Demodex folliculorum

See also: Demodex and Demodex brevis

Demodex folliculorum
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Arachnida
Order: Trombidiformes
Family: Demodecidae
Genus: Demodex
Species:
D. folliculorum
Binomial name
Demodex folliculorum
(Simon, 1842)
Synonyms[1]
  • Acarus folliculorum Simon, 1842

Demodex folliculorum is a microscopic mite that can only survive on the skin of humans.[2][3] Most people have D. folliculorum on their skin. Usually, the mites do not cause any harm, so are considered an example of commensalism rather than parasitism;[4] but they can cause disease, known as demodicosis.[5]

Anatomy[edit]

Due to being adapted to live inside hair follicles, D. folliculorum is thin and worm-like, with short legs.[6] As an adult, D. folliculorum measures 0.3 to 0.4 mm (0.012 to 0.016 in) long.[7] Adults have four pairs of legs; larvae and nymphs have only three pairs.[7] D. folliculorum has a rudimentary gut and anus.[4][8]

Reproduction and life cycle[edit]

The entire life cycle of D. folliculorum takes 14–16 days.[9] Adult mites copulate at the top of the hair follicle, near the skin surface.[10] Eggs are deposited in the sebaceous gland inside the hair follicle.[10] The heart-shaped egg is 0.1 mm (0.0039 in) long, and hatches into a six-legged larva.[11] In seven days the larva develops into a mature adult,[5] with two intervening nymph stages.[11] The adult lives for four to six days.[9]

Ecology[edit]

D. folliculorum prefers areas where sebum production is high,[7] and is typically found in hair follicles on the human face,[12] generally in greater numbers around the cheeks, nose, and forehead, but also elsewhere on the face, eyelids, and ears.[9] The mites may also be found on other parts of the body, such as the chest and buttocks.[7]

Within the hair follicle, D. folliculorum is found above the sebaceous gland,[2] positioned head downward, with the end of the abdomen often protruding from the hair follicle.[11] Inhabited follicles usually contain 2–6 mites, but numbers can be greater.[11]

In one hour, D. folliculorum can travel 8 to 16 mm (0.31 to 0.63 in);[9] they usually travel at night.[9]

The mites are obligate commensals of humans,[7] and can only live on the skin; they soon dry out and die if they leave the host.[10] Higher numbers of D. folliculorum are found in the spring and summer than at other times of year.[10]

Relationship with humans[edit]

D. folliculorum is not found on newborn babies, but is acquired shortly after birth, most likely from maternal contact.[11] Few mites are found on children under 10 years of age, but nearly all elderly people have them.[10] 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.[10]

High numbers of D. folliculorum are associated with blepharitis and acne rosacea.[13] The 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 response, or an innate immune response.[10] 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.[7] Populations of D. folliculorum are higher in people with immunosuppression.[4]

History[edit]

The first report of Demodex folliculorum was made by German scientist Jakob Henle in 1841, but his presentation to the Natural Sciences Society of Zurich, reported in a local newspaper, attracted little attention at the time.[14]

In 1842, German dermatologist Gustav Simon was investigating the formation of acne pustules and blackheads using hair follicle samples under a microscope, when he noticed movement within the sample. He described himself as surprised to find a tiny "head, legs, fore and hind body" pressed between the two glass plates of the sample, and was then able to replicate his discovery with other samples.[15] Simon gave a full report of the appearance of D. folliculorum, naming it Acarus folliculorum.[11]

The following year, 1843, the genus was named Demodex by English scientist Richard Owen.[7] From Simon's initial description of D. folliculorum onwards, two forms were recognized, a long form and a short form.[9] 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, with the larger mite retaining the name D. folliculorum.[9] It was not until 1972 that the existence of two separate species was confirmed.[16]

See also[edit]

References[edit]

  1. ^ Simon, Gustav (1842). "Ueber eine in den kranken und normalen Haarsäcken des Menschen lebende Milbe" [About a mite that lives in the diseased and normal hair sacs of humans]. Archiv für Anatomie, Physiologie und Wissenschaftliche Medicin (in German). 1842: 231.
  2. ^ a b Thoemmes, Megan S.; Fergus, Daniel J.; Urban, Julie; Trautwein, Michelle; Dunn, Robert R. (27 August 2014). "Ubiquity and diversity of human-associated Demodex mites". PLOS ONE. 9 (8): e106265. Bibcode:2014PLoSO...9j6265T. doi:10.1371/journal.pone.0106265. PMC 4146604. PMID 25162399.
  3. ^ "Meet The Mites That Live on Your Face". NPR.org. 21 May 2019. Retrieved 23 May 2019.
  4. ^ a b c Lacey, N; Ní Raghallaigh, S; Powell, FC (2011). "Demodex mites – commensals, parasites or mutualistic organisms?". Dermatology. 222 (2): 128–30. doi:10.1159/000323009. PMID 21228556.Open access icon
  5. ^ a b Viswanath, V.; Gopalani, V.; Jambhore, M. (2015). "Chapter 16. Infestations. Demodicosis". In Singal, Archana; Grover, Chander (eds.). Comprehensive Approach to Infections in Dermatology. JP Medical Ltd. pp. 442–447. ISBN 9789351527480.
  6. ^ Prundeanu Croitoru, Anca G.; Chen, Helen M.; Ramos-e-Silva, Marcia; Busam, Klaus J. (2010). "Chapter 3. Infectious diseases of the skin. Demodicosis". In Busam, Klaus J. (ed.). Dermatopathology. Saunders. p. 171. ISBN 978-0443066542.
  7. ^ a b c d e f g Monsel, G.; Delaunay, P.; Chosidow, O. (2016). "Chapter 34.1 Arthropods. Follicle mites (Demodicidae)". In Griffiths, Christopher; Barker, Jonathan; Bleiker, Tanya; Chalmers, Robert; Creamer, Daniel (eds.). Rook's Textbook of Dermatology Volume 1 (9th ed.). John Wiley & Sons. pp. 34.52–32.54. ISBN 9781118441176.
  8. ^ Smith, Gilbert; Manzano-Marín, Alejandro; Reyes-Prieto, Mariana; Antunes, Cátia Sofia Ribeiro; Ashworth, Victoria; Goselle, Obed Nanjul; Jan, Abdulhalem Abdulsamad A.; Moya, Andrés; Latorre, Amparo; Perotti, M Alejandra; Braig, Henk R. (2022). "Human Follicular Mites: Ectoparasites Becoming Symbionts". Molecular Biology and Evolution. 39 (6). doi:10.1093/molbev/msac125. PMC 9218549. PMID 35724423.
  9. ^ a b c d e f g Meinking, Terri; Taplin, David; Vicaria, Maureen (2011). Schachner, Lawrence A.; Hansen, Ronald C. (eds.). Pediatric Dermatology. Elsevier. p. 1575. ISBN 9780723435402.
  10. ^ a b c d e f g Lacey, N; Kavanagh, K; Tseng, SC (1 August 2009). "Under the lash: Demodex mites in human diseases". The Biochemist. 31 (4): 2–6. doi:10.1042/BIO03104020. PMC 2906820. PMID 20664811.
  11. ^ a b c d e f Burns, DA (May 1992). "Follicle mites and their role in disease". Clinical and Experimental Dermatology. 17 (3): 152–5. doi:10.1111/j.1365-2230.1992.tb00192.x. PMID 1451287. S2CID 29309265.
  12. ^ Dirk M. Elston (2010). "Demodex mites: Facts and controversies". Clinics in Dermatology. 28 (5): 502–504. doi:10.1016/j.clindermatol.2010.03.006. PMID 20797509.
  13. ^ Palopoli, MF; Minot, S; Pei, D; Satterly, A; Endrizzi, J (16 December 2014). "Complete mitochondrial genomes of the human follicle mites Demodex brevis and D. folliculorum: novel gene arrangement, truncated tRNA genes, and ancient divergence between species". BMC Genomics. 15 (1): 1124. doi:10.1186/1471-2164-15-1124. PMC 4320518. PMID 25515815.
  14. ^ Ortiz-Hidalgo, C (2015). "The professor and the seamstress: an episode in the life of Jacob Henle" (PDF). Gaceta Medica de Mexico. 151 (6): 819–27. PMID 26581541.
  15. ^ Simon, Gustav (1842). "Ueber eine in den kranken und normalen Haarsäcken des Menschen lebende Milbe" [On a Mite Living in the Diseased and Normal Hair Follicles of Humans]. In Müller, Johannes (ed.). Archiv für Anatomie, Physiologie und Wissenschaftliche Medicin [Archive for Anatomy, Physiology and Scientific Medicine] (in German). Berlin: Verlag von Veit & Comp. p. 221.{{cite book}}: CS1 maint: date and year (link)
  16. ^ Nutting, Wm. B.; Firda, Karen E.; Desch, Jr., Clifford E. (1989). "Topology and histopathology of hair follicle mites (Demodicidae) of man". In Channabasavanna, G. P.; Viraktamath, C.A. (eds.). Progress in Acarology Volume 1. Leiden, The Netherlands: E. J. Brill. pp. 113–121. ISBN 978-9004085268.

External links[edit]

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