Different species of animals host different species of Demodex. Demodex canis lives on the domestic
dog. The presence of Demodex species on mammals is common and usually does not cause any symptoms. Demodex is derived from
Greekδημός (dēmos) 'fat' and δήξ, δηκός (dēx, dēkós) 'woodworm'.[4][5][6]
D. folliculorum and D. brevis are typically found on humans. D. folliculorum was first described in 1842 by German physician and dermatologist
Gustav Simon,[7] with English biologist
Richard Owen naming the genus Demodex the following year.[8]
D. brevis was identified as separate in 1963 by Akbulatova. D. folliculorum is found in hair follicles, while D. brevis lives in
sebaceous glands connected to
hair follicles. Both species are primarily found in the face — near the nose, the
eyelashes, and
eyebrows — but also occur elsewhere on the body. D. folliculorum is occasionally found as a cause of
folliculitis, although most people with D. folliculorum mites have no obvious ill effects.
The adult mites are 0.3–0.4 mm (3⁄256–1⁄64 in) long, with D. brevis slightly shorter than D. folliculorum.[9] Each has a
semitransparent, elongated body that consists of two fused parts. Eight short, segmented legs are attached to the first body segment. The body is covered with
scales for anchoring itself in the hair follicle, and the mite has pin-like mouthparts for eating
skin cells and oils that accumulate in the hair follicles. The mites can leave the follicles and slowly walk around on the skin, at a speed of 8–16 mm (3⁄8–5⁄8 in) per hour, especially at night, as they try to avoid light.[9]: 2 The mites are transferred between hosts through contact with hair, eyebrows, and the sebaceous glands of the face.
Females of D. folliculorum are larger and rounder than males. Both male and female Demodex mites have a genital opening, and fertilization is internal.[10] Mating takes place in the follicle opening, and eggs are laid inside the hair follicles or sebaceous glands. The six-legged
larvae hatch after 3–4 days, and the larvae develop into adults in about 7 days. The total lifespan of a Demodex mite is several weeks.[11]
Prevalence
Older people are much more likely to carry face mites; about a third of children and young adults, half of adults, and two-thirds of elderly people carry them.[12] The lower rate in children may be because children produce less
sebum, or simply have had less time to acquire the mite. A 2014 study of 29 people in North Carolina, USA, found that all of the adults (19) carried mites, and that 70% of those under 18 years of age carried mites.[13] This study (using a DNA-detection method, more sensitive than traditional sampling and observation by microscope), along with several studies of cadavers, suggests that previous work might have underestimated the mites' prevalence. The small sample size and small geographical area involved prevent drawing broad conclusions from these data.
Research
Research about human infestation by Demodex mites is ongoing:[14][15][16][17]
Evidence of a correlation between Demodex infestation and
acne vulgaris exists, suggesting it may play a role in promoting acne, including in immunocompetent infants displaying
pityriasis and
erythema toxicum neonatorum, or simply that Demodex mites thrive in the same oily conditions where acne bacteria thrive.[16][18]
Several preliminary studies suggest an association between mite infestation and
rosacea.[19][20]
Demodex mites causing a reaction in healthy individuals depends on genealogy. Mites may evolve differently with different bloodlines.[24][25]
New studies suggest demodex mites are involved in psoriasis, allergic rhinitis, and seborrheic dermatitis in immunosuppressed individuals.[26][27][28]
Atopic triad is widely known as atopic dermatitis, allergic rhinitis and asthma."[29]
Consequently, it has been suggested that effective management of atopic dermatitis could deter the progression of the atopic march, therefore preventing or at least reducing the subsequent development of asthma and allergic rhinitis"[30]
The natural host of D. canis is the domestic
dog. D. canis mites can survive on
immunosuppressed human skin and human mites can infect immunosuppressed dogs, although reported cases are rare.
Ivermectin is used for demodex mites requiring up to four treatments to eradicate in humans; only one treatment is usually given to dogs to reduce mite count. Naturally, the D. canis mite has a
commensal relationship with the dog, and under normal conditions does not produce any clinical signs or disease. The escalation of a commensal D. canis infestation into one requiring clinical attention usually involves complex immune factors.
Under normal health conditions, the mite can live within the dermis of the dog without causing any harm to the animal. However, whenever an immunosuppressive condition is present and the dog's immune system (which normally ensures that the mite population cannot escalate to an infestation that can damage the dermis of the host) is compromised, it allows the mites to proliferate. As they continue to infest the host, clinical signs begin to become apparent and
demodicosis/demodectic
mange/red mange is diagnosed.
Since "D. canis" is a part of the natural fauna on a canine's skin, the mite is not considered to be contagious. Many dogs receive an initial exposure from their mothers while nursing, during the first few days of life.[31] The immune system of the healthy animal keeps the population of the mite in check, so subsequent exposure to dogs possessing clinical demodectic mange does not increase an animal's chance of developing demodicosis. Subsequent infestations after treatment can occur.
^"Demodex". Medical Dictionary (medicine.academic.ru).
^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.
^Griffiths, Christopher E. M.; Barker, Jonathan; Bleiker, Tanya; Chalmers, Robert J. G.; Creamer, Daniel; Rook, Graham Arthur, eds. (2016). Rook's textbook of dermatology (9th ed.). Chichester, West Sussex Hoboken, NJ: John Wiley & Sons Inc.
ISBN978-1-118-44117-6.
^
abRufli, T.; Mumcuoglu, Y. (1981). "The hair follicle mites Demodex folliculorum and Demodex brevis: biology and medical importance. A review". Dermatologica. 162 (1): 1–11.
doi:
10.1159/000250228.
PMID6453029.
^Sengbusch, H. G.; Hauswirth, J. W. (1986). "Prevalence of hair follicle mites, Demodex folliculorum and D. brevis (Acari: Demodicidae), in a selected human population in western New York, USA". Journal of Medical Entomology. 23 (4): 384–388.
doi:
10.1093/jmedent/23.4.384.
PMID3735343.
^Douglas, Annyella; Zaenglein, Andrea L. (September 2019). "A case series of demodicosis in children". Pediatric Dermatology. 36 (5): 651–654.
doi:
10.1111/pde.13852.
PMID31197860.
S2CID189817759. Papulopustular lesions predominate, prompting the advice 'pustules on noses, think demodicosis!'
^Miller, William H. Jr.; Griffin, Craig E.; Campbell, Karen L. (2013). "Canine demodicosis". Muller & Kirk's small animal dermatology (7th ed.). St. Louis, Mo.: Elsevier. pp. 304–313.
ISBN978-1-4160-0028-0.