demodex

Demodex is a genus of tiny parasitic mites in the family Demodicidae. The two species most commonly associated with humans are Demodex folliculorum and Demodex brevis. They are ectoparasites inhabiting the keratinized lining of hair follicles and sebaceous glands.

Morphology and life cycle: They are among the smallest arthropods visible only under magnification, measuring roughly 0.3-0.4 mm. Adults have an elongated, worm-like body with eight legs; Demodex folliculorum resides primarily in hair follicles, Demodex brevis in sebaceous glands. The life cycle comprises egg, larva (six legs), nymph, and adult (eight legs), typically completed in a few weeks. Mite activity often peaks at night, when movement to the skin surface occurs.

Prevalence and ecology: Demodex mites are common in humans, with colonization increasing with age; most healthy adults harbor these mites without symptoms. They are transmitted by close contact, likely from mother to child and between partners, and can be spread via shared cosmetics or bedding.

Clinical relevance: In some individuals, high mite density or immune dysfunction can contribute to inflammatory conditions such as blepharitis (eyelid inflammation) and rosacea-like dermatitis. Demodex can be detected by microscopic examination of skin scrapings, standardized skin surface biopsy, or eyelash sampling.

Management: Treatment is usually reserved for symptomatic cases and may include topical acaricides (for example, permethrin or ivermectin), and eyelid hygiene. Tea tree oil preparations have been used anecdotally but may cause irritation. The goal is to reduce mite numbers and control inflammation.