removing the mites), the inflammation of the bronchi slowly subsides, and the likelihood of an attack being provoked decreases. However, there may be no noticeable improvement in health for several months, so do not assume that the changes are ineffective because the asthma does not disappear overnight.
For further information on House Dust Mites please contact Ben Hall, Acarologist firstname.lastname@example.org