''Demodex'' is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of tiny
mite
Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear evid ...
s that live in or near
hair follicles
The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...
of
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s. Around 65 species of ''Demodex'' are known. Two species live on
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s: ''
Demodex folliculorum
''Demodex folliculorum'' is a microscopic mite that can only survive on the skin of humans. 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 par ...
'' and ''
Demodex brevis
''Demodex brevis'' is one of the two species of face mite that inhabit humans (the other being ''Demodex folliculorum''). They are very similar to ''Demodex folliculorum'', with just a few differences. They are usually found in the sebaceous gla ...
'', both frequently referred to as eyelash mites, alternatively face mites or skin mites. Different species of animals host different species of ''Demodex''. ''
Demodex canis
''Demodex'' is a genus of tiny mites that live in or near hair follicles of mammals. Around 65 species of ''Demodex'' are known. Two species live on humans: ''Demodex folliculorum'' and ''Demodex brevis'', both frequently referred to as eyela ...
'' lives on the domestic
dog
The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
. The presence of ''Demodex'' species on mammals is common and usually does not cause any symptoms, although occasionally some
skin diseases
A skin condition, also known as cutaneous condition, is any medical condition that affects the integumentary system—the organ system that encloses the body and includes skin, nails, and related muscle and glands. The major function of this sy ...
can be caused by the mites. ''Demodex'' is derived from
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
δημός ''dēmos'' "fat" and δήξ ''dēx'', "woodworm".
Notable species
''D. folliculorum'' and ''D. brevis''
''D. folliculorum'' and ''D. 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 gland
A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest number ...
s connected to
hair follicle
The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...
s. Both species are primarily found in the face near the nose, the
eyelash
An eyelash (also called lash) (Latin: ''Cilia'') is one of the hairs that grows at the edge of the eyelids. It grows in one layer on the edge of the upper and lower eyelids. Eyelashes protect the eye from debris, dust, and small particles and p ...
es, and
eyebrow
An eyebrow is an area of short hairs above each eye that follows the shape of the lower margin of the brow ridges of some mammals. In humans, eyebrows serve two main functions: first, communication through facial expression, and second, prevent ...
s but also occur elsewhere on the body. ''D. folliculorum'' is occasionally found as a cause of
folliculitis
Folliculitis is the infection and inflammation of one or more hair follicles. The condition may occur anywhere on hair-covered skin. The rash may appear as pimples that come to white tips on the face, chest, back, arms, legs, buttocks, or head.
A ...
, although most people with ''D. folliculorum'' mites have no obvious ill effects.
The adult mites are long, with ''D. brevis'' slightly shorter than ''D. folliculorum''.
Each has a
semitransparent, 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
Scale or scales may refer to:
Mathematics
* Scale (descriptive set theory), an object defined on a set of points
* Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original
* Scale factor, a number w ...
for anchoring itself in the hair follicle, and the mite has pin-like mouthparts for eating
skin cells
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different de ...
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 per hour, especially at night, as they try to avoid light.
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. Mating takes place in the follicle opening, and eggs are laid inside the hair follicles or sebaceous glands. The six-legged
larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
The ...
e 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.
Health issues
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. The lower rate in children may be because children produce less
sebum
A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest nu ...
, 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.
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:
* Evidence of a correlation between ''Demodex'' infestation and
acne vulgaris
Acne, also known as ''acne vulgaris'', is a long-term skin condition that occurs when dead skin cells and oil from the skin clog hair follicles. Typical features of the condition include blackheads or whiteheads, pimples, oily skin, and po ...
exists, suggesting it may play a role in promoting acne, including in immunocompetent infants displaying
pityriasis
Pityriasis commonly refers to flaking (or scaling) of the skin. The word comes from the Greek πίτυρον "bran".
Classification
Types include:
* Pityriasis alba
* Pityriasis lichenoides chronica
* Pityriasis lichenoides et varioliformis acut ...
and
erythema toxicum neonatorum
Erythema toxicum neonatorum is a common, non-threatening rash in newborns. It appears in 4-70% of newborns within the first week of life, and it typically improves within 1–2 weeks. It only occurs during the newborn period, but may appear slightl ...
, or simply that ''Demodex'' mites thrive in the same oily conditions where acne bacteria thrive.
* Several preliminary studies suggest an association between mite infestation and
rosacea
Rosacea is a long-term skin condition that typically affects the face. It results in redness, pimples, swelling, and small and superficial dilated blood vessels. Often, the nose, cheeks, forehead, and chin are most involved. A red, enlarge ...
.
*Demodex mites can cause
blepharitis
Blepharitis is one of the most common ocular conditions characterized by inflammation, scaling, reddening, and crusting of the eyelid. This condition may also cause swelling, burning, itching, or a grainy sensation when introducing foreign objects ...
, which can be greatly reduced with solutions of
tea tree oil
Tea tree oil, also known as melaleuca oil, is an essential oil with a fresh camphoraceous odor and a colour that ranges from pale yellow to nearly colourless and clear. It is derived from the leaves of the tea tree, ''Melaleuca alternifolia'', ...
containing 50% concentrations or greater.
* Demodex mite eradication responds to topical treatments better than oral and intravenous treatments alone. More than one treatment may be required for individuals with prolonged Demodex mite infestation.
* Demodex mites causing a reaction in healthy individuals depends on genealogy. Mites may evolve differently with different bloodlines.
* New studies suggest demodex mites involved in psoriasis and asthma in immunosuppressed individuals.
''D. canis''
![Demodex mite 1](https://upload.wikimedia.org/wikipedia/commons/0/03/Demodex_mite_1.JPG)
The natural host of ''D. canis'' is the domestic
dog
The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
. ''D. canis'' mites can survive on
immunosuppressed
Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reacti ...
human skin and human mites can infect immunosuppressed dogs, although reported cases are rare.
Ivermectin
Ivermectin (, '' EYE-vər-MEK-tin'') is an antiparasitic drug. After its discovery in 1975, its first uses were in veterinary medicine to prevent and treat heartworm and acariasis. Approved for human use in 1987, today it is used to treat inf ...
is used for demodex mites requiring up to four treatments to eradicate in humans; although only one treatment is usually given to dogs to reduce mite count. Naturally, the ''D. canis'' mite has a
commensal
Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
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
Demodicosis , also called Demodex folliculitis in humans and demodectic mange () or red mange in animals, is caused by a sensitivity to and overpopulation of ''Demodex, Demodex spp.'' as the host's immune system is unable to keep the mites under ...
/demodectic
mange
Mange is a type of skin disease caused by parasitic mites. Because various species of mites also infect plants, birds and reptiles, the term "mange", or colloquially "the mange", suggesting poor condition of the skin and fur due to the infection ...
/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. 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.
''D. canis'' was first described by
Franz Leydig
Franz von Leydig, also Franz Leydig (; 21 May 1821 – 13 April 1908), was a German zoologist and comparative anatomist."Franz von Leydig" (biography), Ole Daniel Enerson, 2006, WhoNamedIt.coWNI-675-Leydig
Life
Franz Leydig was born on 21 May ...
in 1859.
References
External links
*
*
''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
*
''Demodetic Mange in Dogs'', by T. J. Dunn, Jr. DVM
{{Taxonbar, from=Q292297
Parasitic acari
Parasitic arthropods of mammals
Trombidiformes genera
Human eyelashes