Three things you didn't know about the arachnids that live on your face

August 28, 2014 by Matt Shipman, North Carolina State University
Demodex brevis. Credit: Dan Fergus and Megan Thoemmes.

You are not alone. Your body is a collection of microbes, fungi, viruses… and even other animals. In fact, you aren't even the only animal using your face. Right now, in the general vicinity of your nose, there are at least two species of microscopic mites living in your pores. You would expect scientists to know quite a lot about these animals (given that we share our faces with them), but we don't.

Here is what we do know: Demodex are microscopic arachnids (relatives of spiders and ticks) that live in and on the skin of mammals – including humans. They have been found on every mammal species where we've looked for them, except the platypus and their odd egg-laying relatives.

Often mammals appear to host more than one species, with some poor field mouse species housing four mite species on its face alone. Generally, these mites live out a benign coexistence with their hosts. But if that fine balance is disrupted, they are known to cause mange amongst our furry friends, and skin ailments like rosacea and blepharitis in humans. Most of us are simply content – if unaware – carriers of these spindly, eight-legged pore-dwellers.

Scientists from NC State, the North Carolina Museum of Natural Sciences, and the California Academy of Sciences have just published a study that uncovers some previously unknown truths regarding these little-known mites – all the while providing a glimpse into even bigger mysteries that have yet to be solved.

1. Everyone has mites.

One of our most exciting discoveries is that these mites are living on everyone. Yes everyone (even you). This hasn't always been obvious because it can be hard to find a microscopic mite living on one's face. Traditional sampling methods (including scraping or pulling a piece of tape off your face) only return mites on 10-25 percent of adults. The fact that mites are found at a much higher rate on cadavers (likely because the dead are easier to sample more extensively and intrusively) was a hint that they might be much more ubiquitous.

his is a Demodex folliculorum. It lives on your face. Credit: USDA, Confocal and Electron Microscopy Unit.

As it turns out, you don't have to actually see a mite to detect its presence. Dan Fergus, a mite molecular biologist at the North Carolina Museum of Natural Sciences, discovered that mite DNA could be sequenced from face scrapings regardless of whether a mite could be found under the microscope. And mite DNA was sequenced from every adult we sampled. Meaning that if you let us scrape your face, we'd find mite DNA on you as well. And where mite DNA is found, you'll find mites.

2. Humans host two mite species that aren't closely related to each other.

One of the most intriguing (and unsolved) face mite mysteries is how humans acquired these beasties. Perhaps these mites are a model system of co-evolution. It's possible that as every species of mammal evolved, so did their mites – each one particularly adapted to its changed environs. In such a case, we would expect that we acquired our mites from our ape ancestors, and that the two species of human mites would be more closely related to each other than to any other mite species.

However, we've learned that the two mite species on our faces Demodex folliculorum (the long skinny one, pictured at the top of this post) and Demodex brevis (the short, chubby one, photo to the right) are actually not very close relatives to each other at all. Our analyses actually show that brevis is more closely related to dog mites than to folliculorum, the other human mite. This is interesting because it shows us that humans have acquired each of these mite species in different ways, and that there are two separate histories of how each of these mite species came to be on our face.

Though we don't have enough evidence to say that we got one of our mites from man's best friend, it does seem possible that one of the domestic animal species that we've long shared our lives with (be it dogs, goats or otherwise) may have gifted us their mites.

3. Mites can tell us about the historical divergence of human populations

How we acquired our mites is just one part of the story. We are also curious about how our mite species have evolved since they became our constant companions.

Demodex have likely been living with us for a long, long time; as early humans walked out of Africa and found their way around the globe, they probably carried their mites with them. So we want to know if Demodex DNA can provide a reflection of our own evolutionary history by allowing us to retrace those ancient paths of human migration.

So far, our analyses look promising. When looking at the DNA from one of our mite species, D. brevis, we found that mites from China are genetically distinct from mites from the Americas. East Asians and European populations diverged over 40,000 years ago and so far it looks like their mites did as well. On the other hand, D. folliculorum from China is indistinguishable from that of the Americas. Of the two Demodex species associated with humans, D. brevis lives deeper in your pores than folliculorum and is probably shared between people less readily, whereas D. folliculorum appears to enjoy global domination.

But as exciting as these results are, China and the US are just a small piece of the picture. We can't wait to see what happens when we sample D. brevis from people all over the world! The ancient journey of Homo sapiens as retold by mites.

If reading this made your face a little itchy, rest easy. In an evolutionary perspective, humans and Demodex are old, old friends. You are in good company. And so are your mites.

Explore further: Bacterial cause found for skin condition rosacea

More information: Thoemmes MS, Fergus DJ, Urban J, Trautwein M, Dunn RR (2014) "Ubiquity and Diversity of Human-Associated Demodex Mites." PLoS ONE 9(8): e106265. DOI: 10.1371/journal.pone.0106265

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5 / 5 (7) Aug 28, 2014
So the cell count - ten foreign cells for every one of our own cells is included with this additional family member as we play host to the microbiome?

Why stop at human migration? Mites appear willing to reveal ALL mammalian life migrations hosting the guests your wonderful research has reveal.
4.6 / 5 (5) Aug 28, 2014
reveal = revealed
Last word is victim to an attention deficit in the comment posted above therefore the typo correction here - see equation.
5 / 5 (6) Aug 28, 2014
@russel_russel - these mites would contribute very little to the cell count because they are eukaryotes and thus have big cells.

Most of the non-human cells in a human are bacterial cells (and mostly living in our guts).
While the mass of a human's internal bacteria is very roughly 1% of the mass of a human,
bacteria are prokaryotes that typically have cells very roughly 100x smaller than eukaryote cells, so by NUMBER of cells they outnumber our human cells by roughly 10x,

However mites are eukaryotes and thus have cells roughly the size as human cells, so even if we had a kilogram of such mites, they would contribute only ~1% of the eukaryote cell count and ~0.1% of the total cell count in a human.

The fungi in our guts are also eukaryotes and hence don't contribute much to the total number of cells.

We also have archaea (other prokaryotes), but no where near as many as bacteria.

Viruses are also present in large numbers, but viruses are not cells.
not rated yet Aug 29, 2014
I am wondering about the possibility of using those animals for medicine, for example treating acne, or to diagnose skin conditions.
not rated yet Aug 30, 2014
Your readers will thank you for that comment. The answer to the question asked is not answered.
Think about what you wonder. What life forms will harbor or host a microbiome where the guests are harmful to the hosts?
The life forms than went "prematurely" extinct.

The conjecture implied here is that the mites have already exhibited a co-existent that at the very least is not detrimental to the host. The conclusion from the implied conjecture is any other condition other than favorable or a benign condition will eventually lead to premature extinction.
Why "premature" extinction?
Because statistically all life forms become extinct.
not rated yet Aug 30, 2014

Viruses are a form of life.


Lifeform or not?

Viruses have no ability to metabolize on their own, but depend upon a host organism for replication and manufacture of chemicals needed for such replication. Rybicki has characterized viruses as a form "at the edge of life."[3] Viruses are found in modern taxonomy, which considers viruses as a totally separate form of life from cellular organisms—some would say that they are merely complex molecules with a protein coating and not a lifeform at all. Since viruses are capable of self-replication, they are clearly some type of lifeform, and likely involved with the early evolutionary development of such other simple lifeforms as bacteria and protists.

Viruses differ, however, from the simpler autonomous replication of chemical crystals. This is due to the fact that a virus can inherit a genetic mutation and is also subject to similar natural selection processes of cellular organisms.

not rated yet Aug 30, 2014

A virus cannot be labelled simply, therefore, as inanimate or lifeless. Here, we consider it a lifeform, but we adhere to current taxonomy and do not credit it with a parallel domain to other recognized cellular lifeforms.

As a proponent of current taxonomy I have no choice. I adhere.
5 / 5 (1) Aug 30, 2014
The original question referred to cells, and viruses are not cells and do not have cells so they do not affect the ratio of ten foreign cells for every one of our own cells.

I agree that viruses are alive - that they borrow a cell's translation machinery rather than bringing their own is a relatively minor detail compared with replicating and evolving. Someone can be a driver without owning a car, or a violinist without owning a violin.
And some viruses have several times more genes than some bacteria do.

Viruses have evolved in families and even orders, similar to cellular life.
For example, single-stranded positive-sense viruses already have three orders and 33 families recognized.
And there are double-stranded RNA viruses and DNA viruses, too, so just as 'bacteria' were found to consist of two groups (bacteria and archaea) that have shared an ancestor in billions of years, viruses may be found to be several distinct kingdoms of life.
5 / 5 (1) Aug 31, 2014
Imagine you had the ratios of foreign guest life forms vs. life forms defined as those belonging to a host. For all life forms.

Can these ratios shed light on a host's evolution? Can such ratios shed light on all of life's evolutionary facets such as natural selection, adaptability, migration, etc., including extinction?

Is evolution possible without a host/guest relationship?
Is population genetics possible without a host/guest reationship?
Can current taxonomy classifications become easier with such ratios?

That's a glimpse of bigger pictures on the horizon. And we were both lucky enough to see the beginnings from research posted here in just one regard to the bigger pictures of evolution. Migration.

Drivers and players are no minor detail. They built us. We, who call ourselves hosts.
not rated yet Aug 31, 2014
Even more numerous than our bacterial inhabitants are mitochondria. While not INDEPENDENT cells today, evidence suggests that mitochondria were independent bacteria that entered an endosymbiotic relationship with eukaryotes (although whether eukaryotes had nuclei before mitochondria were captured is debated).
Our mitochondria have their own genomes, and, in contrast to viruses, still produce and use their own ribosomes, and still have their own membranes, and still divide on their own, so they have the key attributes of cells.

And our mitochondria outnumber our cells by over 100x, and hence outnumber our 'guests' by an order of magnitude.

Evolution and population genetics are possible without a host/guest relationship - viruses and some prokaryotes do not have guests, and they evolve and can be studied with population genetics.

The ratio of 'guests' to host cells should be related to evolution, but also moderated by diet (hard-to-digest food increases bacterial 'guests').
not rated yet Sep 01, 2014
[ - viruses and some prokaryotes do not have guests, and they evolve and can be studied with population genetics - rs

All viruses need a replicate. What evolves without replication? And becomes a population with reproducing? I can not follow you.
not rated yet Sep 01, 2014

- viruses and some prokaryotes do not have guests, and they evolve and can be studied with population genetics - rs

All viruses need a replicate. What evolves without replication? And becomes a population with reproducing? I can not follow you.

That viruses need a host to replicate shows that viruses ARE guests, not that viruses HAVE guests.

I do not know of any viruses that HAVE guests, although nature is so diverse that it wouldn't surprise me.
After all, some viruses have tag-along viroids and virusoids and can even be infected by other viruses (virophages). While most of the known ones are parasites on the viruses, others are like guests in that they don't hurt the virus. If one of these were to integrate itself inside the shell of the virus, it would then be a 'guest'.

You might like this article:http://math.ucr.e...lar.html
And also http://www.merlin...viruses/
not rated yet Sep 01, 2014
- continued -

The closest that I know of to a 'guest' within a virus is the 'Sputnik' virophage of the giant mimi viruses (these giant viruses have more genes than some bacteria, and even have their own replication centers).

The Sputnik virus has been found WITHIN the replication machinery of mimi virus and uses that machinery rather than of the cell that the mimi virus is infecting, so it really a virus of the mimi virus rather than a satellite virus (in spite of its Sputnik name).

However it appears that it is harmful to the mimi virus, so it is more of a parasite of the giant virus rather than a 'guest' of the virus.

(I am using 'guest' for internal life forms that are helpful or at least are not harmful, and parasite for harmful ones, although this is a grey area as there are many that are harmful at some times and helpful at others).

Here is another article:
not rated yet Sep 02, 2014
Evolution and population genetics are possible without a host/guest relationship - rs

Simply put I can not envision evolution and population genetics without a host/guest relationship. Makes no difference what is guest or host. I don't see the exceptions you see - the life forms without this relationship no matter where which label is place on the life forms involved.

Independent life forms - life forms lacking any relationships to other life forms makes no sense. Do you have a broader generalization and easier labels to categorize all life?

Thank you very much for supplying additional information for all.

not rated yet Sep 02, 2014
Simply put I can not envision evolution and population genetics without a host/guest relationship.

It's probably just a matter of definition - I'm only calling something a host/guest relationship if:
1) one species lives IN or ON another species (otherwise I'd call them co-inhabitants or neighbors rather than host/guest), and
2) only if it is a mutually beneficial or at least not harmful relationship (otherwise I'd call one a parasite).

I agree that almost every life form depends on others, but I would hesitate to say EVERY life form because nature has a habit of proving wrong statements with 'every ' and 'never'. For example, the prokaryotes that live deep in South African mines and depend on radioactive decay for their energy may be almost entirely dependent on other life forms (although even there some of the minerals that they use might have been concentrated by other life forms eons ago).
not rated yet Sep 08, 2014
There was no need to constrict or extend the definitions of host or guest.
Without the constrictions or extensions the objections raised disappear.

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