Demodex folliculorum

Demodex folliculorum is a microscopic mite that can only survive on the skin of humans. Most people have D. folliculorum on their skin. Usually, the mites do not cause any harm, and are therefore considered an example of commensalism rather than parasitism. If D. folliculorum does cause disease, this is known as demodicosis. D. folliculorum is adapted to live inside hair follicles, and therefore is thin and worm-like, with short legs. As an adult, D. folliculorum measures 0.3 to 0.4 mm (0.012 to 0.016 in) long. Adults have four pairs of legs, larvae and nymphs have only three pairs. D. folliculorum has a rudimentary gut, and no anus. The entire life cycle of D. folliculorum takes 14–16 days. Adult mites copulate at the top of the hair follicle, near the skin surface. Eggs are deposited in the sebaceous gland inside the hair follicle. The heart-shaped egg is 0.1 mm (0.0039 in) long, and hatches into a six-legged larva. It takes seven days for the larva to develop into a mature adult, with two intervening nymph stages. The adult lives for 4–6 days. Demodex folliculorum prefers areas where sebum production is high, and is typically found in hair follicles on the human face, generally in greater numbers around the cheeks, nose, and forehead, but also elsewhere on the face, eyelids and ears. The mites may also be found on other parts of the body, such as the chest and buttocks. Within the hair follicle, D. folliculorum is found above the sebaceous gland, positioned head downwards, with the end of abdomen often protruding from the hair follicle. Infested follicles usually contain 2–6 mites, but greater numbers can occur. In one hour, D. folliculorum can travel 8 to 16 mm (0.31 to 0.63 in); the mites usually travel at night. The mites are obligate commensals of humans, and can only live on the skin; they soon dry out and die if they leave the host. Higher numbers of D. folliculorum are found in the spring and summer than at other times of the year. D. folliculorum are not found on newborn babies, but are acquired shortly after birth, most likely due to maternal contact. Few mites are found on children under ten years of age, but nearly all elderly people are infested. The increasing population over time may be due to a small initial infestation gradually growing over time, or may be because levels of the mite's food, sebum, increase with age.