Scientists have known for quite some time — since 1842, to be exact — that tiny mites, invisible to the naked eye, like to live on human skin, mostly within hair follicles on the face.
In fact, two species of facial mites have been identified as “belonging” to humans: Demodex brevis and Demodex folliculorum. (Considering that there are more than 48,000 known species of mites in the world, we actually got off pretty easy.)
Scientists don’t really understand exactly why these tiny arthropods make their homes on our skin. They may be helping us by eating bacteria or other microbes, although there’s not yet any good evidence for this. It’s more likely that the mites are simply opportunistic little buggers. They know a comfortable neighborhood when they see one.
One of the other perplexing questions about Demodex mites has to do with their prevalence. Just how many of us have them crawling about on our skin? Studies on cadavers have suggested that these mites show up on everybody. But studies on living people have suggested that they can be found on only about 10 to 20 percent of us.
A better detection tool
It’s an answer that’s likely to elicit many “eww” responses.
Using a recently developed search technique — one that looked for the mites’ DNA rather than the creatures themselves — researchers at North Carolina State University have found that everybody appears to host Demodex on their skin.
Why the need for the DNA search tool? Because, say the authors of the study, the old methods of collecting mites from people’s skin, which included lifting them off with Scotch tape and plucking them out along with eyelashes and eyebrow hairs, was too hit and miss. Not only do mites tend to burrow deep within our pores (and, thus, are hard to extract), but they also tend to colonize in patches. It’s easy, therefore, to miss them.
The mites’ DNA, on the other hand, sticks around on the skin even when the creatures themselves are hidden.
‘Meet Your Mites’
For their study, the North Carolina State University researchers recruited 253 volunteers through “Meet Your Mites” face-sampling events at the university and at the North Carolina Museum of Natural Sciences.
“We had really good responses,” Megan Thoemmes, a graduate student and the study’s lead author, told science writer Ed Yong. “People act grossed out at first, but they get excited when they see the mites under the microscope.” (You can view a gallery of the volunteers’ mites online.)
Using an older mite-searching method (gently scraping the skin with a metal laboratory spatula along the creases of the nose and the surrounding cheek area), Thoemmes and her team identified mites on 14 percent of the volunteers — a number similar to previous estimates. But when the researchers looked for mite DNA among 19 of the volunteers, they found it on all of them. (Yong reports that the researchers have subsequently doubled this sample group, with the same result.)
That is, the researchers found the DNA on all of the adults over the age of 18. When they tested the skin of ten 18-year-olds, only seven had detectable amounts. This finding fits in with other research that suggests that mites become increasingly common as we move through childhood. Just how the creatures get transmitted is not known, but there’s some evidence that one method is through breastfeeding. In a very old (1946) study, Demodex mites were found in 77 percent of nipple tissue examined after mastectomies.
Another interesting finding of the study came when Thoemmes and her colleagues compared their mite DNA sequences to those collected in other parts of the world. Writes Yong (with British spellings):
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 globalization.
“This is an arthropod that’s likely living on everyone’s body,” Thoemmes told Yong. “That’s a huge deal. They could tell interesting stories about the spread of humans across the world.”
You can read the mite study in full on the PLOS ONE website. Yong’s discussion of its findings can be found at his always-wonderful “Not Exactly Rocket Science” blog at National Geographic. I also recommend an earlier article of his on this topic (“Everything You Never Wanted to Know About the Mites that Eat, Crawl, and Have Sex on Your Face”).