Around 65 species of Demodex are known. Two species living on humans have been identified: Demodex folliculorum and Demodex brevis, both frequently referred to as eyelash mites. Different species of animals host different species of Demodex. 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 is derived from Greek dēmos fat + dēx, a wood worm.
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 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 mm (0.012–0.016 in) 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 for anchoring itself in the hair follicle, and the mite has pin-like mouthparts for eating skin cells 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 mm 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 larvae hatch after three to four days, and the larvae develop into adults in about seven days. The total lifespan of a Demodex mite is several weeks.
Older people are much more likely to carry the mites; about a third of children and young adults, half of adults, and two-thirds of elderly people carried them. The lower rate in children may be because children produce less sebum. Recently, a study of 29 adults (18 and over) in North Carolina, USA, found that 70% of those 18 years of age carried mites, and that all adults over 18 (n = 19) carried them. 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 may have underestimated the mites' prevalence. However, the small sample size and small geographical area involved prevent drawing broad conclusions from these data.
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. Evidence of a correlation between Demodex infection and acne vulgaris exists, suggesting it may play a role in promoting acne. Only one zoonosis of Demodex is known.
The species D. canis lives predominantly on the domestic dog, but can occasionally infest humans. Although the majority of infestations are commensal, and therefore subclinical, they can develop into a condition called demodectic mange.
Due to the mites' habitat being deep in the dermis, transmission usually occurs only by prolonged direct contact, such as mother-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 numerous red 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 because Demodex is thought to suppress the normal T-lymphocyte response. Also, certain breeds—such as the Dalmatian, the American Bulldog, and the American Pit Bull Terrier—appear to be more susceptible.
While direct treatment for severe cases is possible by oral administration of 1% ivermectin in ivermectin-tolerant breeds, other breeds can be successfully treated by applying the antiparasitic drug amitraz to the skin, concomitant with improved nutrition and addressing any possible underlying immune system-suppressing diseases. Commercial preparations including fipronil, amitraz, and (S)-methoprene are efficient. The secondary bacterial infection associated with pustular demodicosis may require treatment with antibiotics.
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|Wikispecies has information related to: Demodex|
- 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
- Demodicosis, an article by Manolette R Roque, MD
- Demodetic Mange in Dogs, by T. J. Dunn, Jr. DVM
- High resolution images of Demodex folliculorum