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Allergists and their patients are well familiar with house dust mites, a common cause of allergic rhinitis, allergic asthma, and some cases of allergic eczema. They may be less familiar, however, with the Demodex mite, which inhabits the follicle at the base of the eyelashes, and which is increasingly recognized as a cause of chronic eyelid inflammation.

The Demodex mite has a strikingly different appearance from the house dust mite. As with all acarids, and in contrast to insects, it is non-segmented, and has eight legs in the adult stage. But it differs from the dust mite in having a very long body that extends well past its last pair of legs, giving it something of the appearance of an elongated slug when seen under the microscope on the length of the eyelash. Its long and narrow form allows it to inhabit the narrow hair follicle, where it feeds on the fatty sebum produced there.

Controversy has surrounded the role of this organism in producing eyelid inflammation (blepharitits), primarily because it is almost universally present, even in individuals without disease. The incidence of colonization rises with age, so that by 60 years of age, over 80% of people will have colonization of their eyelashes by these mites. However, demodex mite numbers appear to be increased in patients with eyelid inflammation, as well as in those with some inflammatory skin diseases including rosacea.

Reverting for the moment to dust mites, it must be remembered that dust-mite allergic patients have symptoms not because they are necessarily exposed to more mite allergen, but rather because they are more sensitive to the allergen to which they are exposed. The same may apply to eyelid or skin symptoms from Demodex mites—although there is reason to think that it may be the more evolutionarily primitive innate immune system, rather than acquired antibody-mediated immunity, that is to blame.

Tea-tree oil, also a product of evolution as a natural substance that protects its producer from infestation by plant eating mites and other microorganisms, has been of therapeutic benefit; it kills demodex mites just as it kills house dust mites. However, it is best used under the supervision of an ophthalmologist. It must be diluted to a 50% concentration, and care must be taken to apply it to the lid margins without it getting into the eye, where it is an irritant.

Allergists and allergy patients should thus realize that, just as dust mite allergy can cause eyelid eczema (among other symptoms), an inflammatory response to demodex mite infestation can cause eyelid blepharitits and possibly rosacea.

Jeffrey Miller MD, FACAAI
Mission: Allergy, Inc.
Hawleyville, CT