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Why do demodex mites like human skin?


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Image of Demodex Folliculorum courtesy of National Geographic - by Darlyne A. Murawski
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Basically everyone has demodex mites and it has been thought that the mites have some sort of symbiotic  or commensal relationship with humans, i.e., the mites eat sebum which helps the mites and helps the humans keep sebum stasis. One report states, "....Demodex mites were originally perceived to be commensals, having a symbiotic relationship with the human host." - See Jarmuda et al published in the Journal of Medical Microbiology (second article mentioned in this post). While this same report says that 'most human populations' have NOT been sampled for demodex mites the general belief is that demodex are common throughout humanity and pose no problem as a pathogen except in the case of demodectic rosacea as far as known. 
A Russian study on the mites says, "Demodex folliculorum shows signs of parasitism, while Demodex folliculorum brevis is a saprophyte."  It is comparable to bacteria which humans have a relationship with, there is good bacteria and bad bacteria. The probiotic bacteria and the pathogen bacteria. The demodex mites usually pose no problem with the vast majority of humans since they are possibly, or probably, on everyone. Why they become more numerous seems to be of more importance. What are the numbers showing?
For some unknown reason the mites are in higher density in rosacea patients. We don't know if the rosacea cause this increase in mites or does the increase in mites cause the rosacea, the old chicken or egg conundrum? There is evidence that reducing the mite density count improves rosacea.  It is clear that the mites like human skin since they consume sebum. One report states, "Demodex mites feed on the sebum and cellular proteins that are obtained by protease containing the salivary enzymes of the mites. The lipase enzymes of Demodex are also thought to play a role in digesting bacteria or other microorganisms in addition to the digestion of lipid material." [1]

Dr. Frank C. Powell who wrote the book on rosacea said at the 72nd annual meeting of the American Academy of Dermatology in Denver which was quoted in the NRS Rosacea Review (Spring 2014) the following and is related to this subject:

“The presence of Demodex is likely to confer some sort of benefit on us, because human physiology is such that we wouldn’t tolerate something like this unless there was something to be gained for us,” he said. Whatever benefit that might be, in rosacea patients something causes the mites to proliferate, possibly triggering an inflammatory response. Large quantities of mites have been found in biopsies of rosacea papules and pustules, leading Dr. Powell to wonder whether the papules and pustules might not be 'gravestones to dead Demodex.' ” [2]

Maybe the increase of sugar/carbohydrate in the diet increases sebum which in turn increases the mite population, and voila, the inflammation of rosacea
Not all rosacea is demodectic. GUT Rosacea is a different variant, but may be connected or associated.  The list of systemic comorbidities with rosacea keeps growing. The gut microbiome is obviously connected with skin microbiome (see post on this).  
Could it be that H Pylori has a symbiotic relationship with humans and at times runs amuck and causes issues? Could demodex have this same symbiotic relationship in principle with humans that sometimes runs amuck? Or is demodex a parasite? [2]
"Our results suggest close interactions between the mite, sebaceous gland size and function, and subtle variations of immune status." [3]

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End Notes

[1] An Bras Dermatol. 2020 Mar-Apr; 95(2): 187–193.
Demodex folliculorum infestations in common facial dermatoses: acne vulgaris, rosacea, seborrheic dermatitis
Ezgi Aktaş Karabay and Aslı Aksu Çerman
[2] "Parasitism is a non-mutual symbiotic relationship between species, where one species, the parasite, benefits at the expense of the other, the host." Symbiosis is a controversial subject, but it is generally accepted that the definition is "the living together of unlike organisms." Wikipedia

The classic example is Helicobacter Pylori which has been reported to be in 50% of the human population. Why the other half the human population doesn't have H Pylori is an intriguing question. And this is interesting, "Over 80% of individuals infected with the bacterium are asymptomatic." *

*Am J Clin Dermatol. 2002;3(4):273-82.
Helicobacter pylori infection in skin diseases: a critical appraisal.
Wedi B, Kapp A.
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