Skin parasites: Your face crawls with life – literally

While you are reading this, your face is likely crawling with life: hundreds of eight-legged, worm-like parasites are swarming through glands and follicles, eating your dead skin, mating, and pooping on facial hair while dying. An entire, satisfying life cycle on your lips, in your ears, gently swinging on your eyelash.

Not micro at all: meet the Demodex mite

You might have heard that our skin hosts various lifeforms. Bacteria and other microorganisms constitute a healthy community in and on our skin, called the microbiota. But the Demodex mites are not “micro” at all: the two species that dwell on our skin reach up to 0.4 and 0.3 millimeters. On average, you could find seven Demodex mites per 10 cm2 of skin, meaning that 50 to 70 mites could currently be hiding on your forehead. They can move at quite an amazing speed, too: up to about 10 cm per hour, usually during nighttime.

Forget about seeing them live though: not only are they very small and semi-transparent, they actually spend most of their time inside your face! They usually live in hair follicles, anchoring themselves with their body scales and eating dead skin and accumulated sebaceous oil (the lubricant of our air and skin). Quite the binge eater, to the point that their name literally combines two Greek words for “fat” and “woodworm”. Except that they are not worms at all, which brings us to their discovery.

Demodex mites are not new to researchers: their very first observations date back to 1841, when two scientists observed them independently. Why these gentlemen were observing human sebaceous leftovers under the microscope is one of the wonders of science; anyhow, there they discovered the Demodex. The first researcher to understand that they were mites was Gustav Simon in 1842, who, in memoriam of their place of discovery, baptized them “grease-digging worms”. Quite the imaginative scientist!

Over time, about 65 Demodex species were discovered. Most species dwell on dogs and other animals, while only two species chose humans as playground: D. folliculorum and D. brevis (the short guy). Despite not being extraordinary famous to the general audience, these Demodex have their own fans in the scientific community. In fact, a good number of researchers has extensively studied these interesting – though arguably disgusting – mites.

Demodex mites in research: from evolution to skin health

To begin with, Demodex mites interest biologists for their evolutionary habits. It was discovered that they only live in adult’s skin: babies and kids do not produce enough sebum for their hungry appetites. But wait until teenage! And where do they come from? Most likely, they migrate from person to person. Once settled, they’ll colonize hair follicles for the rest of the host’s life.

Researchers also observed an astonishing evolutionary divergence from other mites: instead of having round-ish bodies, they become elongated to the point of resembling small worms. Why? To better crawl into your follicles, of course! Thanks to their shape, a whole party can thus co-habit a follicle, giving rise to amazing mating opportunities.

Also, differently from most mites, that indirectly transfer sperm “packages” to females, Demodex males fecundate their partners with a proper penetration. Then, after the fecundation, the female lays her eggs inside of the follicles – your follicles. Interestingly, the young larvae have only 6 legs, but will soon develop another pair during their development. The adults can live up to some weeks until dying – yes, in your follicles as well. Although glutton, Demodex do not really defecate during their lifetime, but they accumulate leftovers in their body. Unfortunately, their exoskeleton opens when they die, with obvious consequences for your skin…

Eventually, the main driver for most of the current scientific interest in Demodex is, indeed, the well-being of our skin. In small quantities, they might be beneficial for the maintenance of our skin: through their diet, they get rid of dead cells and oil build-up. However, if too numerous, they might cause skin irritation, rosacea, itch, and scales around the hair follicles. In case of severe symptoms, your dermatologist may prescribe some cream to get rid of the majority and their eggs. Further research is ongoing to assess their connection with other skin problems, to relate their proliferation to immune variations and gland status, and to better understand their role in regulating skin microbiota.

Till death do us apart

Even their death harbours yet another intriguing application of Demodex research: in forensic. The mites only live a few days after the hosts’ death. Hence, forensic doctors may look for their presence on a dead body to estimate the departure time. A small service from the little ones, in exchange for the luxurious, lifelong buffets.

Author: Daniele Proverbio (University of Luxembourg)
Editor: Michèle Weber (FNR)