Since 1989 doctors, supported by the World Health Organisation, have warned that house dust mites are a major cause of disease worldwide and have called for tight control of mite infestations indoors. In order to understand how to control mites a basic knowledge of how the creature evolved, its habitat and how man has responded to mite related diseases, is essential. This article is set out to perform that task.
There may be over 100 million different species of mites living on land, in the air and under the sea. They can be found in such diverse hiding places as the airways of bees, in the quills of birds, tree bark, plant stems, frozen sub-soils, thermal springs, grain stores, kitchen cupboards, and library books. Each species has adapted an intricate system of survival to suit their environment. All are masters of self-preservation. One of the most curious is the tiny mite that lives in one ear of a moth. It has learned not to infest both ears or risk making the moth deaf and an easy dinner for bats. One of the essential tasks that mites perform is soil maintenance. Soil mites break down decayed organic matter thus enriching and benefiting life on earth. Considering the diversity of mites it should come as no surprise that they can also cause problems for animals and mankind.
Areas of concern for man are: # Agriculture: Infestation and damage to crops # Stored Products: Destruction of harvested crops # Health: Disease, reduced quality of life and cost to society # Medical Research - To seek remedies to mite and tick related diseases
Mites and ticks are considered as one animal kingdom that comes under the umbrella of 'mites' by entomologists. Of the many diseases mites cause, allergic diseases are the most commonly seen and recorded and the most common offenders are house dust mites. As long ago as 1662 it was realised that the inhalation of dust particles in the air could lead to symptoms of allergic reaction, but it wasn't until 1960 that doctors established that the house dust mite was the major cause of allergy in house dust. It was not just the mite alone but also its droppings that were the problem and within the droppings were packed over 20 different potential allergens. Clearly, for mankind this was a creature to respect and avoid.
The formal Latin name given to UK's most common house dust mite is 'Dermatophagoides pteronyssinus'. This was the name give to the mite by the Victorians who first identified them by looking through a microscope. Mites live in colonies and are barely visible to the naked eye. They have eight legs each of which has hooks and end in a sucker. These are designed to maintain a firm grip in the mite's environment and help when travelling to a new location. Under a microscope, adult mites appear transparent due to the fact that they are up to 75% water in weight and must maintain this weight in order to breed. It is a creature small enough to sit on the tip of a pin and has evolved with no sight, no respiratory system, never drinks and cannot control its own body temperature. To counter these disadvantages the mite has developed an exquisite sense of smell, which it uses for breeding purposes and for finding food, including nourishment from its matured droppings. For a colony to succeed its members depend upon the surrounding habitat for life giving support. The habitat conditions must be dark and damp with plenty of food available which, for mites, is organic matter such as bacteria, pollen grains, and moulds. Its favourite food, however, is discarded skin scales that have become mouldy due to the damp conditions. It is important to note here that selected fungi is 'allowed' to pass through the mite's toxic gut to grow again, if the condition are favourable. They are 'Aspergillus pecilloides' and W sebi.
From egg to adult the mite goes through five stages of life and can live for approximately three months with females laying up to 40 eggs during that lifespan. All healthy adult mites can produce up to 20 droppings a day. The droppings are tightly wrapped in a dry covering that can dissolve on moist services such as nasal passages, lung tissue or broken skin.
House dust mites thrive in countries with warm, moist temperate climates. Modern homes with poor ventilation, warmth and high humidity from cooking, baths and showers have created an indoor climate prefect for house dust mite to thrive no matter where they are in the world. Common nesting sites for established colonies are known to be in mattresses, pillows, duvets, sofas, soft toys and deep piled carpets. An established colony will breed successfully at temperatures of 25C and 75% relative humidity. In this warm, humid indoor environment mites will be able to maintain their critical water weight and a colony will flourish. How to prevent this from happening will be discussed later in this article.
Sensitisation to mite droppings is common in man and usually presents no health problems. However, once an allergy is diagnosed doctors strongly recommend that their patients avoid mite allergens. When the word allergen is used it infers that mite droppings are no longer 'dust' but are of clinical importance to the patient. Tissue vulnerable to mite allergens can be in the respiratory system, on skin either broken or heavily exposed to droppings, or in the eye either through direct contact or transferred in tears.
Allergens To date there are twenty formally classified allergens from house dust mites. Of these only a handful are considered major allergens, that is, affecting the majority of mite allergic patients. The remaining allergens are considered of lesser importance by allergists, but if taken collectively, they can cause up to 50% of the allergic reactions attributed to mites. Of these twenty allergens only two are known to cause allergic dermatitis in dogs.
In 1989 the first mite allergen was genetically identified and cloned by scientists. It was numerically tagged as Der p1 and remains the most studied and described of all the mite's allergens. Der p1 is a powerful digestive enzyme with properties similar to papain, a plant enzyme successfully used as a meat tenderiser in the food industry. In 1997 scientists identified the mechanisms by which this digestive enzyme could work its way into the body. Once on the surface of the lungs, Der p1 attacks and dissolves the 'glue' that holds the cells together. If significant, this breach of lung defences, combined with spillage from undigested contents of the mite's gut, can raise an alarm in the body's immune system which may lead to a 'full-blown' allergic reactions. Following the discovery of the activity of Der p1, doctors soon found that it actually travels within the body and is capable of reaching the fluid surrounding unborn children.
Today, the pathway from inhalation of the mite's dropping, to the release of the allergens on delicate tissue, leading to absorption in the body is fairly well described by scientists who warn that Der p1 is capable of immune disruption for those allergic to mites and is best avoided if possible.
Control As long ago as 1990 doctors successfully demonstrated that mite infestation in beds could easily be controlled. They did this by using an extra fine woven material that was designed to cover mattresses and pillows. The material was so fine that even the very tiny mite droppings, which were encased in an old mattress, could not pass through. The house dust mite and its droppings could neither get in nor out once the covers were in place. It was a great breakthrough in mite control. This material is now on the market as micro-porous, anti-mite, bedding that can cover all items on the bed, including duvets.
The first step in mite control should always be to consider covering beds and bedding, especially for children. The length of time spent in bed and the importance of a good night's sleep, particularly for developing children, make this an obvious choice. Following this anti mite technique there are a series of steps that need to be taken, many of which have cost considerations attached to them. House dust mite control indoors needs to be planned and executed according to the needs of the patient and family. It would be insensitive to demand that all allergen avoidance changes were made at once especially if the financial burden was beyond the means of the family. Therefore, doctors devised a method that was reasonably cost effective and then went on to prove that it could work in controlling house dust mites indoors.
1) Washing all bed linen and soft toys in hot water regularly
2) Lowering humidity and increasing ventilation
3) Installing mite resistant flooring (wood etc)
4) Putting mite-proof covers on all bedding
5) Using high filtration vacuum cleaners
They also used a mite killing spay on fabric covered sofas and chairs. Regular vacuuming of such furniture, using a high filtration cleaner, is now recommended in mite control.
Another method, which inhibits the mite's ability to breed, was devised by a team of doctors from Cambridge. By reducing the moisture in the indoor air the active mite is unable to sustain its critical water weight. Consequently, it will not be able to breed or thrive and the mite colony becomes threatened with extinction. Additionally indoor humidity, kept below 50% relative humidity, is considered comfortable for humans but hostile to mites. This level of humidity can be achieved by installing high-efficiency dehumidifiers and/or air conditioning. These measures, combined with good ventilation, help to control mite populations, however, there are pockets of mini-environments that may escape this manipulation where alternative methods of mite control must be applied. These mini-environments include, blankets in a pet's basket, beds and bedding and soft furnishings in regular use. A combination of controls, as described in the previous paragraph, need to be employed.
With regard to mite control, a timely warning has recently come from a medical charity reporting on the results of mite infestation in duvets. The charity's article pointed out that researchers from the University of Worcester examined the contents of 10 typical duvets and found one that contained 300,000 live mites, while the others contained fungal spores, bacteria, living and dead mites, eggs and pollen grains. No doubt the owners of the duvets were unaware of their troublesome bedfellows.
A question frequently asked by the public is 'Will professional mattress cleaning help?' The answer is: By all means clean your mattress whatever way you choose. But be warned! If you clean with steam, make sure all moisture is removed from the mattress or it will 'kick-start' a new colony of mites from surviving eggs. Only use a company that can guarantee this safeguard. If you choose to clean with a powerful vacuum cleaner open all windows to take away any harmful dirt that may be disturbed by the process. Once cleaned a micro-porous cover should then be put on the mattress to stop new opportunistic mites from getting into the mattress to colonise. Then a heavy-duty cotton quilted cover should be placed over the protected mattress to 'mop up' perspiration and discarded skin scales and to encourage any travelling mites to 'think' they've found a home.
Each month the cotton cover should be hot washed to kill 'hopeful' mites and to remove any shed skin scales, moulds or bacteria that may have accumulated. This cover must be dried thoroughly before it is replaced on the bed. While the cotton cover is off the mattress it is important to wipe down the micro-porous cover with a damp cloth to remove any unwanted dirt, mites or mite eggs. Make sure pillows and duvets are also covered with micro-porous covers. Hot wash all bedding covers regularly. Beds should not be 'made-up'. Mattresses should be allowed to 'air' all day to release any build-up of moisture. In mite avoidance an anti-mite bed and bedding if the first step, first amongst equals.
Doctors refer to this cleaning regime as 'good sleep hygiene'. When this practice is combined with clean, fresh air in the bedroom and enough time asleep, most people will awake refreshed and be ready to enjoy the day.
The mites that impact upon human health are poorly understood by the general public. This could be remedied by making the mite's biology and ecology a subject for study in schools. Within this subject the reasons why they cause allergies can be explained and discussed. There have been some steps in this direction, such as animations on this site that help in this respect, but much more should be done to help the vulnerable.
'Dust Mite Allergies and Asthma - A worldwide Problem', Platts Mills TAE, de Weck A, UCB Institute of Allergy, Bad Kreuznach September 1987. Reported in The Journal of Allergy and Clinical Immunology, 1989: 83:416-427
Who's been sleeping in your pillow? 1997, Dr Jill Warner, Teaching Support and Medial Services, University of Southampton.
'Detection of House Dust Mite allergen in amniotic fluid and umbilical-cord blood'; Holloway J A et al, 'The Lancet', 2000, Issue 9245, 1900-1902.
'Manchester Asthma and Allergy Study: Low-allergen environment can be achieved and maintained during pregnancy and in early life', Custovic A et al, 2000, J. Clin. Immunol. 105, No.2, Part 1, p252-258.
'The biology of dust mites and the remediation of mite allergens in allergic disease', Professor Larry G. Arlian and Professor Thomas A.E. Platts-Mills, 2001 'Journal of Allergy and Clinical Immunology', Volume 107, Number 3, Pages S406 to S413
'Proteases Are Major Allergens Derived from Various Organisms', Editors, M Capron, F. Trottien, Lille, 2006, Vol. 90, p 48 to 52, Parasites and Allergy, Chemical Immunology and Allergy, ISBN 3 8055 -7974-8
'Determinants of House Dust Mite Allergenicity', Hales BJ, Nora NR Chu, Bosco A, Smith W, A,Tatjana KH, Thomas W R, 'Allergy Clin. Immunol.Int.-J.World Allergy Org, 18/2 (2006) 'Research Trends', pp 65-70