A new hypothesis
THIS IS A HIGH POWERED ELECTRON MICROGRAPH OF A DEMODEX MITE
It is a little known, somewhat uncomfortable fact that Demodex mites live on all humans (and animals for that matter). It's simply life - we all eventually catch them at a certain young age as they are so easily spread (touch, pillows, towels).
They were first discovered 150 years ago (in ear wax), but were thought not to cause disease (ie human commensals, akin to gut bacteria). Occasionally, a doctor (eg Coston see below for link) would wonder about demodex causing blepharitis, but the established view squashed any such theory until it was all but forgotten.
Well, over the last 10 years scientific interest in demodex has been growing. I first came across this 'unknown knowledge' around 2010.
So what do we know about these microscopic critters?
They are ubiquitous, have 65 species, and are related to arachnids. They are small at about 0.3mm length, hate light (they come out at night) and can walk at around 10mm/hr. They eat skin cells & sebum but have no excretory system. They die after a short 21 day life cycle, during which reproduction obviously occurs.
They are though to carry a bacteria (Bacillus oleronius)
or fungi in their gut.
It might be that some people are sensitive to the debris dead mites leave behind deep in the oil glands of the head and neck areas; either that or simply a case of harboring too great a population perhaps.
The pathogenicity of demodex mites has been very controversial in the literature. A Chinese meta-analysis in 2012
found 11 out of the total 13 studies support a significant association (OR 4.89, CI 3.00-7.97). Disease tends to be observed where there is: high parasitic load density index 12.7 vs 5.1; increased density ~10 mites /cm2; > 6 mites per 16 lashes +/- 4 per lash; or
an immune reaction B.oleronius.
Though difficult to prove, I do personally believe demodex are the cause of blepharitis and rosacea. This is based purely on my own experience of treatment response. I do not bother to test for mites anymore but take an empirical approach. With a total anti-demodex treatment strategy I have seen amazing results: mild and moderate cases can be cured within around 6 months; though severe cases seem to take longer, there tends to be significant improvement.