Our face spiders - friends or foes?

You may be surprised (or perhaps horrified) to know that you have spiders on your face right now. In addition to the millions of bacteria, viruses and fungi that make up our skin microbiome (the community of microorganisms on our skin) we have microscopic eight-legged creatures that also make a home in our skin. In humans there are two species; Demodex folliculorum which reside in our hair follicles and Demodex brevis which are found in our sebaceous glands^[1]. They are just two of the 46,000 different species of mites that form the Arachnid family along with spiders and ticks.

Thankfully, the arachnids on your face aren't as big as this Christmas Lights Jumping Spider! Jumping spiders sometimes follow convoluted routes when hunting, even losing sight of their prey. How and why they do this, especially given the size of their brains, is also an open question. Image credit: public domain, via USGS (Flickr)

While studies suggest that we aren’t born with these creatures on our skin, but acquire them over time as a result of skin to skin contact with our mothers, how the spiders get onto us remains a fundamental mystery. Their numbers increase as we get older, but we don’t know why this is^[2,3]. To date scientists have been unable to culture Demodex long term outside the body, as they dry out very easily^[4,5]. As a result they are difficult to research and little is known about their life cycle apart from the observations of Spickett in 1961. He suggested that the mites roam the surface of our skin at night in order to breed. Females lay eggs within our hair follicles where they hatch and develop into adults and the cycle starts again^[6].

Demodex mites are a known cause of mange, a skin condition in dogs and other animals, however their precise role in causing skin disease in humans is unclear. It is generally thought that unlike scabies mites and house dust mites, Demodex normally cause no harm to their human host^[3,7]. For example while scabies mite actively burrow into our skin and cause inflammation, Demodex live out their lifecycle within our hair follicles and sebaceous glands. It has been speculated that they may even play a protective role by ingesting harmful bacteria and viruses - but this is unproven^[8].

Demodex have however been linked to inflammatory conditions affecting the skin (such as rosacea) and the eye (blepharitis) due to the increased numbers of mites present in patients with these diseases^[3,5]. It isn’t known if the mites alone are causing the disease or if they are simply a consequence of the disease. Increased mite numbers have also been reported in immunosuppressed patients such as those with cancer or HIV^[3]. In a skin condition called pityriasis folliculorum patients have higher number of mites than reported in rosacea yet less inflammation is present - suggesting the presence of mites alone is not enough to cause disease^[9].

video of Demodex mite on an eyelash

Successful treatments for rosacea and blepharitis lead to a reduction in mite numbers in most patients^[9]. However it’s not clear if these treatments are killing the mites or simply calming the inflammation caused or worsened by the mites. Rosacea skin is known to be different to healthy skin which may allow the mites to overproliferate. An effective rosacea treatment may return the skin to normal and with a normal population of mites. It’s still unknown what role exactly Demodex mites have in causing rosacea. Demodex mites from patients with rosacea have been shown to carry different bacteria to those from normal skin and bacterial proteins isolated from mites have been shown to cause skin inflammation in laboratory studies, suggesting it may be the bacteria causing the problem. These findings are supported by the fact that rosacea patients respond to antibiotics^{[10, 11]}. The long-term use of antibiotics by patients is not ideal however and can lead to the development of antibiotic resistant bacteria highlighting the need for new therapies. Several new therapies are currently in different stages of development and research is ongoing into the precise role of Demodex in both healthy and rosacea skin.

This article was written by Ellen Margaret Moran, who has just completed a postdoc researching demodex mites in the pathogenesis of rosacea. Ellen carried out her PhD in Rheumatology and has worked in medical research since completion. Ellen enjoys communicating science and tweets at @DrMoranLove

Ellen and the TWDK team wish you a very happy Halloween, and hope this article doesn't give you too many nightmares.

References
why don't all references have links?

[1] Nutting WB, Desch CE. Demodex canis: redescription and reevaluation. The Cornell veterinarian. 1978;68(2):139-49.
[2] Thoemmes MS, Fergus DJ, Urban J, Trautwein M, Dunn RR. Ubiquity and diversity of human-associated Demodex mites. PloS one. 2014;9(8):e106265. DOI: 10.1371/journal.pone.0106265
[3] Lacey N, Ni Raghallaigh S, Powell FC. Demodex mites--commensals, parasites or mutualistic organisms? Dermatology (Basel, Switzerland). 2011;222(2):128-30.
[4] Chen W, Plewig G. Human demodicosis: revisit and a proposed classification. The British journal of dermatology. 2014;170(6):1219-25. DOI: 10.1111/bjd.12850
[5] Lacey N, Kavanagh K, Tseng SC. Under the lash: Demodex mites in human diseases. The biochemist. 2009;31(4):2-6. PMCID: PMC2906820
[6] Spickett SG. Studies on Demodex folliculorum Simon (1842). I. Life history. Parasitology. 1961;51(1-2):181-92.
[7] Fischer K, Walton S. Parasitic mites of medical and veterinary importance--is there a common research agenda? International journal for parasitology. 2014;44(12):955-67. doi:10.1016/j.ijpara.2014.08.003
[8] Wolf R, Ophir J, Avigad J, Lengy J, Krakowski A. The hair follicle mites (Demodex spp.). Could they be vectors of pathogenic microorganisms? Acta dermato-venereologica. 1988;68(6):535-7. PMID:2467494
[9] Forton FM. Papulopustular rosacea, skin immunity and Demodex: pityriasis folliculorum as a missing link. Journal of the European Academy of Dermatology and Venereology : JEADV. 2012;26(1):19-28. DOI: 10.1111/j.1468-3083.2011.04310.x
[10] Murillo N, Aubert J, Raoult D. Microbiota of Demodex mites from rosacea patients and controls. Microbial pathogenesis. 2014;71-72:37-40. DOI: 10.1016/j.micpath.2014.04.002
[11] Lacey N, Delaney S, Kavanagh K, Powell FC. Mite-related bacterial antigens stimulate inflammatory cells in rosacea. The British journal of dermatology. 2007;157(3):474-81. DOI: 10.1111/j.1365-2133.2007.08028.x