This article contains information about the human body that some readers may find uncomfortable. There is nothing graphic, but there is detailed information about a genus of human parasites.
- Demodex mites are arachnids who live their entire life cycle on/in human skin. Basically every human seems to have numerous mites living on them. Rough estimates suggest that there could be thousands or even millions mites living on each human face, meaning trillions living on all human faces.
- It seems likely that Demodex mites are frequently killed by human activities, including bathing, tattoos, the use of light therapy, the use of some medicines, and possibly makeup (though I wasn't able to confirm the effects of makeup with experts).
- Given the scale and neglectedness, this may make Demodex mites a potentially impactful focus for future animal advocacy strategy (though perhaps less impactful than other wild invertebrate interventions). This could be relevant for EAs and researchers interested in wild animal suffering. The first step would be to do a bit more research on some key uncertainties.
- It's unclear whether Demodex mites are sentient because nobody has researched this question. Given evidence of sentience in other groups of invertebrates and the immense number of Demodex mites who are alive at any one time, it seems plausible that we should be giving serious consideration to their interests.
- Research direction: A literature review on arachnid sentience, in the style of Gibbons et al (2022), would be helpful here.
- Another uncertainty is whether Demodex mites are on parts of the bodies that have not been well-sampled in scientific studies (e.g. arms, hands, legs, stomach).
- Research direction: It would be quite easy to resolve this using a lens that you can buy for ~$420 USD plus your smartphone. This could be a great little project for a small team of volunteers, like a university EA group.
- Beyond the implications for animal advocacy movement strategy, Demodex mites may also have implications for the ethics of personal lifestyles. I'm updating in the following ways: I will no longer conduct activities that could kill Demodex mites that I do purely for pleasure (for me, this includes tattoos on my head and upper body, and maybe face makeup). I will continue to do essential activities like bathing and receiving health treatments (e.g. laser therapy, antibiotics) despite the possibility that doing so will kill Demodex mites.
- You may or may not want to do anything about this. After all, many human activities similarly affect the lives of invertebrates (e.g. driving, composting food, gardening, sanitation).
- I know having concern for the interests of skin mites might seem pretty wild to many people. To me, trying not to kill Demodex mites seems comparable to trying not to step on ants when it can be avoided, which is something that many people (particularly EAs and animal advocates) find sensible. Obviously, this hinges on one's beliefs about the moral value of mites, compared to other animals.
What are they?
Demodex mites are arachnids who live on and in human skin.
There are two main species that live on humans: Demodex folliculorum and D. brevis. There are other species that live on other animals.
How common are they?
It is very plausible that 100% of humans have Demodex mites (other than newborn babies).
Many studies that have recorded prevalences around 20-80% using visual identification methods.
But one study found genetic evidence of Demodex mites in 100% of humans participating in the study.
There are two possible explanations: genetic sampling is better than visual identification, so ~100% of humans have Demodex mites; or the humans in the sample in the genetic study happened to have an unusually high prevalence.
The hypothesis that ~100% of humans have skin mites is supported by cadaver studies, as well as a 1993 study using a particularly sensitive method - these studies have also found mites in ~100% of participants.
Mites spread between humans via direct contact or by sharing bedding, towels and cosmetics. Demodex mites depend on their hosts for survival. If they leave the human body, they will die within a few days at most. Mite numbers increase with a human's age.
Where are they found?
Demodex mites are most commonly found on the face, scalp, and neck. Less commonly, they have also been found on the chest and back, buttocks, and around/on the genitals. An expert told me via email that they are probably found on other areas of the body too, but it is difficult to be certain as studies have not specifically tested these (e.g. arms, legs, hands, stomach).
D. folliculorum usually lives in hair follicles, typically in the eyelashes, nose, ears, facial hair, and eyebrows, either as single mites or in groups of up to four mites. D. brevis usually lives in sebaceous glands and ducts, and are found as single mites. A 1985 study found around 200 mites in one meibomian gland. D. folliculorum exists in greater numbers on the human body, but D. brevis is more widely distributed around the body.
Demodex mites seem to be sensitive to light, so they usually burrow down into the follicle and come out only at night. The mites seem to occur on the skin's surface more often during the nymphal live stage, though all life stages are sensitive to light. When they're on the skin surface, they walk at around 8 to 16 cm/hour.
There is one report of an abundance of Demodex being limited to the area of a patient's tattoo, but it's unclear whether this is a common occurrence.
What is their life cycle like?
Demodex mites have two sexes (female and male). Mites mate in the follicle opening. The female moves into the hair follicles or sebaceous glands, where the eggs are laid (~20 eggs).
After 3-4 days, the larvae hatch. The larvae undergo a few stages of development, and in around 7 days transform into adults, which survive for around 5 more days. The total life cycle is around 14-18 days, including the egg, larval, and adult stages. Dead mites decompose inside the hair follicles or the sebaceous glands.
Are Demodex mites sentient?
Strictly speaking, it's unclear whether Demodex mites are sentient. But this is mostly because nobody has researched this question - for many groups of invertebrates where people have looked (e.g. insect groups), the science does support these groups being sentient. To me, it seems plausible enough that Demodex mites are sentient that we should be giving serious consideration to their interests.
To provide a bit more detail: Demodex mites are arachnids, which is the class that also contains spiders and scorpions. Arachnids are arthropods, which is the phylum that also contains insects, crustaceans, millipedes, extinct trilobites, etc.
When it comes to sentience or the capacity to feel pain, there is limited information beyond a passing reference to arachnids (e.g. Mikhalevich and Powell 2020, Sømme 2005).
Brian Tomasik tried to look into the cognitive abilities of mites in 2016. You can watch Brian's video about a 2003 study about learning in spider mites here, or a general discussion on dust mite cognition in this video here (34:00 onwards).
I contacted an invertebrate sentience expert via email, and they told me:
- There has not been a formal scientific review on whether arachnids are sentient,
- There is probably sufficient evidence throughout the scientific literature that arachnids meet at least some of the criteria for consciousness, if somebody were to conduct such a review, and
- This evidence would probably be limited to spiders and scorpions, with very little on mites.
Demodex mites are much smaller than other arachnids and may have different brains / cognitive systems. So even if some arachnids are sentient, this is not necessarily a guarantee that mites are also sentient.
A recent review found strong or substantial evidence for the capacity to feel pain in many groups of insects. The UK government now recognises some crustaceans as sentient, based on scientific research. To be clear, Demodex mites are not insects or crustaceans. But this suggests that it's possible that other groups of arthropods might be capable of feeling pain.
How are Demodex mites affected by different activities?
- Li et al (2022) identify studies that used cleansers to kill Demodex mites. These include baby shampoo and Ocusoft eyelid scrub.
- These generally seem to kill Demodex mites.
- It seems likely to me that using cleansers on your face or other body parts would kill the mites living there, but I'm not certain.
- Li et al (2022) identify these medicines in the systematic review of Demodex treatments: tea tree oil (essential oil used as natural medicine to treat skin conditions), ivermectin (anti-parastic drug, taken as a pill or cream), permethrin (insecticide cream), crotamiton (anti-scabies cream), and metronidazole (antibiotic pill or cream).
- Most of these seem to kill Demodex mites.
- Sędzikowska et al (2021) suggest that mites can spread when people share makeup like eyeliner and lipstick. The authors looked at the average survival times of mites taken from the face and placed into different makeups. However, their study did not actually test whether applying makeup to the face can remove mites from the face in the first place. I was unable to confirm with experts whether applying makeup to the face would remove mites from the face.
- An expert told me via email that Demodex mites would probably be killed if a tattoo needle hits them. The expert was unsure if the ink would affect the mites in any way.
- Sławinska et al (2021) documented a case where a patient had numerous Demodex mites living within the space of a tattoo. I'm speculating here, but if it represents a harm to bring invertebrates like Demodex mites into existence (as people like Brian Tomasik argue, and as I generally agree with), then getting a tattoo that causes more Demodex mites to exist could be morally bad. But it's unclear whether the Demodex-filled tattoo as observed by Sławinska et al is common or rare, as the authors themselves conclude. There is a hypothesis that the ink reduces the effectiveness of the immune system, which can allow the Demodex mites to spread.
- Light therapy is commonly used to kill Demodex mites in patients.
- Li et al (2022) identify a few different types of light therapy: intense pulsed light (multiple wavelengths), pro-yellow laser (wavelength of 577 nm), and pulsed dye laser (585 nm).
- Scientists aren't sure how this kills the mites - it could be because of the high temperatures, or it could be because of the mites' biological vulnerabilities to light. Personally, I've had laser hair removal treatment on my face, and it feels very hot on the skin.
- It seems likely to me that laser hair removal would kill the mites on one's face, but I'm not certain. I don't know if the mites would be affected differently by different wavelengths or intensities of light.
Even if killing face mites is morally bad (which I think it is), it is unclear whether killing them would reduce their populations - and hence their suffering - in the long run. If so, then under purely utilitarian considerations, the suffering prevented by reducing face mites' populations in the long run might outweigh the suffering caused by killing additional mites in the short term.
What do they look like?
D. folliculorum is around 0.3 to 0.4 mm long, and D. brevis is around 0.2 to 0.3 mm long.
There is a video available here, taken with a smartphone and lens by Kaya and Gürdal 2018.
Here are some neat photos, which I've copied from Bitton et al 2021, Chang and Huang 2017, and Jing et al 2005.
Kaya and Gürdal (2018) published a way to photograph/video Demodex mites using a smartphone and a lens. The lens you need is this one, and it seems to sell for ~$420 USD. This could be a relatively cheap way to run future studies on Demodex mites (e.g. if you wanted to apply for a microgrant from one of the grantmakers in animal advocacy to buy a lens like this and do some research on yourself and your friends). If you do this, please be careful to minimise the effects of your research on the mites.
That said, even if we limit ourselves to highly neglected groups of invertebrates, there might be other areas of research that are a higher priority than face mites. Interventions focused on soil invertebrates or zooplankton might have a much higher impact than interventions focused on face mites.
Do they cause disease in humans?
Demodex mites are associated with a range of skin and eye diseases. However, their role in actually causing these diseases is debated by scientists. In normal circumstances, the mites probably don't cause symptoms - after all, every human seems to have them.
What are some other neat Demodex facts?
I'm so glad you asked:
- Demodex mites are the most common microscopic ectoparasite on the human skin.
- Different types of mammals have their own species of Demodex mites: cats (Demodex cati), cattle (Demodex bovis), goats (Demodex caprae), sheep (Demodex ovis), rats (Demodex ratti), and others.
- The first scientific records of Demodex mites are from the 1840's from a professor of anatomy in Zurich named Jakub Henle and a German dermatologist named G. Simon.
- Clanner-Engelshofen (2022) were the first to discover a way to cultivate a population of Demodex mites in the lab.
Also, a really neat genetic study found that:
- The genetic divergence between D. folliculorum and D. brevis seems to be relatively ancient.
- D. folliculorum shows less genetic diversity than D. brevis. In fact, there was more genetic diversity between D. brevis mites collected from the same human than between D. folliculorum mites collected from the Americas and China.
- The high genetic diversity in D. brevis might be because this species lives deeper in the human skin (sebaceous glands) than D. folliculorum does (hair follicles), so D. brevis likely experiences greater reproductive isolation.
- The genetic diversity between D. brevis mites collected from the same human suggests that each human is likely colonised multiple times.
Acknowledgements and references
Thank you to the experts I consulted via email.
Thank you to Brian Tomasik for introducing me to Demodex mites in this article. This is the only source that I'm aware of that expresses concern for the interests of Demodex mites.
While writing this article, I consulted these academic sources:
- Bitton, Etty, and Sarah Aumond. 2021. “Demodex and Eye Disease.” Clinical & Experimental Optometry: Journal of the Australian Optometrical Association 104 (3): 285–94.
- Chang, Yin-Shuo, and Yu-Chen Huang. 2017. “Role of Demodex Mite Infestation in Rosacea: A Systematic Review and Meta-Analysis.” Journal of the American Academy of Dermatology 77 (3): 441–47.e6.
- Cheng, Anny M. S., Hosam Sheha, and Scheffer C. G. Tseng. 2015. “Recent Advances on Ocular Demodex Infestation.” Current Opinion in Ophthalmology 26 (4): 295–300.
- Clanner-Engelshofen, B. M., L. M. Ständer, T. Steegmüller, T. Kämmerer, L. H. Frommherz, P-C Stadler, A. Gürtler, and M. Reinholz. 2022. “First Ex Vivo Cultivation of Human Demodex Mites and Evaluation of Different Drugs on Mite Proliferation.” Journal of the European Academy of Dermatology and Venereology: JEADV 36 (12): 2499–2503.
- Dopytalska, Klaudia, Katarzyna Lipa, Piotr Sobolewski, Elżbieta Szymańska, and Irena Walecka. 2019. “Role of Demodex Folliculorum in Dermatology.” Przeglad Dermatologiczny 106 (5): 507–14.
- Elston, Dirk M. 2010. “Demodex Mites: Facts and Controversies.” Clinics in Dermatology 28 (5): 502–4.
- Fromstein, Stephanie R., Jennifer S. Harthan, Jaymeni Patel, and Dominick L. Opitz. 2018. “Demodex Blepharitis: Clinical Perspectives.” Clinical Optometry 10 (July): 57–63.
- Hom, Milton M., Katherine M. Mastrota, and Scott E. Schachter. 2013. “Demodex.” Optometry and Vision Science: Official Publication of the American Academy of Optometry 90 (7): e198–205.
- O’Donel Alexander, John. 1984. “Hair Follicle Mites in Man.” In Arthropods and Human Skin, edited by John O’Donel Alexander, 293–302. London: Springer London.
- Rather, Parvaiz Anwar, and Iffat Hassan. 2014. “Human Demodex Mite: The Versatile Mite of Dermatological Importance.” Indian Journal of Dermatology 59 (1): 60–66.
- Rusiecka-Ziółkowska, Jolanta, Marta Nokiel, and Małgorzata Fleischer. 2014. “Demodex - an Old Pathogen or a New One?” Advances in Clinical and Experimental Medicine: Official Organ Wroclaw Medical University 23 (2): 295–98.
- Thoemmes, Megan S., Daniel J. Fergus, Julie Urban, Michelle Trautwein, and Robert R. Dunn. 2014. “Ubiquity and Diversity of Human-Associated Demodex Mites.” PloS One 9 (8): e106265.
- Jing, Xu, Guo Shuling, and Liu Ying. 2005. “Environmental Scanning Electron Microscopy Observation of the Ultrastructure of Demodex.” Microscopy Research and Technique 68 (5): 284–89.
- Kaya, Abdullah, and Canan Gürdal. 2018. “Office-Based Diagnosis of Demodex Using Smartphone.” Eye & Contact Lens 44 (6): e25–26.
- Li, Jiahua, Erdong Wei, Amin Reisinger, Lars Einar French, Benjamin M. Clanner-Engelshofen, and Markus Reinholz. 2022. “Comparison of Different Anti-Demodex Strategies: A Systematic Review and Meta-Analysis.” Dermatology , October, 1–20.
- Mikhalevich, Irina, and Russell Powell. 2020. “Minds without Spines: Evolutionarily Inclusive Animal Ethics.” Animal Sentience 29 (1). https://scholarworks.rit.edu/article/1993/.
- Sędzikowska, Aleksandra, Katarzyna Bartosik, Renata Przydatek-Tyrajska, and Monika Dybicz. 2021. “Shared Makeup Cosmetics as a Route of Demodex Folliculorum Infections.” Acta Parasitologica / Witold Stefanski Institute of Parasitology, Warszawa, Poland 66 (2): 631–37.
- Sømme, Lauritz S. 2005. “Sentience and Pain in Invertebrates.” Report to Norwegian Scientific Committee for Food Safety. https://www.seafoodnews.com/news/images/norwegianinvertebrate.pdf.
- Gibbons, Matilda, Andrew Crump, Meghan Barrett, Sajedeh Sarlak, Jonathan Birch, and Lars Chittka. 2022. “Chapter Three - Can Insects Feel Pain? A Review of the Neural and Behavioural Evidence.” In Advances in Insect Physiology, edited by Russell Jurenka, 63:155–229. Academic Press.
Wow, thank you for writing this. I'm really interested in parasites and their potential suffering generally and this seems like an area where a greater understanding and reasonable, welfare-promoting/population-abolishing intervention would also possibly have public appeal from a "yuck" perspective.
Thanks for writing this. I mentioned to someone that EA hasn't made a lot of progress on finding "cause x" and they said "oh really?" and linked to this post.
Interesting, thanks for writing this!
If there are 1 k to 1 M Demodex mites in each human, and each mite has 1 k neurons (spiders have 100 k according to this), mites in each human total 1 M to 1 G neurons. According to this, fruit flies and chickens have 100 k and 221 M neurons, so the mites in each human have as many neurons as:
Moreover, the number of neurons severely underestimates the moral weight of arthropods. According to Rethink Priorities' recent estimates, the median moral weight of black soldier flies is 0.013, which is 10 k times as large as the weight of 10^-6 (= 100 k / 100 G) suggested by direct comparison with the 100 G neurons of each human.
For these reasons, I would not be surprised if the scale of the welfare of Demodex mites exceeds that of factory-farmed animals. One could say this latter problem is more tractable, but mites being way more neglected makes me think they are super underrated.
I was pretty sure this was written by Brain Tomasik, until he was mentioned in the text :)
Very mindblowing piece, thanks to the author!
At an event recently someone gave the example of 'we shouldn't walk on grass because we might kill ants' and how intial reactions were to sort of scoff but after the first person made his case it was clear they had thought about this lots. I remember the example as illustrating that tension between intial somewhat emotional reactions and then really sitting and thinking about it.
Not sure where I fall on demodex mites, since I think sentience and ability to feel pain are big ones for me but this was still a cool text to use as a reflection!