Demodex
Demodex /ˈdɛmədɛks/ is a genus of tiny mites that live in or near hair follicles of mammals. Around 65 species of Demodex are known.[2] Two species live on humans: Demodex folliculorum and Demodex brevis, both frequently referred to as eyelash mites, alternatively face mites or skin mites.[3] Different species of animals host different species of Demodex. Demodex canis lives on the domestic dog. The presence of Demodex species on mammals 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".[4][5][6]
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. D. folliculorum is occasionally found as a cause of folliculitis, although most people with D. folliculorummites have no obvious ill effects.
The adult mites are only 0.3–0.4 mm (3⁄256–1⁄64 in) long, with D. brevis slightly shorter than D. folliculorum.[7] 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 that accumulate in the hair follicles. The mites can leave the follicles and slowly walk around on the skin, at a speed of 8–16 mm (3⁄8–5⁄8 in) per hour, especially at night, as they try to avoid light.[7]: 2 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.[8] 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.[citation needed]
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.[9] The lower rate in children may be because children produce less sebum, or simply have had less time to acquire the mite. A 2014 study of 29 people in North Carolina, USA, found that all of the adults (19) carried mites, and that 70% of those under 18 years of age carried mites.[10]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. The small sample size and small geographical area involved prevent drawing broad conclusions from these data.
