Think of all the adults you know. Think of your parents and grandparents. Think of the teachers you had at school, your doctors and dentists, the people who collect your rubbish, and the actors you see on TV. All of these people probably have little mites crawling, eating, sleeping, and having sex on their faces.
There are more than 48,000 species of mites. As far as we know, exactly two of those live on human faces. While their relatives mostly look like lozenges on spindly legs, face-mites are more like wall plugs—long cones with stubby legs at one end. They don’t look like much, and most of us have never looked at one at all. But these weird creatures are almost certainly the animals we spend the most time with.
They live in our hair follicles, buried head-down, eating the oils we secrete, hooking up with each other near the surface, and occasionally crawling about the skin at night. They do this on my face. They probably do it on yours. A group of scientists led by Megan Thoemmes and Rob Dunn at North Carolina State University found that every adult in a small American sample had face-mites on their faces—something that was long suspected but never confirmed. If you want to find humanity’s best friend, ignore dogs; instead, swab a pore and grab a microscope.
As I wrote back in 2012, the mites were discovered in 1841, but only properly described a year later by German dermatologist Gustav Simon. He was looking at acne spots under a microscope when he noticed a “worm-like object” with a head and legs. Possibly an animal? He extracted it, pressed it between two slides, and saw that it moved. Definitely an animal. A year later, Richard Owen gave the mite its name, from the Greek words ‘demo’, meaning lard, and ‘dex’, meaning boring worm. Demodex: the worm that bores into fat. We host two species: Demodex folliculorum (bigger, round-bottomed) and Demodex brevis (smaller, short-bottomed).
Scientists have since found Demodex in every ethnic group where they’ve have cared to look, from white Europeans to Australian aborigines to Devon Island Eskimos. In 1976, legendary mite specialist William Nutting wrote “One can conclude that wherever mankind is found, hair follicle mites will be found and that the transfer mechanism is 100% effective! (One of my students noted it was undoubtedly the first invertebrate metazoan to visit the moon!)”
But it’s always been hard to say exactly how common they are. The first estimate came from a 1903 study, which found the critters in 49 out of 100 French cadavers. The next count, from 1908, found them in 97 out of 100 German cadavers. Most studies since then have fallen in the range of 10 to 20 percent.
But these censuses were all based on visual counts. Someone would apply cellophane tape to skin to pull the mites off, or scrape an oily patch of face with a small spatula, or pluck eyelashes and eyebrows. But the creatures live in our pores and aren’t easy to extract. They’re also unevenly distributed. You might have a population living in your cheek; I might have one on my forehead. Unless you’re scraping and taping and plucking all over someone’s face, you might miss their mites.
So Thoemmes did something different. She searched for their DNA. The mites have this helpful habit where they… er… have no anus and never poo. Instead, they release a lifetime’s worth of waste when they die. That contains their DNA, which gives away the presence of the mites even when the creatures themselves are inaccessibly hidden.
Thoemmes developed a test for Demodex DNA and recruited willing volunteers at “Meet Your Mites” face-sampling events. “We had really good responses,” she says. “People act grossed out at first, but they get excited when they see the mites under the microscope.” She recruited 253 volunteers and saw the actual mites on 14 percent of them, in line with previous estimates. She also checked for mite DNA in 19 adults… and found it on all of them. Those results are published today in PLOS ONE, but Thoemmes tells me that the team has continued their work and more than doubled their sample size. Same result.
Obviously, this is a small and unrepresentative sample, but it clearly shows that visual counts grossly underestimate the proportion of people with mites. That, combined with over a century of other studies, strongly suggests that the mites are to faces as smoke is to fire.
Or, at least, on adults. Thoemmes also sampled ten 18-year-olds and found Demodex DNA on just 70 percent of them. This fits with what earlier studies had shown—the mites seem to become more common with age. They’re rare on babies, more common on teenagers, and universal in adults. No one really knows where we get them from. Dogs get their face-mites during nursing, and humans might do the same—after all, one study found a lot of Demodex living in nipple tissue. But the fact that some teens aren’t colonised suggests that we pick up these creatures throughout our lives.
The team also compared their mite DNA to sequences from other parts of the world. They found that D.follicorum doesn’t have a lot of genetic diversity. The ones living on someone in China are probably very similar to those living on an American face. D.brevis, on the other hand, is much more diverse, and a single face can house many different lineages.
These differences probably reflect the lifestyles of the two species. D.brevis snuggles deeply in our pores and stays there. As we travelled the world, it hitched along and co-evolved with us, giving rise of many distinct lineages. D.folliculorum is a shallower resident, and may move between people more easily. Brevis epitomises insularity, folliculorum symbolises globalisation. “This is an arthropod that’s likely living on everyone’s body,” says Thoemmes. “That’s a huge deal. They could tell interesting stories about the spread of humans across the world.”
Considering how common these creatures are, there’s still so much we don’t know about them. We don’t know where our two face-mite species came from, or what their closest relatives are. We also don’t know how many other face-mites exist. Each Demodex species seems to stick to one mammal host, and humans, dogs, and cats all have more than one. There are over 5,000 species of mammals, which means that there could potentially be 10,000 species of Demodex left to discover.
For more on the habits of these wonderful creatures, and how they might affect our health: Everything you never wanted to know about the mites that eat, crawl, and have sex on your face
Reference: Thoemmes, Fergus, Urban, Trautwein & Dunn. 2014. Ubiquity and diversity of human-associated Demodex mites. PLOS ONE, citation tbc.