Demodex folliculorum

Demodex folliculorum
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Arachnida
Order:	Trombidiformes
Family:	Demodicidae
Genus:	Demodex
Species:	D. folliculorum[1]
Binomial name
Demodex folliculorum Simon, 1842

Demodex folliculorum is a species of face mite. D. folliculorum is one of the parasitic/commensal face mites that occur on people (other than Demodex brevis).^[1][2] When large numbers of D. folliculorum are found on humans, the infestation is known as "demodicosis".^[3]

History

Demodex was first discovered in 1840 in dogs to cause mange, a skin disease caused by parasitic mites affecting animals. Tulk was the first to publish about Demodex in dogs. In 1842, a Frenchman named Berger discovered Demodex folliculorum in earwax.^[4]

Behavior

These mites are found in the human hair follicles, normally found in greater numbers around the cheeks, nose, eyebrows, eyelashes, and forehead. They could also be found in other parts of the body such as arms, chest, and ears. It is a species of tiny mites that have been thought to contribute to hair loss (though this hypothesis is under debate)^[5] and lives in pores and hair follicles. The mites vary in size from 0.1 mm to 0.4 mm long.^[6] Mites do not invade internal organs. Under normal conditions, they are not harmful, and classified as commensals (the mite benefits but there is no harm or benefit to the host) rather than parasites (where the host is harmed), though under outbreak conditions (demodicosis) they can be harmful.

Morphology

D. folliculorum is semi-transparent elongated organism consisting of a head, neck, body, and tail.^[3] D. folliculorum is worm-like, with tiny claws, and scales over their entire body, which allows it to anchor itself in the hair follicles. As an adult, D. folliculorum can measure 0.1 mm to 0.4 mm in length and possess four pairs of short legs near its head and neck region.^[3] However as a larva and/or nymph, D. folliculorum resembles the adult but has three pairs of short legs near its head and neck region.^[3] The body and tail region of D. folliculorum is striated.^[3]

Demodex folliculorum v. Demodex brevis

Both Demodex folliculorum and Demodex brevis are typically found on the human face.^[7] Demodex folliculorum resides in hair follicles, whereas Demodex brevis survives in sebaceous glands adjacent to hair follicles. For instance, around the eye area, Demodex folliculorum is found in the follicles of the eyelashes. In contrast, Demodex brevis inhabits the sebaceous gland of the eyelashes and the meibomian glands. Demodex brevis was first separated from Demodex folliculorum in 1963 by Akbulatova.

Life cycle

The entire life cycle of D. folliculorum takes place in the time span of 18–24 days on its host.^[3] During the day, they feed on dead skin cells within hair follicles. At night, they emerge onto the surface to mate and lay eggs in hair follicles. A female adult lays 20–24 eggs in a single hair follicle, as they grow they become tightly packed, and develop into larvae.^[3] The larva is then washed by a sebaceous flow, produced by the host’s sebaceous glands, into the mouth of the hair follicle where they mature into adults.^[3] It takes seven days for the larva to develop into an adult that is ready to reproduce sexually.^[8]

Whom it affects

Demodex mites are acquired shortly after birth and are considered to be normal skin fauna that increase in number as people age. Almost everyone has a certain degree of the Demodex mite on his or her skin.^[9] Heavy infestations of Demodex can arrive in adolescence, and could last up to middle age. The increase of their food supply (sebaceous glands proliferate during puberty) explains the increase of infestations of mites during adolescence. 25% of the people who are up to 20 years old have mites, and 30% of people up to the age of 50 have mites. Of people that are between 80 and 100 years old, between 50% and 100% have mites.^[6] Hair follicles in all adults are infested, but the distribution of mites varies, which has a different impact on each person. Men are often more likely to be infested by mites than women because they have more sebaceous glands, thus producing more food for the mites.^[7]

A 2014 study conducted at North Carolina State University found that 100 per cent of 253 people over age 18 sampled by her team had mite DNA on their faces, suggesting that the mites could be universal inhabitants of adult humans.^[10]

References

Further reading

• Thoemmes, Megan S.; Fergus, Daniel J.; Urban, Julie; Trautwein, Michelle; Dunn, Robert R. (August 27, 2014). "Ubiquity and Diversity of Human-Associated Demodex Mites". PLoS ONE 9 (8): e106265. doi:10.1371/journal.pone.0106265. PMID 25162399. Lay summary – National Geographic (August 27, 2014).