Asthma

A key cause of asthma – the house dust mite

While asthma is at base an allergic response to environmental triggers, and many factors can contribute to symptoms, it is known that specific allergens trigger both sensitization and acute attacks. The house dust mite (HDM) is recognised as the source of the major allergens responsible for asthma.

The actual cause is proteins present in the faeces of the dust mite which stimulate an allergic response. Between 50% and 90% of asthmatics who react to airborne material are sensitive to this material, and in one British study 10% of the general population reacted to mite allergens.

House dust mites are small, with adults approximately 0.3mm in length. One gram of dust can contain 500 mites, while a mattress can hold more than two million. The amount of mite material present increases with age. One tenth of the weight of a six-year old pillow can consist of mites and mite debris. In a carpet, there can be between 1,000 and 10,000 mites per square metre. In the three months of her life, a female mite will lay 25-100 eggs.

An average mite will produce 20 feces each day of its life: twice its own body weight. These adhere to fibres and materials in the environment. Both mites and feces are very difficult to remove, and any mechanical attempt to remove the feces will break many of them up, with the resulting fine, allergenic dust distributed by air movement.

Despite the many areas affected, two mites are responsible for the majority of the problem:

Mites are Arachnids, closely related to spiders and more distantly related to insects.

As well as causing problems in adults, exposure to dust mite material early in life is also responsible for sensitising children to mite allergen. Such sensitisation is strongly linked to the future development of asthma.

Since dust mites require humidity above 50%, some regions are not as affected, including much of the western United States. But the main areas which are affected include the eastern half of the United States (and major western coastal cities), populous areas of Canada, almost all of western Europe, India, Japan, Korea, and coastal areas of South America, Australia and South Africa.

The total population of these areas is in excess of two billion people. Almost two hundred million Americans live in areas severely affected by house dust mite infestation. The total costs of asthma within the United States and Europe are estimated as in excess of $100 billion per annum.

In addition, one study showed a similar frequency of sensitisation to house dust allergens among asthmatics in Kuwait, an extremely dry country. According to recent data, Kuwait actually shows a far higher rate of asthma deaths than does Western Europe or the United States. Thus, mites are present in other areas at significant levels, apparently due to human modifications of the indoor environment. These include carpets, soft furnishings, humidifiers and air conditioning. In summary, when humans make their environment comfortable for themselves, they make it comfortable for dust mites.

Current control methods

Control of mites by any of the currently available methods is of limited efficacy in reducing numbers overall and their use in a co-ordinated fashion can be very demanding.

Current methods for controlling mites and limiting exposure to allergen are as follows:

Washing infested material at temperatures above 55°C will kill mites, but is impractical for many of the favoured locations for mites, such as carpets, mattresses, and soft furnishings. In no case is there any barrier to immediate recolonization from nearby sources of mites.

Steam cleaning can treat mattresses, but can be destructive to the materials used and is disruptive since it is best carried out immediately after the bed is vacated to catch the highest number of mites near the surface of the mattress. In addition, there is no residual effect so that immediate recolonisation from nearby sources of mites (such as the base of the treated mattress) is both possible and likely.

Rigorous vacuum cleaning is often used in an attempt to reduce the amount of mite faeces present, but is limited to surfaces and even there is generally ineffective due to the adherent nature of the faeces, reducing the amount of allergen present by only 5-10 % and having very little effect on live mites. In addition, vacuuming requires the use of machines equipped with HEPA filters if the allergenic dust is not to be released, and machines with such filters can be expensive to buy and to maintain. They also incur regular costs for filter replacement if they are to remain effective. Even if such a machine is used, the physical action of cleaning and emptying may stir up allergen-loaded dust and so increase exposure.

Filtering or dehumidifying room air is possible, but in a standard domestic setting the reintroduction of allergens or humidity from outside is both frequent and almost impossible to prevent.

Barrier methods that block the access of mites to mattresses and bedding are in common use, but are also of limited value. No such method can be used on carpets and other soft furnishings, which are a major source of exposure to allergens. In addition, such methods can be expensive. Despite this they are often cited as the best available option and are widely used.

Denaturing solutions intended to inactivate allergens in carpets or on surfaces are also available. However, they require re-application at frequent intervals and are only able to treat accessible surfaces.

Acaricides are chemical agents intended to kill the mites themselves, including both organophosphates and carbamates. However, use of such pesticides can be problematical. One major product was recalled after the U.S. Environmental Protection Agency (EPA) documented more than 400 cases of adverse side effects among users. Other compounds have been associated with adverse environmental effects. Unsurprisingly, opinion is often against the use of such chemical pesticides, particularly in the domestic environment. In addition, work with other mites, notably storage mites in grain, has shown that resistance to chemical pesticides can develop rapidly and can become total at acceptable dose levels.

Chemical acaricides usually cannot be used effectively to treat furniture or other padded items, which are a major source of dust mites, leaving anoxic fumigation (requiring removal of the item to a specialist facility at a high cost) as the only available option. Immediate recolonization can of course occur following such treatment. The limitation of individual treatments to specific items or areas can also allow rapid recolonization from untreated areas once treatment is completed.

No single current treatment can be used for all infested items, and combined use can be complex, demanding, and expensive. A 2008 Cochrane systematic review of 56 trials of such methods concluded that “Chemical and physical methods aimed at reducing exposure to HDM allergens cannot be recommended”.

Thus, the current situation is one of unmet need.

A biological acaricide

A reduction in mite populations is regarded as beneficial both to established asthmatics and in the prevention of sensitisation in children and other potential asthmatics. A reduction in mite levels and, thus, in allergen exposure is thus predicted to be beneficial and this underlies the multi-billion dollar spend on existing approaches.

However, mites infest a wide range of domestic habitats, and there is no broadly effective control option for house dust mite infestations currently available

But there is another approach that has the ability to exert long-lasting control across the range of dust mite habitats, spreading out from limited initial applications. This technology is biological control.

As well as its unique capabilities, the “green” and ecologically friendly nature of biological control has the potential to provide a major positive for a biological acaricide. It should be noted that regulators of organic products have been receptive towards such approaches in food hygiene, while the US Environmental Protection Agency (EPA) website has stated that “Since biopesticides tend to pose fewer risks than conventional pesticides, EPA generally requires much less data to register a biopesticide”. In this case, a biopesticide which has the potential to being direct benefits to human health.