''Demodex folliculorum'' is a microscopic

mite Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear e ...

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, so are considered an example of

commensalism Commensalism is a long-term biological interaction ( symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fr ...

rather than

parasitism Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...

. ''D.folliculorum''-caused disease, though, is known as

demodicosis Demodicosis , also called Demodex folliculitis in humans and demodectic mange () or red mange in animals, is caused by a sensitivity to and overpopulation of '' Demodex spp.'' as the host's immune system is unable to keep the mites under control. ...

Anatomy

Due to being adapted to live inside hair follicles, ''D. folliculorum'' is thin and worm-like, with short legs. As an adult, ''D.folliculorum'' measures long. Adults have four pairs of legs; larvae and nymphs have only three pairs. ''D.folliculorum'' has a rudimentary gut and anus.

Reproduction and life cycle

The entire

life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesis ...

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 A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest nu ...

inside the hair follicle. The heart-shaped egg is long, and hatches into a six-legged larva. Seven days are needed for the larva to develop into a mature adult, with two intervening nymph stages. The adult lives for 4–6 days.

Ecology

''D. folliculorum'' prefers areas where

sebum A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest nu ...

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

, positioned head downward, with the end of abdomen often protruding from the hair follicle. Inhabited follicles usually contain 2–6 mites, but greater numbers can occur. In one hour, ''D. folliculorum'' can travel ; 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.

Relationship with humans

''D. folliculorum'' is not found on newborn babies, but is acquired shortly after birth, most likely due to maternal contact. Few mites are found on children under 10 years of age, but nearly all elderly people have them. The increasing population over time may be due to a small initial presence gradually growing over time, or may be because levels of the mite's food, sebum, increase with age. High numbers of ''D. folliculorum'' are associated with blepharitis and

acne rosacea Acne, also known as ''acne vulgaris'', is a long-term skin condition that occurs when dead skin cells and oil from the skin clog hair follicles. Typical features of the condition include blackheads or whiteheads, pimples, oily skin, and p ...

. The exact mechanism by which the mites cause disease is unknown; they may physically block the hair follicle, carry disease-causing bacteria, or after death, their bodies may cause either a

delayed hypersensitivity Type IV hypersensitivity, often called delayed-type hypersensitivity, is a type of hypersensitivity reaction that can take a day or more to develop. Unlike the other types, it is not humoral (not antibody-mediated) but rather is a type of cell-medi ...

response, or an innate immune response. Controversy exists over whether high numbers of ''D. folliculorum'' cause rosacea, or whether the skin environment caused by rosacea is more hospitable to mites than normal skin, allowing them to flourish. Populations of ''D. folliculorum'' are also increased in people with

immunosuppression Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reacti ...

History

The first report of ''Demodex folliculorum'' was made by German scientist

Jakob Henle Friedrich Gustav Jakob Henle (; 9 July 1809 – 13 May 1885) was a German physician, pathologist, and anatomist. He is credited with the discovery of the loop of Henle in the kidney. His essay, "On Miasma and Contagia," was an early argument for ...

in 1841, but his presentation to the Natural Sciences Society of Zurich, reported in a local newspaper, attracted little attention at the time. In 1842, German dermatologist Gustav Simon gave a full report of the appearance of ''D. folliculorum'', naming it ''Acarus folliculorum''. The following year, 1843, the genus was named ''

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 eyela ...

'' by English scientist Richard Owen. From Simon's initial description of ''D.folliculorum'' onwards, two forms were recognized, a long form and a short form. In 1963, it was suggested that these long and short forms were two subspecies of ''D.folliculorum'', and that the smaller mite be named ''

Demodex brevis ''Demodex brevis'' is one of the two species of face mite that inhabit humans (the other being ''Demodex folliculorum''). They are very similar to ''Demodex folliculorum'', with just a few differences. They are usually found in the sebaceous gla ...

'', with the larger mite retaining the name ''D. folliculorum''. It was not until 1972 that the existence of two separate species was confirmed.

References

External links

* {{Taxonbar, from=Q4171774 Trombidiformes Animals described in 1842 Parasitic_arthropods_of_humans