Demodex folliculorum

Due to being adapted to live inside hair follicles, D. folliculorum is thin and worm-like, with short legs.{{cite book|last1=Prundeanu Croitoru|first1=Anca G.|last2=Chen|first2=Helen M.|last3=Ramos-e-Silva|first3=Marcia|last4=Busam|first4=Klaus J.|editor1-last=Busam|editor1-first=Klaus J.|title=Dermatopathology|date=2010|publisher=Saunders|isbn=978-0443066542|page=171|chapter=Chapter 3. Infectious diseases of the skin. Demodicosis}} As an adult, D.{{nbsp}}folliculorum measures {{convert|0.3|to|0.4|mm|abbr=on}} long. Adults have four pairs of legs; larvae and nymphs have only three pairs. D.{{nbsp}}folliculorum has a rudimentary gut but lacks an anus, so waste accumulates within the body until it dies.{{Cite journal|doi=10.1093/molbev/msac125 |doi-access=free |title=Human Follicular Mites: Ectoparasites Becoming Symbionts |year=2022 |last1=Smith |first1=Gilbert |last2=Manzano-Marín |first2=Alejandro |last3=Reyes-Prieto |first3=Mariana |last4=Antunes |first4=Cátia Sofia Ribeiro |last5=Ashworth |first5=Victoria |last6=Goselle |first6=Obed Nanjul |last7=Jan |first7=Abdulhalem Abdulsamad A. |last8=Moya |first8=Andrés |last9=Latorre |first9=Amparo |last10=Perotti |first10=M Alejandra |last11=Braig |first11=Henk R. |journal=Molecular Biology and Evolution |volume=39 |issue=6 |pmid=35724423 |pmc=9218549 }}

The entire life cycle of D.{{nbsp}}folliculorum takes 14–16 days. Adult mites copulate at the top of the hair follicle, near the skin surface.{{cite journal|last1=Lacey|first1=N|last2=Kavanagh|first2=K|last3=Tseng|first3=SC|title=Under the lash: Demodex mites in human diseases.|journal=The Biochemist|date=1 August 2009|volume=31|issue=4|pages=2–6|doi=10.1042/BIO03104020|pmid=20664811|pmc=2906820}} Eggs are deposited in the sebaceous gland inside the hair follicle. The heart-shaped egg is {{convert|0.1|mm|abbr=on}} long, and hatches into a six-legged larva. In seven days the larva develops into a mature adult,{{cite book|last1=Viswanath|first1=V.|last2=Gopalani|first2=V.|last3=Jambhore|first3=M.|editor1-last=Singal|editor1-first=Archana|editor2-last=Grover|editor2-first=Chander|title=Comprehensive Approach to Infections in Dermatology|date=2015|publisher=JP Medical Ltd|isbn=9789351527480|pages=442–447|chapter=Chapter 16. Infestations. Demodicosis}} with two intervening nymph stages. The adult lives for four to six days.

D. folliculorum prefers areas where sebum production is high, and is typically found in hair follicles on the human face,{{cite journal |author=Dirk M. Elston |year=2010 |title=Demodex mites: Facts and controversies |journal=Clinics in Dermatology |volume=28 |issue=5 |pages=502–504 |pmid=20797509 |doi=10.1016/j.clindermatol.2010.03.006}} 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 downward, with the end of the abdomen often protruding from the hair follicle. Inhabited follicles usually contain 2–6 mites, but numbers can be greater.

In one hour, D. folliculorum can travel {{convert|8|to|16|mm|abbr=on}}; they usually travel at night.

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

D. folliculorum is not found on newborn babies, but is acquired shortly after birth, most likely from 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.{{cite journal|last1=Palopoli|first1=MF|last2=Minot|first2=S|last3=Pei|first3=D|last4=Satterly|first4=A|last5=Endrizzi|first5=J|title=Complete mitochondrial genomes of the human follicle mites Demodex brevis and D. folliculorum: novel gene arrangement, truncated tRNA genes, and ancient divergence between species|journal=BMC Genomics|date=16 December 2014|volume=15|issue=1 |pages=1124|doi=10.1186/1471-2164-15-1124|pmid=25515815|pmc=4320518 |doi-access=free }} 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. 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 higher in people with immunosuppression.

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.{{cite journal|last1=Ortiz-Hidalgo|first1=C|title=The professor and the seamstress: an episode in the life of Jacob Henle|journal=Gaceta Medica de Mexico|date=2015|volume=151|issue=6|pages=819–27|pmid=26581541|url=https://www.anmm.org.mx/GMM/2015/n6_english/2331AX156_151_2015_UK6_762-769.pdf}}

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.{{Cite book |last=Simon |first=Gustav |url=https://www.biodiversitylibrary.org/item/49986 |title=Archiv für Anatomie, Physiologie und Wissenschaftliche Medicin |publisher=Verlag von Veit & Comp. |year=1842 |editor-last=Müller |editor-first=Johannes |volume= |location=Berlin |pages=221 |language=DE |trans-title=Archive for Anatomy, Physiology and Scientific Medicine |chapter=Ueber eine in den kranken und normalen Haarsäcken des Menschen lebende Milbe |trans-chapter=On a Mite Living in the Diseased and Normal Hair Follicles of Humans}} Simon gave a full report of the appearance of D. folliculorum, naming it Acarus folliculorum.{{cite journal|last1=Burns|first1=DA|title=Follicle mites and their role in disease|journal=Clinical and Experimental Dermatology|date=May 1992|volume=17|issue=3|pages=152–5|doi=10.1111/j.1365-2230.1992.tb00192.x|pmid=1451287|s2cid=29309265|doi-access=free}}

The following year, 1843, the genus was named Demodex by English scientist Richard Owen.{{cite book|last1=Monsel|first1=G.|last2=Delaunay|first2=P.|last3=Chosidow|first3=O.|editor1-last=Griffiths|editor1-first=Christopher|editor2-last=Barker|editor2-first=Jonathan|editor3-last=Bleiker|editor3-first=Tanya|editor4-last=Chalmers|editor4-first=Robert|editor5-last=Creamer|editor5-first=Daniel|title=Rook's Textbook of Dermatology Volume 1|pages=34.52–32.54|date=2016|publisher=John Wiley & Sons|isbn=9781118441176|edition=9th|chapter=Chapter 34.1 Arthropods. Follicle mites (Demodicidae)}} From Simon's initial description of D.{{nbsp}}folliculorum onwards, two forms were recognized, a long form and a short form. In 1963, it was suggested by LK Akbulatova that these long and short forms were two subspecies of D.{{nbsp}}folliculorum, and that the smaller mite be named Demodex brevis, with the larger mite retaining the name D. folliculorum.{{cite book|last1=Meinking|first1=Terri|last2=Taplin|first2=David|last3=Vicaria|first3=Maureen|editor1-last=Schachner|editor1-first=Lawrence A.|editor2-last=Hansen|editor2-first=Ronald C.|title=Pediatric Dermatology|date=2011|publisher=Elsevier|isbn=9780723435402|page=1575}} It was not until 1972 that the existence of two separate species was confirmed.{{cite book|last1=Nutting|first1=Wm. B.|last2=Firda|first2=Karen E.|last3=Desch, Jr.|first3=Clifford E.|editor1-last=Channabasavanna|editor1-first=G. P.|editor2-last=Viraktamath|editor2-first=C.A.|title=Progress in Acarology Volume 1|date=1989|publisher=E. J. Brill|location=Leiden, The Netherlands|isbn=978-9004085268|pages=113–121|chapter=Topology and histopathology of hair follicle mites (Demodicidae) of man|chapter-url=https://eurekamag.com/research/002/533/002533027.php}}

• Human microbiome

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{{Mite-related diseases and infestations}}

{{Taxonbar|from=Q4171774}}

Category:Trombidiformes

Category:Animals described in 1842

Category:Parasitic_arthropods_of_humans