Demodex

Demodex
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Arachnida
Subclass:	Acari
Order:	Trombidiformes
Family:	Demodicidae Nicolet, 1855
Genus:	Demodex Owen, 1843
Species
Demodex brevis Demodex bovis Demodex canis Demodex caprae Demodex cati Demodex equi Demodex folliculorum Demodex ovis Demodex phyloides

Demodex is a genus of tiny parasitic mites that live in or near hair follicles of mammals. About 65 species of Demodex are known; they are among the smallest of arthropods. Two species living on humans have been identified: Demodex folliculorum and Demodex brevis, both frequently referred to as eyelash mites. Demodex canis lives on the domestic dog. Infestation with Demodex is common and usually does not cause any symptoms, although occasionally some skin diseases can be caused by the mites.

Demodex folliculorum and Demodex brevis

Demodex folliculorum and Demodex brevis are typically found on humans. D. folliculorum was first described in 1842 by Simon; 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.

The adult mites are only 0.3–0.4 millimetre (0.012–0.016 in) long, with D. brevis slightly shorter than D. folliculorum.^[1] They have a semi-transparent elongated body that consists of two fused segments. 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 mouth-parts for eating skin cells, hormones and oils (sebum) which accumulate in the hair follicles. The mites can leave the hair follicles and slowly walk around on the skin, at a speed of 8–16 centimetres (3.1–6.3 in) per hour—especially at night as they try to avoid light.^[1]

Female Demodex folliculorum are somewhat shorter and rounder than males. Both male and female Demodex mites have a genital opening, and fertilization is internal.^[2] 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 it takes about seven days for the larvae to develop into adults. The total lifespan of a Demodex mite is several weeks. The dead mites decompose inside the hair follicles or sebaceous glands.

Older people are much more likely to carry the mites; it is estimated that about a third of children and young adults, half of adults, and two-thirds of elderly people carry the mites.^[3] The lower rate of children may be because children produce much less sebum. It is quite easy to look for one's own Demodex mites, by carefully removing an eyelash or eyebrow hair and placing it under a microscope.

The mites are transferred between hosts through contact of hair, eyebrows and of the sebaceous glands on the nose. Different species of animals host different species of Demodex; only one zoonosis of Demodex is known.

In the vast majority of cases, the mites go unobserved, without any adverse symptoms, but in certain cases (usually related to a suppressed immune system, caused by stress or illness) mite populations can dramatically increase, resulting in a condition known as demodicosis or Demodex mite bite, characterised by itching, inflammation and other skin disorders. Blepharitis (inflammation of the eyelids) can also be caused by Demodex mites.

There is some evidence linking Demodex mites to some forms of the skin disease rosacea, possibly due to the bacterium Bacillus oleronius found in the mites.^[4]

Demodex canis

Demodex canis

The species Demodex canis lives predominantly on the domestic dog, but can occasionally infect humans. Although the majority of infestations are commensal, and therefore sub-clinical, it can develop into a condition called demodectic mange.

Due to their habitat being deep in the dermis, transmission is only usually possible via prolonged direct contact, such as bitch-to-pup transmission during suckling. As a result, the most common sites for early appearance of demodicodic lesions are the face, muzzle, forelimbs and periorbital regions. Demodicosis can manifest as lesions of two types: squamous—which causes dry alopecia and thickening of the skin; and pustular—which is the more severe form, causing secondary infection (usually by Staphylococcus) resulting in the characteristic red, numerous pustules and wrinkling of the skin.

The escalation of a commensal D. canis infestation into one requiring clinical attention usually involves complex immune factors. Demodicosis can follow immunosuppressive conditions or treatments, or may be related to a genetic immune deficiency. This is complicated by the fact that Demodex itself is thought to suppress the normal T-lymphocyte response. There are also certain breeds, such as the Dalmatian, which appear to be more susceptible.^[5]

While direct treatment for severe cases is possible by applying the antiparasitic drug Amitraz to the skin, improved nutrition also helps and addressing any possible underlying immune-system suppressing diseases. The secondary bacterial infection associated with pustular demodicosis requires treatment with antibiotics.

References

1. ^ ^{a b} T. Rufli & Y. Mumcuoglu (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. PMID 6453029. 
2. ^ Aisha Rush (2000). "Demodex folliculorum". Animal Diversity Web. University of Michigan. http://animaldiversity.ummz.umich.edu/site/accounts/information/Demodex_folliculorum.html. Retrieved January 7, 2007. 
3. ^ H. G. Sengbusch & J. W. Hauswirth (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. PMID 3735343. http://www.ingentaconnect.com/content/esa/jme/1986/00000023/00000004/art00009. 
4. ^ New Study Shows Role for Bacteria in Development of Rosacea Symptoms, National Rosacea Society press release, 3 May 2004
5. ^ G. M. Urquhart (1996). Veterinary Parasitology (2nd ed.). Blackwell Publishing. ISBN 0-632-04051-3. 

External links

Arthropods portal

• Demodex, an inhabitant of human hair follicles, and a mite which we live with in harmony, by M. Halit Umar, published in the May 2000 edition of Micscape Magazine, includes several micrographs
• Description and pictures
• Demodicosis, an article by Manolette R Roque, MD
• Mites might cause mighty problems, USA Today (Magazine), February 2004
• Demodex in the Dog, by T. J. Dunn, Jr. DVM
• High resolution images of Demodex folliculorum
• Ted's mange cure.