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!
This is a very interesting article, I learned a lot of new things, thank you very much! The only error in the speed of movement of the Demodex is not cm, but mm.
1) I am interested in information about how demodex mites affect body odor. What shade of odor do they give to the body in the absence of demodicosis and with good hygiene?
2) Is it possible to force demodex to live in containers with old food fat and expired cosmetic creams + meat broth so that they eat this fat and process it into something else that can be useful? By analogy with tick-borne delicacy cheeses, as well as spider excrement, which is completely different from ordinary animal excrement.
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 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.
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.
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.
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.
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:
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.
Cleaners:
Medicines:
Makeup:
Tattoos:
Light therapy:
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.
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.
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.
I'm so glad you asked:
Also, a really neat genetic study found that:
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:
Excellent.
Excellent.