Field of Science

Your Little Friends that are With You Always


Demodex folliculorum, male above and female below, from Desch and Nutting (1972).


At all times, you are surrounded by life. Micro-organisms swarm on your skin, swim in your gut, and set up shop in your organs. Indeed, at any one time, there are considerably more microbial cells around your person than there even are of your own cells. The micro-organisms are not living on you - you are living among the micro-organisms.

They're not all bacteria, either. I just thought that I'd briefly introduce you to one of the more distinctive micro-organisms that you are almost certainly carrying about with you - the follicle mite. I also challenge you to keep from scratching yourself while reading this.


Demodex brevis, male on left and female on right, from Desch and Nutting (1972).
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Demodex, the follicle mite, is a specialised inhabitant of the follicles and pores of mammalian skin. A significant number of species appear to have been described from different hosts, ranging from humans to dogs to cattle to marsupial mice to honest-to-goodness mice. The most distinctive feature of Demodex is its elongate body shape, which allows it to live head-first inside the follicles of its host, feeding on cells within the follicle. Humans are actually inhabited by two species, Demodex folliculorum and D. brevis (Desch & Nutting, 1972). The more elongate D. folliculorum lives in the follicles themselves, and feeds on the epithelial lining. The rarer D. brevis is shorter, and is found within the sebaceous gland on which it feeds. Demodex brevis is found on less people, and also at lower numbers - multiple D. folliculorum may be found in a single follicle (as shown below in an image from here), but usually only one D. brevis.




The question of whether Demodex causes any harm to its human host is a difficult one. The sheer universality of Demodex within the human population implies that its presence is usually of no concern to the host. However, another species, D. canis, is widely connected with mange in dogs, and Demodex has been connected with skin disorders (demodicidosis) in a number of humans. The difficult question is whether Demodex is a direct causative agent or not. As one might reasonably expect Demodex to be present anyway, it's mere presence at an infection site does not automatically indicate its responsibility for the infection. It seems likely that Demodex may be a facultative agitator of problems arising from other ultimate causes, such as a suppressed immune system (Jansen et al., 2001) or an already-damaged follicle (Pena & Andrade Filho, 2000). It is noteworthy in this light that prevalence of demodicidosis varies seasonally (it is most common in spring), but the prevalence of Demodex itself does not (Desch & Nutting, 1972). Demodex folliculorum may cause damage when more than six individuals are present in the same follicle (Desch & Nutting, 1972).

REFERENCES

Desch, C., & W. B. Nutting. 1972. Demodex folliculorum (Simon) and D. brevis Akbulatova of man: redescription and reevaluation. Journal of Parasitology 58 (1): 169-177.

Jansen, T., U. Kastner, A. Kreuter & P. Altmeyer. 2001. Rosacea-like demodicidosis associated with acquired immunodeficiency syndrome. British Journal of Dermatology 144 (1): 139–142.

Pena, G. P., & J. de S. Andrade Filho. 2000. Is Demodex really non-pathogenic? Revista do Instituto de Medicina Tropical de São Paulo 42 (3): 171-173.

11 comments:

  1. Yeah, there was a great deal of itching and scratching while reading that article. Weird looking critters, though. I'd imagine that a few dozen (hundred? thousand?) individuals are hitchhiking around on my body RIGHT NOW, eh?

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  2. Supposedly, every species of mammal (except whales) has its own Demodex species, reportedly up to 16 on some species. Each species lives in its own follicular or glandular niche.

    There are some good instructions for viewing your own Demodex in E.O.Wilson's "The Diversity Of Life" (from when I showed him mine.) Whenever I need a handy science demonstration, that one is sure to please.

    One of the nicer things about these (and if I remember right, it's characteristic of the entire Prostigmata), is that the gut doesn't end in an anus: it's a dead end. They are terminally constipated. This is a handy thing for a parasite: no wastes irritating the host.

    The other genera in the family Demodicidae live in a diverse variety of niches in their hosts. A strange one that I saw and photographed was an Ophthalmodex that lived in the vitreous humor of cats. It was the most transparent mite I'd ever seen, and almost impossible to see mounted on the slide. Judging from the list linked above, it was never published: I think the discoverer, Lukoshus (who showed it to me), died shortly after finding it.

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  3. Interestingly, Demodex zalophi is found on sea lions, so its the lack of hair in cetaceans rather than the aquatic lifestyle that means they miss out. One thing that I couldn't find when I was searching yesterday was any sort of phylogenetic study of Demodex - it would be interesting to see if the relationships between species line up with the phylogeny of the mammalian hosts, or if there's any noticeable pattern to when transfer between species can occur.

    I did find a number of references to the blind gut, but all were on sites such as Wikipedia. I couldn't find a proper reference to back it up, so I decided to leave it. I would have liked to have known how excretion does take place - I'm guessing that all the wastes produced are able to be handled by the nephridia?

    Do you know what the best method of getting Demodex specimens is? I'm considering it for an arachnid diversity lab next week.

    Finally:

    Judging from the list linked above, it was never published: I think the discoverer, Lukoshus (who showed it to me), died shortly after finding it.

    The tragedy of the Great Taxonomy Crisis strikes again. It's cases like that that show why the agin population of taxonomists is such a problem. And we're not talking mere academic interest here - I suspect that anything living in the vitreous humour of a cat's eye would be well worth investigating to see if it had any veterinary significance.

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  4. Supposedly, every species of mammal (except whales) has its own Demodex species, reportedly up to 16 on some species. Each species lives in its own follicular or glandular niche.

    So that's 5k species of Demodex minimum? Talk about big genera ...

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  5. One of these days I am going to look for Demodex on my own face. Perhaps, a few may be lurking under my eye brows.

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  6. No waste products? How's THAT work?

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  7. I'm pretty certain that Demodex would produce waste products of some kind. What it doesn't necessarily produce is solid waste products that require evacuation direct from the digestive system via an anus. Demodex's diet is almost entirely liquids or semi-liquids, and if only soluble wastes are produced then, as I said, the nephridia could probably handle all excretion.

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  8. David Marjanović12 April 2008 08:13

    In the vitreous humor? Does that mean in the eyeball?!?

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  9. To extract Demodex, I place my fingers of the left hand on my forehead and stretch the skin to my left side. I scrape slowly with my right hand away from my fingers, with the 3 inch edge of a glass slide held perpendicular to my forehead, applying substantial pressure. The goal is to squeeze sebum from deep in the follicles out onto the edge of the slide without smearing it back onto the forehead. View at roughly 100x after mixing with some oil.

    Maybe it's practice, maybe it's my skin. I have no trouble extracting them from myself. The few times I've demonstrated how to do it, not everybody has been able to, and I haven't been able to extract them from everybody. However Desch and Nutting have reported finding Demodex by sectioning on essentially every corpse they've inspected, if I recall correctly.

    One caution: applying pressure to your forehead with a thin piece of glass is risky. When I gave a demo to the NY Microscopical Society, I accidentally
    snapped the slide between the three fingers holding it, and cut my forehead in front of 70 members who I was expecting to try this. :-) Fortunately, I didn't react except with an oops, dab, don't do that, and didn't have any problems. Since then, I use two fingers....

    I really ought to demonstrate this on youtube.

    What I remember from roughly 30 years ago is that Demodex's waste products are stored as guanine: they are not excreted. When you examine living specimens, there's often crystalline-looking material inside. Too bad my Krantz's "Acarology" is packed away.

    And yes, the Ophthalmodex I saw was from the vitreous humor of cat eyeballs. The specimens are at the Ohio State Acarology Collection, if anybody wants to follow up.

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  10. is there any way to kill these things, like by rubbing alcohol or citric acid on your skin?

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  11. Note: I am not a doctor.

    I'm sure that if you did smear rubbing alcohol over your forehead it would kill off a large number of Demodex. However, I suspect that doing so would be a really, really bad idea, because (A) even if you did remove your own Demodex population, you'd probably be recolonised pretty quickly, so it's rather futile; and (B) seeing as Demodex are usually all but harmless, any benefit to removing them would be far outweighed by the strong negative effects of rubbing a potent dehydrating solution on your forehead.

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