Demodex

***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*

Main articles: Demodex folliculorum, Demodex 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.

### Health issues

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 carried them. The lower rate in children may be because children produce less sebum. Recently, a 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.

#### Research

Research about human infestation by *Demodex* mites is ongoing:

* 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.

## *D. canis*

See also: Demodicosis
The natural host of *D. canis* is the domestic dog. Although it can temporarily infect humans, *D. canis* mites cannot survive on the human skin, and will die shortly after exposure and are considered not 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.