Demodex folliculorum

This parasite was first discovered in 1841 by a doctor Henle - a famous chap in hair biology circles who has a component of the hair follicle named after him (the Henle layer of the inner root sheath since you ask). Demodex folliculorum is a microscopic creature in the form of an elongated and jointed worm, with clearly defined cephalic, thoracic, and abdominal portions; with eight legs, four on each side of the thorax, each leg with three articulations, and terminating in three small hooklets. The posterior extremity of the body is a vermiform appendage, terminating in a conical point. In short, it looks nasty.

Demodex folliculorum is sometimes found on the surface of the skin, particularly on those parts of the skin where the sebaceous glands are large, and on individuals affected with acne or seborrhea oleosa. Demodex feeds on the skin sebum and oils so the more sebum around the greater the number of Demodex folliculorum that can survive. While it is sometimes found on the surface of the skin, it is more commonly encountered in the substance of hair follicle comedo plugs (blackheads), where five to twenty worms may be discovered in a single follicle. A similar species which is considered to be a variety of that discovered upon the skin of man infests dogs, mice and other mammals. However, none of these related demodex species are known to be transmissible to humans.

You might not like the idea, but Demodex folliculorum infection is very common. Around 80% of the adult population, both men and women, have a Demodex folliculorum infection. It is believed that the frequency Demodex folliculorum is less in children. It is rarely found in children under 5 years old. Between 5 and 10 years, it is found in around 50% of children, while between the ages of 10 and 20 the percentage is about the same as in adults. So it would seem that as we get older we become infected with Demodex folliculorum probably through contact with adults. As such, it is more or less impossible to conciously avoid infection by Demodex folliculorum.

Demodex folliculorum and hair loss

Unfortunately over many years there have been sporadic claims made about Demodex folliculorum and hair loss. Quite recently a theory has been put forward that infection of hair follicles by the Demodex folliculorum parasite is correlated with pattern baldness. By infiltrating the sebaceous gland of hair follicles the parasite causes an immune response and inflammation of surrounding tissue - so it is claimed. Through long term invasion, the parasite "exhausts" the hair bulb and shifts the hair cycle from anagen to telogen. This theory is promoted by certain companies that just happen to have a range of products that kill off Demodex folliculorum! At least one of the companies has conducted research that allegedly showed 88% of 240 men with pattern baldness had Demodex folliculorum infection in their hair follicles. However, there are several problems with this theory.

First, as above, just about everyone has Demodex folliculorum whether they have hair loss or not. The company conducting the research conveniently forgot to study normal haired people to find the frequency of Demodex folliculorum in people with a full head of hair. Second, research has shown that there are frequently excessive numbers of the Demodex folliculorum parasite in eyelash follicles. However, people with pattern baldness do not lose their eyelash hairs. Third, there is a clear bias towards men having pattern baldness hair loss even though women are equally susceptible to Demodex folliculorum infection and at least some children are also infected. If Demodex folliculorum infection was causing inflammation that pushed hair follicles into telogen then one would expect to see some children with pattern baldness and that women would be equally affected with hair loss. Indeed, women have a stronger immune system and so one might expect more women than men to have pattern baldness, but this is not the case. Fourth, if Demodex folliculorum was a key cause of pattern baldness it would be impossible for hair follicle transplants to work. Given thousands of transplant procedures are done every year and 80% of men have Demodex folliculorum infection, then most of the men with hair transplants must also have a Demodex folliculorum infection. In a hair transplant, follicles are moved from the back of the head to the top of the same individual. If the individual is infected with Demodex folliculorum one would expect that the transplanted follicles would either already be infected, or become infected in their new location. However, transplanted follicles grow in every individual who has had the procedure done and pattern baldness does not redevelop. Fifth, there is a widely available treatment to Demodex folliculorum infection called pilocarpine gel. However, using this gel on the skin does not promote any hair growth.

The more likely explanation for Demodex folliculorum infection in people with pattern baldness is that the sebaceous glands of alopecia affected hair follicles become larger and more active, producing oils at a faster rate, under the influence of dihydrotestosterone (DHT). The oils combine with dead cells from the hair follicle to make sebum. The sebum is a rich source of nutrients and this is the food that Demodex folliculorum eats. The oil food supply increases in most hair follicles affected by pattern baldness so these hair follicles can accommodate a greater number of Demodex folliculorum parasites. Rather than the parasites causing pattern alopecia, they simply take advantage of the increased food produced. The parasite infection is a consequence of pattern baldness rather than a cause of the hair loss.

Demodex folliculorum references

• Baima B, Sticherling M. Demodicidosis revisited. Acta Derm Venereol. 2002; 82(1): 3-6.
• Georgala S, Katoulis AC, Kylafis GD, Koumantaki-Mathioudaki E, Georgala C, Aroni K. Increased density of Demodex folliculorum and evidence of delayed hypersensitivity reaction in subjects with papulopustular rosacea. J Eur Acad Dermatol Venereol. 2001 Sep; 15(5): 441-4.
• Aydingoz IE, Dervent B, Guney O. Demodex folliculorum in pregnancy. Int J Dermatol. 2000 Oct; 39(10): 743-5.
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• Forton F. Demodex-associated folliculitis. Am J Dermatopathol. 1998 Oct; 20(5): 536-7.
• Fulk GW, Murphy B, Robins MD. Pilocarpine gel for the treatment of demodicosis--a case series. Optom Vis Sci. 1996 Dec; 73(12): 742-5.
• Vollmer RT. Demodex-associated folliculitis. Am J Dermatopathol. 1996 Dec; 18(6): 589-91.
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• Huismans H. [Demodex folliculorum] Klin Monatsbl Augenheilkd. 1988 Sep; 193(3): 304-6.
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• Sengbusch HG, Hauswirth JW. Prevalence of hair follicle mites, Demodex folliculorum and d. brevis (Acari: Demodicidae), in a selected human population in western New York, USA. J Med Entomol. 1986 Jul 28; 23(4): 384-8.
• Forton F. [Demodex and perifollicular inflammation in man: review and report of 69 biopsies] Ann Dermatol Venereol. 1986; 113(11): 1047-58.
• Crosti C, Menni S, Sala F, Piccinno R. Demodectic infestation of the pilosebaceous follicle. J Cutan Pathol. 1983 Aug; 10(4): 257-61.
• Mills OH Jr, Kligman AM. The follicular biopsy. Dermatologica. 1983; 167(2): 57-63.
• Westermark P. Demodex folliculorum in human skin. Acta Derm Venereol. 1983; 63(2): 183-4. No abstract available.
• Aylesworth R, Vance JC. Demodex folliculorum and Demodex brevis in cutaneous biopsies. J Am Acad Dermatol. 1982 Nov; 7(5): 583-9.
• Norn MS. Incidence of Demodex folliculorum on skin of lids and nose. Acta Ophthalmol (Copenh). 1982 Aug; 60(4): 575-83.
• Andrews JR. The prevalence of hair follicle mites in caucasian New Zealanders. N Z Med J. 1982 Jul 14; 95(711): 451-3.
• Rufli T, Mumcuoglu Y. The hair follicle mites Demodex folliculorum and Demodex brevis: biology and medical importance. A review. Dermatologica. 1981; 162(1): 1-11.
• Roth AM. Demodex folliculorum in hair follicles of eyelid skin. Ann Ophthalmol. 1979 Jan; 11(1): 37-40.
• Riechers R, Kopf AW. Cutaneous infestation with Demodex folliculorum in man. J Invest Dermatol. 1969 Jan; 52(1): 103-6.
• Breckenridge RL. Infestation of the skin with Demodex folliculorum. Am J Clin Pathol. 1953 Apr; 23(4): 348-52.