Demodex and Dry Eye

Demodex are microscopic mites that live on human skin. Two species are commonly found around the eyes:

• •Demodex folliculorum
• •Demodex brevis

They tend to occupy slightly different environments. D. folliculorum is primarily associated with eyelash follicles, while D. brevis can live deeper in sebaceous and meibomian glands.

You cannot normally see the mites themselves just by looking in the mirror. And simply having them does not mean you have an infection or disease.

No. This distinction is essential. Demodex can be present without causing clinically significant disease.

TFOS DEWS III cites studies finding Demodex in a substantial proportion of the population, particularly among older adults. So Demodex being present is not the same as Demodex blepharitis. Demodex blepharitis refers to eyelid disease associated with the mites. The diagnosis depends on clinical findings rather than simply proving that one mite exists.

One of the most important signs of Demodex blepharitis is the presence of collarettes. These are small cylindrical cuffs of debris that accumulate around the base of the eyelashes. You may also hear them called cylindrical dandruff. They can look like tiny sleeves wrapped around individual lashes.

TFOS DEWS III describes cylindrical dandruff at the eyelash bases as a particularly characteristic finding of Demodex blepharitis. This is useful because Demodex symptoms themselves are not very specific.

People may experience:

• •itching
• •burning
• •eyelid irritation
• •redness
• •foreign-body sensation
• •crusting around the eyelashes
• •watery eyes
• •dry-eye-like discomfort

Some people have relatively few symptoms despite visible eyelid disease. And many of these symptoms overlap with ordinary dry eye, allergy, other forms of blepharitis and ocular rosacea. So symptoms alone cannot tell you that Demodex is the cause.

Several mechanisms have been proposed. The mites can mechanically irritate eyelash follicles and surrounding tissue. Their presence and biological material may also provoke inflammatory responses. Changes in the microbial environment around the eyelids may potentially contribute as well.

The result, in susceptible individuals, can be chronic inflammation of the eyelid margin: Demodex blepharitis. But it is important not to turn this into an overly simple story. Demodex can exist without obvious disease, and the relationship between mite density, inflammation and symptoms differs between people.

No. Demodex blepharitis is an eyelid condition. Dry eye disease is a multifactorial disease characterized by symptoms together with loss of tear-film and/or ocular-surface homeostasis. The two can coexist.

Eyelid inflammation may contribute to an unhealthy ocular-surface environment and can potentially act as one of several dry-eye drivers. But finding Demodex blepharitis does not automatically establish that Demodex explains all of a patient’s dry-eye symptoms.

No. Again, these are different things. Demodex blepharitis is eyelid disease associated with Demodex mites. Meibomian gland dysfunction (MGD) describes abnormal function of the meibomian glands.

They can coexist. In particular, D. brevis has been identified within meibomian glands, and Demodex-associated disease has been linked with meibomian gland abnormalities. But this does not mean that Demodex equals MGD, or that every case of MGD is caused by Demodex. MGD has many possible contributors and associations.

Meibomian Gland Dysfunction (MGD): what actually happens inside the glands — coming soon

This is where wording matters. You may see very simple explanations online saying that Demodex mites:

“Block the glands and cause MGD.”

That can make a complicated relationship sound much more certain than it is. Demodex, particularly D. brevis, can inhabit structures associated with sebaceous and meibomian glands, and Demodex disease has been associated with MGD. But in an individual patient, finding Demodex does not prove that it caused their gland obstruction or gland dysfunction.

The clinically useful question is: are both conditions present, and does Demodex-associated eyelid disease appear to be an important contributor?

Demodex is also associated with rosacea. Studies have reported increased Demodex density in people with rosacea, and the mites may participate in inflammatory mechanisms involved in the disease. But once again, association does not mean that Demodex is the single cause of rosacea. Rosacea is a complex inflammatory condition.

A person may therefore have overlapping:

• •ocular rosacea
• •Demodex blepharitis
• •MGD
• •dry eye

This is a good example of why dry-eye disease often cannot be reduced to one simple root cause.

Ocular Rosacea and Dry Eye: how rosacea can affect the eyelids and ocular surface

The eyelids and eyelashes are usually examined closely, often using a slit lamp. The clinician looks particularly for collarettes around the base of the lashes, as well as eyelid-margin redness and other characteristic findings. In some circumstances, eyelashes can be removed and examined under a microscope to visualize and count mites. This method was also used in major Demodex treatment trials to measure mite density.

But routine diagnosis does not necessarily require pulling out eyelashes from every patient. Characteristic clinical signs can provide strong evidence.

How Dry Eye Is Diagnosed

Collarettes at the base of the eyelashes are considered a highly characteristic clinical sign of Demodex blepharitis. TFOS DEWS III describes cylindrical dandruff as pathognomonic in its discussion of Demodex-associated blepharitis. That makes collarettes much more useful diagnostically than vague symptoms such as burning or dryness. It also illustrates an important principle from our diagnostic section: look for a finding that actually points toward the suspected condition rather than diagnosing it from nonspecific symptoms.

Yes. And this area has changed significantly in recent years. Historically, many different lid-cleaning products and approaches have been used to try to reduce Demodex populations, with varying levels of evidence. More recently, a treatment specifically targeting Demodex mites has been studied in large randomized controlled trials.

Lotilaner ophthalmic solution 0.25% significantly reduced collarettes and mite density compared with vehicle in the Saturn-1 and Saturn-2 trials. A pooled analysis of 833 participants found substantially greater collarette reduction and mite eradication after six weeks of treatment than with vehicle alone. TFOS DEWS III consequently includes anti-Demodex therapy among approaches to lid disease management. Availability and approved indications differ between countries.

Treating Meibomian Gland Dysfunction

Not necessarily. This distinction is extremely important. A treatment can successfully reduce:

• •mites
• •collarettes
• •eyelid inflammation

without proving that every dry-eye symptom in that patient was caused by Demodex. Someone with Demodex blepharitis may simultaneously have:

• •MGD
• •aqueous deficiency
• •ocular rosacea
• •incomplete blinking
• •exposure
• •ocular-surface inflammation
• •or another dry-eye driver

Treating a confirmed contributor makes sense. But removing one contributor does not automatically normalize the entire tear-film system.

Dry Eye Is Often Mixed

No. Dry eye alone is not evidence that Demodex treatment is needed. There should first be a reason to believe that clinically significant Demodex-associated disease is actually present. This is particularly important because Demodex mites themselves are common. The goal is not necessarily to:

“Eliminate every mite because mites exist.”

The goal is to identify and treat Demodex blepharitis when it is clinically relevant.