The past several decades have seen an alarming increase in the incidence of allergic disorders including allergic rhinitis and allergic asthma. This is particularly true in the western world and in developed nations. The exact reasons for these increases have not been entirely elucidated and cannot be explained solely on the basis of genetic factors which would typically require much longer intervals of time to have significant effects. The Hygiene Hypothesis as first proposed by David P. Strachan in 1989 (ref. 1) is one attempt to explain these increases. This hypothesis theorizes that early exposure to certain environmental and infectious agents (parasites, symbiotic organisms, multiple siblings and farm animals to name a few) supports the development of a more Th-1 rather than Th-2 dominated immune response. This, in turn, is thought to protect against the development of allergic disorders such as allergic rhinitis and allergic asthma. The hygiene hypothesis has recently come under greater scrutiny and more studies in this area are needed. (ref. 2)

There exists a considerable body of literature regarding the issue of patient compliance with asthma, and to a lesser degree, allergy medications. However, little is known about patient adherence to environmental control measures and, furthermore, whether such adherence results in improved quality of life through better symptom control. Although classically taught as the initial modality in the triad approach to the management of allergic patients (environmental control, pharmacotherapy, antigen specific immunotherapy) many of these teachings seem to derive more from common sense and intuition rather than sound clinical science. With the possible exception of cat dander, it has been thought that the potential for the greatest benefits to be derived from antigen avoidance were those directed at house dust mites (HDM). Both antigenic load (millions of HDM per bed each producing 10 fecal balls per day) and prolonged direct exposure (average of 6-8 hours per night) would seem to support this view. Furthermore in most countries including the United States and the United Kingdom HDM are the most common trigger for perennial allergic rhinitis.

Dust mites (D. farinae and D. pteronyssinus) are members of the arachnid family of insects (ref. 3). As adults they are roughly 250-300 � in length. Dust mites are blind and unable to drink therefore they must absorb water from the air around them to survive. As a result they require greater than 50% relative humidity to survive. They can only live at altitudes less than 3500 feet and within a temperature range of 65-84�. Their diet consists of organic human debris such as hair and exfoliated skin cells and, as such, find homes in bedding, pillows, linens, carpeting, drapery, as well as stuffed and live animals. Their fecal balls contain the antigenic proteins that humans are capable of mounting an IgE -mediated allergic response to if genetically predisposed.

Meta-analysis is a quantitative statistical method applied to separate but similar studies of different and usually independent researchers. Data is then pooled from various studies and tested for statistical significance often allowing for greater statistical power than that of individual studies. There are now a total of 4 such meta-analyses and updates from the Cochrane Library examining the issue of house dust mite avoidance measures for perennial allergic rhinitis (2001, 2003, 2007, 2010). These reviews have surveyed a total of 23 international electronic data bases providing 327 potentially relevant papers. The most recent update (ref. 4) reported in the European Journal of Allergy and Clinical Immunology ultimately evaluated 9 clinical trials involving 501 patients. Various environmental control measures for house dust mites were examined including: house dust mite impermeable bed covers (n=4), acaracides (n=2), high efficiency particulate air filters (n=2), and the use of acaracides and impermeable bedding in isolation and combination (n=1).

The authors concluded that trials to date have been of relatively poor methodological quality. In turn, this makes for difficulty in making definitive recommendations regarding the role, if any, of house dust mite avoidance measures in the management of the house dust mite sensitive patient. Seven of nine clinical trials demonstrated that the interventions studied (acaracides most notably) resulted in a significant reduction in house dust mite load, but this did not consistently translate into improved patient symptoms scores. Nonetheless, acaracides still appear to be the most promising form of mono-intervention and further pragmatic, randomized, controlled studies are warranted. The authors further concluded that the use of impermeable bedding as an isolated intervention was unlikely to offer significant clinical benefit. High efficiency particulate air filters, as a stand alone measure, were also shown not to have significant clinical effect.

So what is it, then, that we should be telling our house dust mite allergic patients about environmental control measures? Perhaps words from the Hippocratic Oath serve to advise us best:

"I will prescribe regimens for the good of my patients according to my ability and my judgement and never do harm to anyone."

Mark J. Hoy , M.D.