Demodex is a genus of tiny parasitic mites that live in or near hair follicles of mammals. Around 65 species of Demodex are known; they are among the smallest of arthropods. Two species living on humans have been identified: Demodex folliculorum and Demodex brevis, both frequently referred to as eyelash mites. Demodex canis lives on the domestic dog. Infestation with Demodex is common and usually does not cause any symptoms, although occasionally some skin diseases can be caused by the mites.
D. folliculorum and D. brevis 
Demodex folliculorum and Demodex brevis are typically found on humans. D. folliculorum was first described in 1842 by Simon; D. brevis was identified as separate in 1963 by Akbulatova. D. folliculorum is found in hair follicles, while D. brevis lives in sebaceous glands connected to hair follicles. Both species are primarily found in the face, near the nose, the eyelashes and eyebrows, but also occur elsewhere on the body.
The adult mites are only 0.3–0.4 millimetre (0.012–0.016 in) long, with D. brevis slightly shorter than D. folliculorum. Each has a semitransparent, elongated body that consists of two fused segments. Eight short, segmented legs are attached to the first body segment. The body is covered with scales for anchoring itself in the hair follicle, and the mite has pin-like mouth-parts for eating skin cells and oils (sebum) which accumulate in the hair follicles. The mites can leave the hair follicles and slowly walk around on the skin, at a speed of 8–16 mm per hour, especially at night, as they try to avoid light.
Females of Demodex folliculorum are larger and rounder than males. Both male and female Demodex mites have a genital opening, and fertilization is internal. Mating takes place in the follicle opening, and eggs are laid inside the hair follicles or sebaceous glands. The six-legged larvae hatch after three to four days, and the larvae develop into adults in about seven days. The total lifespan of a Demodex mite is several weeks. The dead mites decompose inside the hair follicles or sebaceous glands. Recent research has indicated that the common skin malady rosacea may be caused by the decomposing mites, possibly due to the bacterium Bacillus oleronius found in their bodies.
Older people are much more likely to carry the mites; about a third of children and young adults, half of adults, and two-thirds of elderly people are estimated to carry the mites. The lower rate of children may be because children produce much less sebum. It is quite easy to look for one's own Demodex mites, by carefully removing an eyelash or eyebrow hair and placing it under a microscope.
The mites are transferred between hosts through contact of hair, eyebrows and of the sebaceous glands on the nose. Different species of animals host different species of Demodex; only one zoonosis of Demodex is known.
In the vast majority of cases, the mites go unobserved, without any adverse symptoms, but in certain cases (usually related to a suppressed immune system, caused by stress or illness) mite populations can dramatically increase, resulting in a condition known as demodicosis or Demodex mite bite, characterised by itching, inflammation and other skin disorders. Blepharitis (inflammation of the eyelids) can also be caused by Demodex mites. Evidence of a correlation between Demodex infection and acne vulgaris exists suggesting it may play a role in promoting acne.
Demodex canis 
The species Demodex canis lives predominantly on the domestic dog, but can occasionally infest humans. Although the majority of infestations are commensal, and therefore subclinical, they can develop into a condition called demodectic mange.
Due to their habitat being deep in the dermis, transmission is only usually possible via prolonged direct contact, such as mother-to-pup transmission during suckling. As a result, the most common sites for early appearance of demodicodic lesions are the face, muzzle, forelimbs and periorbital regions. Demodicosis can manifest as lesions of two types: squamous—which causes dry alopecia and thickening of the skin; and pustular—which is the more severe form, causing secondary infection (usually by Staphylococcus) resulting in the characteristic red, numerous pustules and wrinkling of the skin.
The escalation of a commensal D. canis infestation into one requiring clinical attention usually involves complex immune factors. Demodicosis can follow immunosuppressive conditions or treatments, or may be related to a genetic immune deficiency. This is complicated because Demodex itself is thought to suppress the normal T-lymphocyte response. Also, certain breeds—such as the Dalmatian, the American Bulldog and the American Pit Bull Terrier—appear to be more susceptible.
While direct treatment for severe cases is possible by p.o. administration of 1% Ivermectin in Ivermectin-tolerant breeds (the general dosage is 0.3–0.6 mg/kg, PO, sid until 2 negative skin scrapings 1 mo apart), other breeds can be successfully treated by applying the antiparasitic drug amitraz to the skin, concomitant with improved nutrition and addressing any possible underlying immune system-suppressing diseases. The secondary bacterial infection associated with pustular demodicosis may require treatment with antibiotics. It has been suggested that infestation of Demodex is a contributory factor in rosacea.
- Ed Yong (August 31, 2012). "Everything you never wanted to know about the mites that eat, crawl, and have sex on your face". Discover. Retrieved April 24, 2013.
- T. Rufli & Y. Mumcuoglu (1981). "The hair follicle mites Demodex folliculorum and Demodex brevis: biology and medical importance. A review". Dermatologica 162 (1): 1–11. doi:10.1159/000250228. PMID 6453029.
- Aisha Rush (2000). "Demodex folliculorum". Animal Diversity Web. University of Michigan. Retrieved January 7, 2007.
- Debora MacKenzie (2012). "Rosacea may be caused by mite faeces in your pores". New Scientist. Retrieved August 30, 2012.
- New Study Shows Role for Bacteria in Development of Rosacea Symptoms, National Rosacea Society press release, 3 May 2004
- H. G. Sengbusch & J. W. Hauswirth (1986). "Prevalence of hair follicle mites, Demodex folliculorum and D. brevis (Acari: Demodicidae), in a selected human population in western New York, USA". Journal of Medical Entomology 23 (4): 384–388. PMID 3735343.
- Zhao YE, Hu L, Wu LP, Ma JX (2012). "A meta-analysis of association between acne vulgaris and Demodex infestation.". Zhejiang Univ Sci B 13 (3): 192–202. PMID 22374611.
- G. M. Urquhart (1996). Veterinary Parasitology (2nd ed.). Blackwell Publishing. ISBN 0-632-04051-3.
|Wikispecies has information related to: Demodex|
- Rosacea may be caused by mite faeces in your pores, by Debora MacKenzie, published on newscientist.com 30 August 2012
- Demodex, an inhabitant of human hair follicles, and a mite which we live with in harmony, by M. Halit Umar, published in the May 2000 edition of Micscape Magazine, includes several micrographs
- Description and pictures
- Demodicosis, an article by Manolette R Roque, MD
- Demodex in the Dog, by T. J. Dunn, Jr. DVM
- High resolution images of Demodex folliculorum