Positive Health Online

House dust mites are found in most houses. Their numbers have increased dramatically in recent years, and there is evidence that this has led to a corresponding rise in the number of asthma sufferers.

Asthma induced by allergy to house dust mites (HDM) is a world-wide problem.[1] Despite advancement in our understanding of asthma and safer medications, patterns of prevalence in the UK and for most industrialised countries show an increased incidence in children. For example, The National Asthma Campaign Audit for 1997/98 puts the UK at the top of the European league for asthma rates with 3.4 million sufferers (14% of all children aged 2–15 years, and 4% of the adult population aged 16 and above).[2] This is a significant increase on previous years. Between 1979 and 1993 general practitioners (GPs) recorded four and five times as many new episodes of asthma in children aged 5–14 and 0–4 respectively; and the numbers of adults visiting their GPs has more than trebled between 1971 and 1991.[2] Medical costs in asthma treatment are enormous. The latest 1993 costs to the National Health Service (NHS) for treating asthma in England alone are estimated to be £588 million: £438 million on the 31.2 million prescriptions dispensed and £150 million on other services.[2] Costs are apparently increasing 5% per year.

These alarming statistics suggest that current asthma management is failing. The UK, New Zealand and Australia, adhere closely to the guidelines recommended for asthma management by a recent international consensus but there are no published longitudinal studies to confirm whether it is effective.[3] The onus of treatment is to prescribe prophylactic drugs to subside symptoms rather than to prevent the onset of asthma.[4] Yet there is now overwhelming evidence to suggest that allergen avoidance can prevent the onset of asthma and reduce the severity of asthma attacks for those individuals already sensitised.

A predisposition is the most common risk factor for developing allergic diseases. But there are a wide range of other risk factors, alone or in combination, which can almost double the risk. These include the month of birth, gender, race, diet, tobacco smoking, pollution and allergens in the air.[5] After genetic factors, exposure in early childhood to HDM allergens is one of the most important determinants of the subsequent development of asthma.[6] World-wide, dust mite exposure is the single most important factor in the development of bronchial reactivity.[7] In Australia, which has the highest national prevalence of childhood asthma, sensitisation to house dust mite allergen, Der p I, has been shown to be the most important risk factor.[8]

In the USA, progressive changes in housing and lifestyles (e.g. higher indoor temperatures, reduced ventilation, cool wash detergents, and widespread carpeting) has been associated with increased allergen sensitisation and asthma.[9] Similarly, prevalence of childhood asthma has increased dramatically over the last 20 years in Ireland and this has been associated with improved household conditions for mite survival.[10]

Although the importance of HDM as triggers has been known for some time, the preferred method of patient management for GPs is to rely exclusively on drug prescription rather than to advise patients on allergen avoidance. The majority of GPs and asthma sufferers are not fully aware of the role of HDM, despite the overwhelming evidence now available: most symptoms occur at night (beds contain a reservoir of HDM allergens), most deaths occur at night, 80% of asthmatics produce IgE to HDM antigen, low asthma rates are found in low HDM areas whereas more sufferers are found in homes with high HDM, and allergen avoidance reduces allergies.[11]

The role of HDM in triggering asthma

In optimum living conditions (25ºC and 80%RH), adult house dust mites live for 3 months during which females lay 2–3 batches of 20–40 eggs that develop into adults in around 25 days.[12] House dust mites feed on shed human skin, and have specific enzymes to digest skin scales. The enzymes are passed out in their faeces and they are highly allergenic to susceptible individuals. An adult produces 20–30 faecal pellets/day (2–3,000/lifetime) and it is these which are breathed in and firstly sensitise people. Further exposure may lead to an asthma attack. The allergens of the two main species of HDM, Dermatophagoides pteronyssinus (European house dust mite) allergen and D. farinae (American house dust mite) allergen are known as Der p I and Der f I, respectively.

Epidemiological studies provide convincing evidence that HDM exposure is not only associated with the majority of cases of childhood asthma, but also that it is causally related to the development of asthma.[13],[14] The causal relationship between allergen exposure and asthma has been recently reviewed.[15] It has been recommended that families who are sensitive to HDM should consider measures to reduce exposure from early childhood to prevent the onset of asthma.[7]

The risk level for genetically predisposed individuals to develop specific IgE to HDM allergen (i.e. sensitisation level) is 2 µg Der p I or 100 mites/g dust and the risk level for acute asthma in mite-allergic patients (i.e. exacerbation level) is 10 µg Der p I or 500 mites/g dust.[1] Frequently, mite counts and allergen levels in homes are above these risk levels.[16]

House dust mites are found in most houses. Their numbers have increased dramatically in recent years, and there is evidence that this has led to a corresponding rise in the number of asthma sufferers. The increase in mite populations has been a direct consequence of our own improved housing conditions. The widespread use of soft furnishings, wall to wall carpets, double glazing and central heating has not only improved our environment but also that of the house dust mite. Although HDM can be present in most areas of the home, they are particularly associated with mattresses and bedding because these remain at an ideal temperature and humidity throughout the year. Hence, many asthmatics notice worsening symptoms during the night or early morning when in close contact with bedding.

HDM control can relieve clinical symptoms

Clinical research trials have shown that reducing the levels of house dust mites can significantly prevent the onset of asthma in one-year-old babies who are at risk of atopy, using a dual approach focussing on foods and aeroallergens,[1]7,[18] and relieve symptoms in mite-allergic asthmatics.[19],[20],[21],[22],[23] The improvement can be sufficient to allow the levels of drug intake by patients to be significantly reduced.[19],[22] There are simple measures to reduce exposure to HDM which may lead to relief from symptoms and even help prevent onset of atopic illnesses in otherwise healthy individuals.

As highlighted in a recent review,[24] measures beside medical treatment should be implemented in a large group at risk of developing asthma, mainly composed of babies who are genetically prone to asthma. The effectiveness of dietary manipulations was questionable in this group, but environmental control, anti-mite measures in particular, was effective. However, as emphasised in another review article,[25] not all clinical trials employing anti-mite measures are successful, especially when only one method of control is used. The measures must be applied consistently for more than 6 months for proven efficacy. Furthermore, an integrated approach, employing measures to kill mites and remove allergen is required.

Anti-mite measures

Firstly, we can make our home less favourable to the mites. The best way to do this is to reduce the humidity, which is their only source of water. The number of mites declines rapidly when moisture in the air falls below 55%. Dehumidifiers and mechanical ventilation may achieve this, and simply opening windows on dry days as often as possible will help. Pillows, duvets and mattresses should be aired whenever possible, the former by hanging them outdoors and by regularly turning mattresses. Mites and allergens can be mechanically removed from the atmosphere, either by vacuuming carpets, mattresses, bedding and soft furnishings or filtering the room air with a high efficiency particulate air cleaner. Several different types of vacuum cleaners have been tested for their ability to remove HDM allergen.[26] Pillows, duvets and cushions can be easily vacuumed by placing them inside a large polythene bag and placing the nozzle of the vacuum cleaner inside the bag (make sure that the bag opening is securely sealed around the nozzle). Hard surfaces should be dusted with an anti-static or damp cloth that retains dust particles rather than a conventional cloth duster which just spreads the dust around the room and makes it become airborne. Allergen removal measures should be undertaken as often as possible, as early in the day as possible (just before bedtime should be avoided) and at least once a week, with particular attention paid to bedrooms. Physically excluding mites from an area, or removing them at source, can also be effective. Bare wooden flooring, or lino, in bedrooms is far better than fitted carpets. Enclosing mattresses and pillows with a semipermeable cover can reduce direct contact with the allergen although not necessarily controlling the mite population itself. As old mattresses and pillows can harbour huge quantities of mites, and their allergens, it is advisable to replace them with new ones wherever possible, particularly if more than 10 years old.

Secondly, we can kill the mites directly. This could be by hot washing or by dry tumble drying pillows, soft toys and bedding at temperatures >55°C. Alternatively, children's stuffed toys can be placed in bags and kept overnight in a deep freeze. Carpets can be treated with liquid nitrogen, although this must be carried out by a specialist company. Chemical control methods are also very successful at reducing or eliminating house dust mites. Some compounds claim to work by killing fungi which facilitate the feeding of mites, although there is little convincing evidence that such products alone can lead to clinical improvements.[27] Other products which act by denaturing or binding allergens are also available and may prove useful if used frequently or in combination with other measures.[28],[29],[30] Several products containing pyrethroid insecticides can kill huge numbers of mites rapidly.[31],[32] The application of these sprays and powders should be avoided by asthmatics, who may react if the chemicals are inhaled.[34] It is very important to remember that only long term control of the mites will lead to any clinical improvements because the allergen itself is relatively stable even after the mites are killed.

With these limitations in mind, my colleague Nigel Hill and myself are involved in the development of more suitable methods of control. In particular, we are examining pyrethroid impregnated bedding (mattress liners and pillow/duvet covers). Once fitted, the products slowly release the active ingredient directly into the bedding and only need changing after a year or more. Laboratory experiments, using chambers constructed to simulate mattress conditions, showed that permethrin-impregnated nets were effective in eradicating HDM populations for at least one year.[35] A field trial is underway to test the efficacy of permethrin- impregnated mattress covers against HDM and, after 2 months in use, HDM were significantly reduced, almost completely eliminated, from test beds. This effect has been maintained for 6 months, and the trial is still in progress. If long-term efficacy is established, such covers, in conjunction with allergen removal methods, may provide a simple, cheap and long-lived means of protection against HDM allergens and will offer alternative treatment for asthma sufferers which may reduce their dependence on drugs.

The way forward

Surely it is time for GPs to advise their patients, and those at risk of developing atopic conditions, to implement the simple household allergen avoidance measures outlined above. Ideally advice should be given as part of antenatal care, particularly where there is any history of atopic allergy in the family. When atopic asthma has been established, and allergy to HDM has been confirmed by a prick test, further treatments to reduce allergen levels in the homes of patients should be provided by specialist companies through the NHS. Specialist companies should only use products which have proven efficacy in reducing HDM allergen and relieving clinical symptoms. There are many products on the market aimed at people with asthma. Unfortunately, as identified by a panel of experts for the Consumers' Association's magazine Health Which?, many make misleading claims in their advertising and exploit the fears of asthma sufferers. Perhaps the reluctance of GPs to advocate anti-mite measures stems from the confusion surrounding certain products and a fear that patients will believe that they no longer require any medication at all and stop taking prescribed drugs. Regulation of products should be implemented, whereby proven efficacy in reducing HDM allergen is required before products can be released on the market. A strategy should be developed, which combines different anti-mite and allergen removal measures, in order to achieve the best possible reduction in allergen exposure. Maybe then GPs will have confidence to recommend allergen avoidance as part of their patient's asthma management. A close liaison between GP and patient will determine if or when the amount of prescribed drugs can be reduced.

References

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