Demodex is a genus of tiny parasitic mites that live in or near hair follicles of mammals. About 65 species of Demodex mites 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 mites is common and usually does not cause any symptoms; occasionally some skin diseases can be caused by the mites.
The adult mites are only between 0.3 mm and 0.4 mm long, with D. brevis slightly shorter than D. folliculorum. They have a semi-transparent 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, hormones and oils (sebum) which accumulate in the hair follicles. The mite's digestive system is so efficient and results in so little waste that there is no excretory orifice. The mites can leave the hair follicles and slowly walk around on the skin, at a speed of about 8–16 cm/hour, especially at night; they try to avoid light.
Female Demodex folliculorum are somewhat shorter and rounder than males. The total lifespan of a Demodex mite is several weeks. 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 it takes about seven days for the larvae to develop into adults. The dead mites decompose inside the hair follicles or sebaceous glands.
Older people are much more likely to carry the mites; estimates range as high as an 96-98% infestation rate in aged people. The lower rate of children may be due to the fact that 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; and demodex is not contagious between different species.
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, characterised by itching, inflammation and other skin disorders. Blepharitis (inflammation of the eyelids) can also be caused by Demodex mites.
There is some evidence linking demodex mites to some forms of the skin disease rosacea, possibly due to the bacterium Bacillus oleronius found in the mites. Some people believe that there is also a link to acne vulgaris, but there is little research to back this up, and quite reasonable experimental evidence linking acne vulgaris to a sensitivity to Propionibacterium acnes.