House dust mite allergen avoidance in asthma

Since faecal pellets of house dust mites were identified as the principal source of allergen in house dust over 30 years ago,¹ the role of mite eradication or allergen avoidance in the management of asthmatic patients has remained controversial.²

Enthusiasts point to studies in which allergic asthmatic patients stayed for several months in hospitals or high altitude Alpine sanatoriums. In these effectively mite free environments their condition improved both symptomatically and in terms of non-specific bronchial responsiveness.³,⁴ These were, however, uncontrolled studies in which patient blindness was impossible, so not all the benefits can be attributed unequivocally to allergen avoidance. Nevertheless, that is the most plausible explanation, implying that radical reduction in mite allergen exposure may be beneficial to at least some asthmatics.

Sceptics emphasise the practical challenge of achieving sustained and substantial reductions in mite allergen exposure in the home, particularly in regions, including Britain, where mite infestation is widespread. In theory, personal exposure may be reduced by eradicating mites from the environment or by preventing their faecal pellets becoming airborne.²,⁵ In practice, each method has important limitations. Killing live mites does not remove the offending allergens, which may persist for months or even years in reservoirs of house dust. Reinfestation is likely to occur unless acaricides are applied regularly or the humidity of the home is kept low throughout the year. On the other hand, physical barriers to allergen dispersion can be applied to bedding—though not so readily to other mite habitats such as carpets, curtains, furnishings, and soft toys.

Many methods of domestic mite eradication or allergen avoidance have been tested in small intervention studies, and these have been the subject of two recent overviews. The first, a narrative review, suggested that there was some evidence of benefit from interventions which reduced allergen exposure.⁵ The second, a quantitative meta-analysis published in this week’s issue,⁶ concludes that “current chemical and physical methods aimed at reducing exposure to house dust mite allergens seem to be ineffective.” The most recent British guidelines on asthma management state that “in those with established asthma avoidance of house dust mite allergen by means of bed covers has proven efficacy in the short term. Other methods such as acaricides remain unproven.”⁷ Whom should we believe?

Part of the confusion is resolved by distinguishing clearly between two measures of effectiveness: reduction in allergen exposure (efficacy) and impact on symptomatic illness (clinical effectiveness). It is not surprising that interventions which lack efficacy are clinically ineffective. However, this does not exclude a potential benefit to patients from measures which actually achieve substantial reductions in personal allergen exposure. Such measures include impermeable (plastic) or semipermeable (microporous) mattress and bedding covers, which dramatically reduce allergen levels on the bedding surface^8–10 and thus protect at least against nocturnal allergen exposure. Recently marketed microporous fabrics are much more acceptable to patients than vapour impermeable plastic covers, but they have yet to be fully evaluated for clinical effectiveness. Should they be recommended to patients, based on the evidence currently available?

The meta-analysis by Gøtzsche et al identified only five trials, involving 123 adults and 86 children, in which there was proved reduction in mite allergen exposure in the intervention group.⁶ Only four of these trials reported changes in morning peak expiratory flow rate, the principal outcome selected for meta-analysis. The results would be consistent either with no clinical benefit or an increase in average peak flow of up to about 45 l/min—a small but potentially useful improvement. This meta-analytic approach may be conservative in that other outcome measures, such as improvements in symptoms or bronchial responsiveness, are not analysed for this small subgroup of trials.

Woodcock and colleagues considered six studies to have used an efficacious intervention.⁵ They point to some evidence of clinical benefit in all of these trials, although the outcome measures differed in each. This narrative evaluation may overestimate effectiveness because the findings reported from a wide range of trial outcomes will tend to be those which were statistically significant, particularly in favour of the intervention. Thus, neither review offers conclusive evidence to guide patient choices.

Most methods of mite eradication or allergen avoidance tested in published trials cannot be recommended to patients simply because they do not materially reduce mite allergen exposure. There are too few data from trials where allergen exposure was substantially reduced to draw any firm conclusion about the potential clinical benefits of newer, more efficacious methods. This position of uncertainty can be resolved only by large trials, preferably double blind and placebo controlled, in which interventions known to reduce allergen exposure are tested in large representative samples of mite sensitised asthmatic patients. At least one such trial has started recently, using encasement of mattress and bedding by semipermeable covers. When its findings are reported we may be able to provide more definite advice to patients and their doctors.

Papers p 1105