Wednesday, 3 September 2014

Demodex-traordinary

An eyelash mite
I had the pleasure of appearing on Radio Scotland yesterday. No, not to discuss the Independence vote, but the matter of eyelash mites.

When I wrote The Universe Inside You, which uses the human body as a starting point for exploring all kinds of science from the nature of light to evolution, I just had to include (with a title like that) the veritable zoo of creatures that call our bodies home. Of course I explored the bacteria, which, with ten times as many bacterial cells in the body than human, are pretty impressive. But I also included Demodex, the eyelash mite.

These tiny little arachnids - typically 1/4 to 1/3 of a millimetre in length - feed on sloughed skin and sebaceous oil, in effect clean-up scavengers. They are transparent and pretty well impossible to see, mostly living at the base of eyelashes and eyebrow hair. What I said in UiY is that it was thought that around half of adults have them, but the reason they had become news, featured in national newspapers and on Radio Scotland, was that a study had shown that all adults had them. (Or at least, that's how it was interpreted. More on this in a moment.)

There was some interesting psychology as to why this change made them news. I suspect it is because it went from feeling like something like head lice that other people have (until there's an outbreak at your children's school) to something you have.

In fact the study is both more interesting and limited that the reporting suggested. The PLOS One paper does not actually say that mites were discovered on 100% of adults - in fact they were only spotted on 14% of adults, as it's hard to do. But what the researchers did was to take a sample of sebum and search for Demodex DNA. They discovered it on 100% of adults over 18 and 70% of eighteen-year-olds. Admittedly this isn't a perfect determinant, but as the paper puts it 'Though it is possible Demodex 16S rDNA could be found on the face of an individual without mites, the likelihood that we detect such transferred DNA in our limited sampling area would be low.'

So an interesting development. One of the conclusions was 'The diversity of D. brevis 18S rDNA found on individual humans suggests that not only do all adult humans have Demodex mites but that colonization is likely to occur more than once.' This is the interpretation that I'm a little worried about. The study is based on DNA testing on 19 adults, all from Raleigh NC. I'm not convinced that this provides sufficient data to make the the sweeping statement that all adult humans have Demodex mites - which then led to the news flurry. It may well be true, but this seems a very small sample to build that conclusion on - though its clear that the mites are significantly more prevalent than previously expected.

A bit of fun, though. Got itchy eyebrows? I thought so.

Image "Haarbalgmilbe". Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons


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