Demodex mites live on nearly every adult’s skin. They’re microscopic parasites that feed on oil and dead skin cells inside your hair follicles, and in small numbers they’re completely harmless. What “causes” them is really two separate questions: how you get them in the first place, and what makes their population grow large enough to cause problems. The answer to the first is simple: close physical contact, starting from infancy. The answer to the second involves your skin’s oil production, your immune system, and sometimes an underlying health condition.
Why Everyone Has Them
Demodex mites are passed from person to person through direct skin contact. Newborns likely pick them up from parents and caregivers shortly after birth. But babies and young children rarely have significant numbers because their skin doesn’t yet produce much oil. The mites depend on sebum, the greasy substance your oil glands secrete, so they can’t establish a real foothold until puberty ramps up oil production.
Two species live on human skin. One inhabits the shallow parts of hair follicles, especially eyelashes, and feeds primarily on skin cells. The other burrows deeper into the oil glands themselves and feeds directly on sebum. Both are too small to see with the naked eye, and both spend their entire life cycle inside your pores.
By older adulthood, infestation rates are nearly universal. About 84% of people over 60 carry detectable mites, and that figure reaches 100% in people over 70. At low densities, most people never notice them.
What Makes Mite Populations Explode
The threshold between normal skin flora and a diagnosable problem (called demodicosis) is a density of more than five mites per square centimeter of skin. Below that, the mites coexist with you quietly. Above it, they can trigger visible inflammation, itching, and skin or eyelid disease. Several factors push populations past that line.
Excess Oil Production
Because Demodex mites feed on sebum and skin cells, anything that increases oil output on your face creates a more hospitable environment for them. The 20 to 30 age range actually has the highest rate of follicle infestation, matching the period when sebum secretion peaks. Hair loss is another contributor: follicles affected by pattern baldness develop larger, more active oil glands under the influence of hormones like dihydrotestosterone, giving mites a richer food supply.
Overusing heavy creams, oily moisturizers, or greasy skincare products can have the same effect. The extra lipids essentially serve as supplemental food, helping mites reproduce faster and in greater numbers. Infrequent face washing compounds the problem by allowing oil and dead cells to accumulate in follicles.
A Weakened Immune System
Your immune system normally keeps Demodex populations in check. When that system is compromised, mite numbers can surge. The conditions most strongly linked to overgrowth include HIV/AIDS, diabetes, cancer, and the use of immunosuppressive medications.
The connection with HIV is especially stark. In one survey of 60 HIV-positive patients, 95% of those with severely depleted immune cells (CD4 counts below 200) had Demodex on their eyelashes, compared to just 20% of those with healthier immune function. People with demodicosis also show a pattern of reduced immune cell activity in their blood, with lower counts of several types of protective white blood cells and a higher proportion of cells involved in programmed cell death. Certain genetic immune profiles also play a role: some people inherit immune markers that make them naturally more susceptible to infestation, while others carry protective markers that help their bodies fight the mites more effectively.
UV light therapy, used for conditions like eczema and psoriasis, can also suppress local skin immunity enough to encourage mite growth. Some topical anti-inflammatory creams prescribed for eczema work by dialing down immune cell activity in the skin, which can inadvertently let Demodex flourish.
Skin Conditions That Create a Favorable Environment
Rosacea and Demodex have a close, circular relationship. Rosacea creates inflamed, oil-disrupted skin that mites thrive in, and high mite densities may worsen rosacea symptoms in return. Studies consistently find higher mite populations on rosacea-affected skin, and the severity of rosacea closely correlates with the severity of eyelid inflammation caused by Demodex.
Dry eye disease may also set the stage. When the tear film on the eye’s surface is compromised, the eyelid environment becomes more favorable for mites to colonize lash follicles. The deeper-dwelling species can physically block the tiny oil glands along your eyelid margins, worsening dry eye and creating a cycle that’s hard to break without treatment.
How Mites Cause Damage
Understanding why overgrowth matters helps explain why the “cause” question is so important. Demodex mites have tiny claw-like structures they use to anchor inside follicles, and these claws physically irritate the surrounding skin. The body responds by producing excess keratin, the tough protein in your outer skin layer, leading to thickened, flaky buildup around hair follicles and eyelash bases.
The mites also lack any way to excrete waste. Instead, they regurgitate undigested material, which mixes with dead skin cells, eggs, and keratin to form crusty deposits. On eyelashes, these appear as distinctive waxy cylinders called collarettes wrapped around the lash base. Proteins inside the mites, along with bacteria they carry, trigger inflammatory responses in the surrounding tissue. Even when mites die inside a follicle, the release of their internal contents can set off a cascade of inflammation involving immune cells and inflammatory signaling molecules.
Conditions Linked to Overgrowth
Demodicosis isn’t just one condition. Depending on where the mites concentrate and how your body reacts, overgrowth can show up as blepharitis (red, itchy, crusty eyelids), rosacea-like facial redness and pustules, a scaly rash around the mouth, or a flaky eruption along the hairline and scalp. In studies of rosacea patients with pustules, skin biopsies showed dense clusters of a specific type of immune cell surrounding infested follicles, a pattern not seen in mite-free skin.
Reducing Mite Populations
For eyelid-specific infestations, tea tree oil has been the most studied over-the-counter option. In laboratory testing, full-strength tea tree oil killed mites within 15 minutes, and a 50% concentration took about 15 minutes as well. A practical regimen of weekly lid scrubs with 50% tea tree oil combined with daily use of diluted tea tree shampoo has been shown to effectively clear mites from eyelashes. Higher concentrations can cause irritation, especially near the eyes, so care is needed to avoid direct contact with the eye surface.
A prescription eye drop containing a compound that paralyzes and kills the mites became available recently. In pooled data from two large clinical trials, 60% of patients using the prescription drops achieved complete mite eradication by day 43, compared to 16% in the placebo group. About 91% of treated eyes had their mite density drop to near zero (half a mite or fewer per lash on average), and 85% saw their crusty lash deposits reduced to minimal levels.
Beyond targeted treatments, managing the underlying conditions that feed mite growth matters just as much. Controlling oil production, treating rosacea, addressing immune deficiencies, and avoiding excessively heavy facial moisturizers all help keep populations from rebounding. Since mites are a normal part of human skin ecology, the goal isn’t permanent elimination but keeping their numbers low enough that they stop causing symptoms.