It’s getting to be summer time in the Northern Hemisphere and I’m starting to see more creepy-crawlies outside, inside and attacking my personal space. Spiders, mosquitoes and cockroaches are becoming an increasingly common, unpleasant sight. Finding these beasties in random corners of my house and ruthlessly killing them had me thinking about the human-insect relationship, in particular the special one we have with ectoparasites. Ectoparasites depend upon mammals for their survival but there are several that rely on humans specifically and have co-evolved over hundreds of years to inhabit our bodies. Examples include the sucking lice (body, head and crab), bed bugs, fleas and mites.

Naturally, I went to the internet to find pictures and to have an up-lifting, skin-crawling session. There’s one notable ectoparasite that induced the greatest chills and hit all of the gross-out buttons – microscopic size, alien-like appearance, infestation fears and, of course, there’s not much you can do about them living within you. Allow me to introduce the follicle mites Demodex folliculorum and Demodex brevis!

Scanning electron micrograph of Demodex folliculorum mites (in blue) in skin follicles. Image: Science Photo Library. Click for source.

Human beings are the one and only host of this ubiquitous mite (a). In fact, these two mites are considered to be the most common ectoparasite of humans (d). Women tend to be the main beneficiary of these minute bugs, with a male:female ratio of 2:5 (c). Sorry, ladies. The rate of infestation seems to be correlated with age, with 84% of people at age 60 harboring mites and increasing to 100% in those 70 years and older (e). Whether those that are immunocompromised are more susceptible to higher infestation rates is unknown, though some studies indicate that AIDs and leukemia patients may be more prone to greater than average numbers (c).

The mites may be found in the scalp, face and upper chest area, with D. folliculorum exhibiting a predilection for the hair follicles and D. brevis for the sebaceous ducts and meibomian glands at the rim of the eyelids (the sebaceous ducts transfer the waxy sebum that lubricates the skin and hair from the sebum glands; the meibonmian glands are a special type of such gland) (b)(c). D. folliculorum are a communal bunch, tending to congregate in the follicle area of the hair or eyelashes with their posterior ends protruding from the follicular pores. D. brevis, on the other hand, tend to be more solitary and will occupy the sebaceous glands singly (d). Both species are tiny, less than 0.4 mm, with elongated, clear bodies and four pairs of stout legs. D. brevis is usually a tad shorter, ~ 0.1 mm, than D. folliculorum. They both have ridged scales along their cephalothorax and sharp, piercing teeth (d).

Short-lived creatures, a mite’s life cycle from egg to larva to adult lasts from 14-18 days. Adults emerge from the follicles and ducts to reproduce at the surface of the skin where females will then deposit eggs in the sebaceous glands. Larva will mature via two nymphal stages in the glands until entering the follicles and ducts as adults to begin the cycle anew (d). It is hypothesized that both species of mites feed upon sebum as a primary food source but may also munch on follicular and glandular epithelia. They are obligate ectoparasites, incapble of living outside their human host.

Scanning electron micrograph of two D. folliculorum mites covered in cellular debris. Their ridged cephalothorax, four pairs of legs and sharp mouth-parts may be seen. Image: Science Photo Library. Click for source.

So what do Demodex mites do in our skin besides frolicking in our follicles and consuming cellular debris? For most people, mites live harmlessly in the skin as a result of either down-regulating host immunity or simply dodging host immune defenses (f). There is vociferous debate in the dermatology crowd as to whether or not they are the causative agents of such skin diseases as rosacea and blepharitis (inflammation of the eyelids). However, some studies have indicated that greater than average mite density, greater than five mites per cm2, certainly do play a role in these two diseases for some patients (d).

Researchers have suggested that blockage of the hair follicles and sebaceous ducts by mites may result in epithelial hyperplasia, elicit a phagocytic, granulomatous reaction or bring about an inflammatory response due to their waste products (d). Due to the fact that treatment with certain antibiotics can reduce the severity of rosacea strongly suggests a microbial component to mite-related diseases. Indeed, in 2007, researchers isolated from D. folliculorum a bacterium Bacillus oleronium that provoked inflammatory responses in 73% of rosacea patients but only 29% of controls (f). These results suggest that patients with rosacea were sensitized to the bacteria and may be immunologically sensitive to the mites, bacteria or both (f).

Two antigenic proteins found on the bacterium’s cell surface in particular appeared to be responsible for the inflammatory response by stimulating peripheral blood mononuclear cell proliferation; one 83 kDa protein showed similarity with heat-shock proteins while the other 62 kDa protein shared amino acid sequence homology with a protease enzyme found to be involved signal transduction as well as carbohydrate metabolism (f). Stronger proof of the pathogenic role of B. oleroniusin rosacea may also be found in the sensitivity of the bacterium to many antibiotics proven to be effective in the treatment of rosacea, specifically tetracycline, doxycycline and minocycline (f).

Scanning electron micrograph of the posteriors of several D. folliculorum mites (in green) extending from a follicle containing a hair (in yellow). Image: Science Photo Library. Click for source.

Closely related species in the Demodexgenus can cause mange in dogs and other mammals, and the reasoning that the two mite species that call humans “home” can cause disease as well may not be so farfetched. As such, more conclusive investigation into the role of Demodex in rosacea and blepharitis is needed. The latest research into B. oleronius and it’s immunoreactive antigens seems promising, especially upon consideration of the role of the bacterium Wolbachia in the inflammatory response of filariaisis. Bacteria that reside within medically important arthropods and nematodes are increasingly being seen as collaborative partners in pathology and B. oleronius seems to fit this pattern. Research into the function of B. oleronius in the mite’s biology should also be a priority.

For those of us content with our long-term, eight-legged guests, let us be thankful we don’t feel them and see them. And let this be a lesson to those who go image hunting on Google for ectoparasites.

Note: On April 30, 2012, the sentence “Women tend to be the greatest harbingers of these minute bugs” was changed to “Women tend to be the main beneficiary of these minute bugs”; the former incorrectly uses the word “harbinger”. Sorry for the mix up!

References

a) Kligman AM & Christensen MS. (2011) Demodex folliculorum: Requirements for Understanding Its Role in Human Skin Disease. Journal of Investigative Dermatology. 131: 8–10 b) Despommier, D, Gwadz RW, Hotez PJ and Knirsch CA. Parasitic Diseases. 5th ed. New York: Apple Trees Production, LLC. 2006 c) Hsu CK, Hsu MM, Lee JY. (2009) Demodicosis: a clinicopathological study. J Am Acad Dermatol. 60(3): 453-62 d) Lacey N, Kavanagh K, Tseng SC. (2009) Under the lash: Demodex mites in human diseases. Biochem (Lond). 31(4): 2-6 e) Liua J, Sheha H, & Tsenga SCG. (2010) Pathogenic role of Demodex mites in blepharitis. Curr Opin Allergy Clin Immunol. 10(5): 505–510. f) Lacey N, Delaney S, Kavanagh K, Powell FC. (2007) Mite-related bacterial antigens stimulate inflammatory cells in rosacea. Br J Dermatol. 157(3): 474-81

Lacey, N., Delaney, S., Kavanagh, K., & Powell, F. (2007). Mite-related bacterial antigens stimulate inflammatory cells in rosacea British Journal of Dermatology, 157 (3), 474-481 DOI: 10.1111/j.1365-2133.2007.08028.x