Demodex is a genus of tiny mites that live in or near hair follicles of mammals. Around 65 species of Demodex are known. Two species live on humans: Demodex folliculorum and Demodex brevis, both frequently referred to as eyelash mites. Different species of animals host different species of Demodex. 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. Demodex is derived from Greek δημός dēmos "fat" and δήξ dēx, "woodworm".
D. folliculorum and D. brevis
D. folliculorum and D. 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 mm (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 mouthparts for eating skin cells and oils 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 cm (3.1–6.3 in) per hour, especially at night, as they try to avoid light. The mites are transferred between hosts through contact with hair, eyebrows, and the sebaceous glands of the face.
Females of D. 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 3–4 days, and the larvae develop into adults in about 7 days. The total lifespan of a Demodex mite is several weeks.
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 carry them. The lower rate in children may be because children produce less sebum. A 2014 study of n = 29 people in North Carolina, USA, found that all the adults (n = 19, over 18 years of age) carried mites, and that 70% of those under 18 years of age carried mites. This study (using a DNA detection method, more sensitive than traditional sampling and observation by microscope), along with several studies of cadavers, suggests that previous work might have underestimated the mites' prevalence. However, the small sample size and small geographical area involved prevent drawing broad conclusions from these data.
- Evidence of a correlation between Demodex infestation and acne vulgaris exists, suggesting it might play a role in promoting acne.
- Several preliminary studies suggest an association between mite infestation and rosacea.
The natural host of D. canis is the domestic dog. Although it can temporarily infect humans, D. canis mites cannot survive on human skin and will die shortly after exposure and are therefore not considered to be zoonotic. Naturally, the D. canis mite has a commensal relationship with the dog and under normal conditions does not produce any clinical signs or disease. The escalation of a commensal D. canis infestation into one requiring clinical attention usually involves complex immune factors. Under normal health conditions, the mite can live within the dermis of the dog without causing any harm to the animal. However, whenever an immunosuppressive condition is present and the dog's immune system (which normally ensures that the mite population cannot escalate to an infestation that can damage the dermis of the host) is compromised, it allows the mites to proliferate. As they continue to infest the host, clinical signs begin to become apparent and demodicosis/demodectic mange/red mange is diagnosed.
Since D. canis is a part of the natural fauna on a canine's skin, the mite is not considered to be contagious. All dogs receive an initial exposure from their mothers during nursing. The immune system of the healthy animal keeps the population of the mite in check, so subsequent exposure to dogs possessing clinical demodectic mange does not increase an animal's chance of developing demodicosis. Subsequent infestations after treatment can occur.
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- Zhao, Ya-e; Peng, Yan; Wang, Xiang-lan; Wu, Li-ping; Wang, Mei; Yan, Hu-ling; Xiao, Sheng-xiang (December 2011). "Facial dermatosis associated with Demodex: a case-control study". Journal of Zhejiang University Science B. 12 (8): 1008–1015. doi:10.1631/jzus.B1100179. PMC 3232434. PMID 22135150.
- Zhao, Ya-e; Hu, Li; Wu, Li-ping; Ma, Jun-xian (March 2012). "A meta-analysis of association between acne vulgaris and Demodex infestation". Journal of Zhejiang University Science B. 13 (3): 192–202. doi:10.1631/jzus.B1100285. PMC 3296070. PMID 22374611.
- "2011-2012 Annual Evidence Update on Acne vulgaris" (PDF). University of Nottingham Centre of Evidence Based Dermatology. 2012. p. 10. Retrieved 23 September 2013.
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- Jarmuda, Stanisław; O'Reilly, Niamh; Żaba, Ryszard; Jakubowicz, Oliwia; Szkaradkiewicz, Andrzej; Kavanagh, Kevin (August 2012). "The potential role of Demodex folliculorum mites and bacteria in the induction of rosacea". Journal of Medical Microbiology. 61: 1504–10. doi:10.1099/jmm.0.048090-0. PMID 22933353.
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- 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
- Demodicosis, an article by Manolette R Roque, MD
- Demodetic Mange in Dogs, by T. J. Dunn, Jr. DVM