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House dust mites -
Evidence against mite control for asthma

October '98.
British Medical Journal suggests house dust mite control does not help.
The British Medical Journal has published a statistical analysis of objective trials of house dust mite control and concluded that there was no evidence of benefit in patients whose asthma was attributed to house dust mites. Before embarking on a lot of expense and trouble you will surely want to know what this report has to say and whether it should be regarded as reliable. This web page discusses this.

Introduction

"Reducing exposure to house dust mite allergen does not improve asthma"

says the British Medical Journal of 28th of October, 1998. But the accompanying article on which this claim is based proves no such thing. The 'BMJ' has fallen for a logical fallacy. What the article does prove is that trials in which mite control was compared with deliberately ineffective measures (controlled trials) demonstrated no overall benefit. This is not the same thing. The fault may lie in the clinical trials and better trials might show a different answer. In fact the authors of the article say exactly this. Furthermore, there is very good evidence that reducing mite exposure does improve asthma; it just doesn't come from the sort of trials which the authors looked at. Nevertheless, the conclusions of the article are important and we should take serious notice of them. It is perfectly true that 'controlled' trials of a treatment are, under most conditions, much more trustworthy than uncontrolled ones. There is an important practical message: in practice the things people have been able to do about mites in their own homes in the context of scientific trials just didn't work. This probably means that what you and I are likely to be able to do in our everyday lives to control mites also won't work, at least in terms of things we can easily measure over the timescale (generally up to a year) of the clinical trials. The research which does show asthma improvement from reducing exposure to house dust mite allergen involved things like moving to another country or living in a clean hospital room, not just changing things in the patient's own house.

We need to rethink our practical advice to avoid inflicting useless expense and trouble on people. We do not, in my opinion, need to abandon the search for ways of achieving worthwhile results in a practical way. Nor do we need to tell people that all attempts at mite control are necessarily a waste of time, though you should understand the limitations and the uncertainties. Go ahead only if you want to in the light of proper scepticism.

The main article

The main article in the British Medical Journal sums up the results of all 23 controlled trials of mite control which the authors could find. No controlled trial showed large benefit, and on balance there was no benefit. The conclusion of the article is that "Current chemical and physical methods aimed at reducing exposure to allergens from house dust mites seem to be ineffective and cannot be recommended as prophylactic treatment for asthma patients sensitive to mites." There is an editorial article about the findings.

The authors do not give up hope altogether that future research may show better results. They state: "We suggest that future studies should be much larger and more rigorous than those analysed here and should use methods to control or eradicate mites other than those used so far."

Where does this leave the detailed advice which we have been giving for years, in common with many other experts in allergy? Should you forget about the whole business?

On the one hand, the article does make an extremely serious point, and is undoubtedly correct as far as it goes. No objective and serious advocate of mite control denies that success is difficult at best. On the other hand it is worth asking whether either the article or the studies on which it is based contain such serious flaws that we should still take mite control seriously. A related question is whether it's still worth your while to attempt it.

One of the flaws in the article is the fact that it mixes up trials which the top experts have long regarded as useless (it has been well recognised that anti-mite chemicals alone have not helped asthma in such trials) with trials which did seem to make more sense. However, the authors dealt with this criticism by subdividing the types of trial, and showed that the others did not show benefit either.

Almost all the serious evidence that elimination of mites from the environment helps asthma comes from research without an untreated ('control') group for comparison. Such studies were automatically excluded from analysis in the article. Such 'uncontrolled' tests are rightly regarded as far less reliable than comparisons with a control group, but this does not mean that they necessarily give wrong results. In fact there is quite a mass of uncontrolled data in favour of mite avoidance, and it would be a foolhardy person who would dismiss it all. For example it has been known since the early part of the 20th century that sending Dutch children with house dust mite sensitive asthma to Davos in Switzerland very often improved them dramatically whilst they were there. The Dutch convincingly linked this improvement to lower house dust mite numbers in the drier atmosphere high in the Alps. A mass of high quality data from similar Alpine resorts in other parts of Europe supports the conclusion. But these observations are not 'controlled' in the sense that they were collected systematically in comparison to observations on children not sent to the mountains. So this information could not be included in the article, which was a summary of controlled trials.

Platts-Mills and others kept a group of house dust mite sensitive asthma patients in a nearly mite-free hospital room for a number of weeks and consistently showed dramatic improvement, with careful measurements to back this up. This experiment was also 'uncontrolled' and therefore could not be included in the British Medical Journal article.

There is also a mass of evidence linking the presence of mites to the development of asthma, but again this does not come in the form of controlled clinical trials.

In the case of new medicines, arguing for uncontrolled evidence would generally be ridiculous. Controlled trials are the gold standard by which medicines are judged, and generally if controlled trials of a drug do not show benefit, then there is either no benefit, or so little that it does not matter.

One can make a case for the situation being different with mite control. Good large trials of mite control are vastly more difficult than those of a medicine. In addition, the huge amount of funding and expertise which good trials need are available from the manufacturers in the case of new drugs, but comparable funds are practically never made available for tests of devices or lifestyle changes. This is surely the reason why the total number of patients considered in the British Medical Journal article is pitifully small compared to numbers tested with any common new drug. In fact it can easily be argued that the numbers are totally insufficient by normal drug test standards. Another reason why trials of lifestyle changes are more difficult than trials of drugs is that it is difficult to ensure that people really do what they were intended to do.

So it could be argued that the article discussed exclusively a type of trial which is almost impossible to do well and has therefore not been done, ignoring compelling evidence from other sources.

I would not want to take a defence of mite control too far in this direction. The fact that real people in real comparisons could not achieve asthma improvement even when they were being supervised and encouraged by research workers is bad news for you if you want to try this with less support in your own home. The upshot may well be, and probably is, that worthwhile asthma improvement does occur under some extreme conditions which cut house dust mite exposure drastically (living on a mountain, or admission to hospital) but that the necessary drastic reductions in mites are just not achievable for ordinary people in our ordinary climate and living conditions.

Not a nihilist

I'm still not a nihilist about mite control. Indeed the authors of the article are not, as their conclusion about the need for more (but larger, longer and better) trials shows. I agree with that conclusion. The fact is that wallowing in more mite dust than we can easily help seems a very bad idea. Medication as we know it is not the whole answer either, and probably never will be. Immunological preventive treatments or changes in lifestyle which achieve immunological changes may be another matter in the future. Meanwhile, if I had housedust mite sensitive asthma, I would do what I could to minimise my exposure without doing myself serious financial or social harm. I would do so with an open mind and the full realisation that what is achievable in practice may at the moment turn out to be as much of a waste of time as the British Medical Journal article, with its selective view of the evidence, seems to show.

Look up the original articles on the web:

House dust mite control measures in the management of asthma: meta-analysis. Gøtzsche, Hammarquist, Burr. British Medical Journal 1998;317:1105-1110 (24 October)

House dust mite allergen avoidance in asthma (editorial article). Strachan. British Medical Journal 1998;317:1096-1097 (24 October)