Mites (demodex mites)

Demodex folliculorum mite next to a human hair

Mites on human skin

Symptoms of mite infestation

Catching a mite infestation

Prevention / remedies / cures / treatment for mites

References

There are about 65 known species of demodex mites. Only two of those species have been identified as regularly living on humans: demodex folliculorum and demodex brevis. Demodex canis lives predominantly on domestic dogs, but can occasionally infest humans.

Mites on human skin

Demodex mites live in hair follicles and sebaceous (oil) glands on the skin. Sometimes they are known as eyelash mites. Both species are primarily found on the face near the nose, the eyebrows and eyelashes. They also occur elsewhere on the body, such as around the ears, on the scalp, and in skin folds.

Demodex folliculorum mites live in the hair follicles. The adult mites are between 0.3 mm and 0.4 mm long, with a semi-transparent body. You can see fully grown adults with the naked eye, but a microscope is useful for a more reliable examination.

Demodex brevis live in the sebaceous glands connected to hair follicles. The adult mites are up to 0.2 mm long, with a semi-transparent body. You need a microscope to examine a plucked hair or thick oil squeezed from a gland to check for infestation.

The mites eat skin-cells, hormones and oils (sebum) which accumulate in the hair follicles. Their digestive system is so efficient and results in so little waste that they have no excretory orifice. At night they leave the hair follicles and come out to mate. Eggs are laid deep in the hair follicles or oil glands, and are impossible to wash out. You have to wait for them to hatch, and then treat the mites.

Mites' eggs, clawed legs, spiny mouthparts, salivary enzymes, and particularly their decaying bodies after they die in a follicle, could all provoke an immune response, but this generally doesn't seem to happen. It is possible that they could even have a beneficial effect by eating bacteria or other micro-organisms in the follicles.

The eggs are laid 15-20 days after mating, and the legged larvae hatch 3-4 days after the eggs are laid. It takes about a week for the larvae to develop into adults. The total lifespan of a demodex mite is about two weeks. The dead mites decompose inside the hair follicles or sebaceous glands. Mite treatment must last 12 weeks.

Catching a mite infestation

Most humans acquire mites shortly after birth, and they form part of the normal commensal skin faunae for the rest of their life. (1) At first there are only a few mites, but during adolescence when the prolific sebaceous glands produce more food for the mites, the mite population can increase. The number of mites on everyone's skin increases with age. (2) About a third of children and young adults, half of adults, and two-thirds of elderly people carry the mites. (3) About a third of children and young adults, half of adults, and two-thirds of elderly people are estimated to carry the mites. (3)

Mites usually only come out of their follicles at night, as they try to avoid light. They move very slowly, about 1 cm (half an inch) per hour. Mites transfer between people by skin and hair contact, particularly by sleeping with a person in close contact at night. Demodex canis resides deep in the dog's skin, transmission from one animal to another is usually only usually possible with prolonged direct contact, such as mother-to-pup transmission during suckling.

Demodex mites can only live for a couple of hours away from their host in a dry environment. In a wet environment like a damp towel they can survive for longer. They can live for up to 58 hours in a drop of edible oil, and perhaps even longer in an animal fat that resembles the sebum from a sebaceous gland.

This means it is possible to catch a demodex mite infestation by sharing recently-used towels and bedding. However, a good airing in the sun or a cycle in the tumble dryer would make it virtually impossible to transmit an infection. It also means that you would not get an infection from chairs, carpets, furniture etc.

Symptoms of mite infestation

For the majority of people, mites cause no symptoms (the mites are commensal). (1) It seems that in older people, those with suppressed immune systems (caused by stress or illness), or possibly those taking antibiotics or with compromised skin faunae, the mite population can sometimes dramatically increase and get out of control. Studies show that mite infestations are more common in people with HIV, children with leukaemia, or patients on immunosuppressive drugs. Men are more prone to mite infestation than women because they have more sebaceous glands which provide food for the mites. (2)

The infestation causes a condition known as demodicosis or mite bite, characterised by itching, inflammation and other skin disorders. The "insects under the skin" feeling is known as formication.

Acarophobia is the fear of mites and other microorganisms that cause itching. People with acarophobia respond well to "getting away from it all", particularly a relaxing vacation with lots of bare skin exposure at a sunny beach. A follow-up examination of their follicles after the holiday usually shows some level of infestation remaining, but the bright light, boost to vitamin D, natural salt water, reduction in stress and boost to the immune system are usually beneficial.

Prevention / remedies / cures / treatment for mites

Demodex brevis mites live deep in the sebaceous glands and don't come to the surface as often as demodex folliculorum. This means d. brevis are more difficult to eliminate. Their ovum and nymphs reside deep in follicles and are covered with a membrane which nothing (including pesticides, tea tree oil and antibiotics such as ivermectin) can penetrate. The time from mating until a new generation hatches is 15-20 days, so it is essential to treat an infestation over a couple of cycles to catch the adults and prevent them from mating again. I recommend a three month treatment period, because you have to assume that you have a d. brevis rather than d. folliculorum infection.

References

1. Albert M Kligman, Michael S Christensen. Demodex folliculorum: Requirements for Understanding Its Role in Human Skin Disease. Journal of Investigative Dermatology (2011) 131, 8-10. doi:10.1038/jid.2010.335.

2. Dirk M. Elston. Demodex mites: Facts and controversies 2010. Clinics in Dermatology 28 (5): 502-504. doi:10.1016/j.clindermatol.2010.03.006. PMID 20797509.

3. H. G. Sengbusch & J. W. Hauswirth. Prevalence of hair follicle mites, Demodex folliculorum and D. brevis (Acari: Demodicidae), in a selected human population in western New York, USA. 1986, Journal of Medical Entomology 23 (4): 384-388. PMID 3735343.

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Mite infestation on human skin, symptoms, feeding, breeding, treatment