Monday, 2 March 2009

Aspergillus, Asthma and the House Dust Mite

The house dust miteHouse dust mites are thought to be the leading cause of asthma in the home. They excrete an allergen in their faeces which is a major trigger for asthma and also contributes to hay fever and atopic dermatitis. They may look fearsome but they are tiny (half a millimetre in length) and otherwise quite harmless.

Dust mites have a particularly intimate relationship with ourselves as they feed on the flakes of skin we shed every day. A human being sheds more than one gram of skin cells per day (up to half a kilo per year!!) so there is plenty of food for them in pretty much every household. Half a kilo is enough to feed a million mites!

Beds are a favourite mite home as they prefer warm moist places. This is also a time when we can be at our most vulnerable to ingesting the allergen via breathing them in as we lie on contaminated pillows and bedlinen - there is some suggestion that we eat a few every night too! Once ingested allergy can slowly build up in some people, and of course for asthmatic people it can act as a trigger for an attack.
The dust mite allergen is easily removed by washing so it is clear that regular washing of the bed linen will keep allergies at bay.

Skin cells are pretty indigestible so it is interesting to note that mites seem to rely on Aspergillus repens to turn them into a more nutritious meal. A. repens is commonly found in house dust and uses its highly efficient digestive enzymes to break down stubborn material such as skin cells prior to absorbing the nutrients - feeding on dead matter is what Aspergillus has evolved to do over millions of years.

The mites live alongside the fungus and eat the partially digested skin cells, thus making it much easier for them to eat and taking advantage of the much higher nutritional content.

This relationship begs a question about which came first, the mite or the fungus! These observations suggest that the mite would do less well if the fungus was not present. There is evidence that mites do less well in drier atmospheres such as those found at higher altitudes, but then the fungus would also struggle to thrive as it is dependent on moisture to grow - perhaps that is part of the reason the mites cannot survive?

Perhaps antifungal bed linen would be a good idea alongside frequent cleaning of the bed (patent pending)!

Moulds may make a larger contribution to asthma than simply as allergen providers - we know that moulds also cause significant allergies on their own but it looks like it is pretty difficult to separate the importance of mould from that of how it supports the life of the mite. Controlling both the fungus and the mites might be a helpful strategy for improving levels of asthma in the home.

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