Gruesome Twosome

There are two distinct species of Demodex mites that inhabit the ocular tissues: Demodex folliculorum and Demodex brevis. D. folliculorum are generally larger and call the base of the eyelash follicles their home, while D. brevis are smaller and reside in the sebaceous glands.¹ These mites are specific to humans.¹ In fact, Demodex are a normal part of the lid environment and are the most common ectoparasite of human skin.^1,2

However, infestation by these mites, called demodicosis, has been implicated in a variety of dermatologic and anterior segment conditions.^1,2 Many of the consequences of demodicosis are thought to occur due to immune response and inflammatory reactions.^1,2Demodex has also been found to act as a mechanical blockage in sebaceous glands and is associated with hordeolum and chalazia formation.¹

A DAY IN THE LIFE

Demodex makes its living by eating sebum with its pin-like mouthparts. Although the mites have no excretory orifice, their digestion occurs internally, driven by lipase enzymes,^1,2 and waste is regurgitated externally through the “mouth.” The death and breakdown of the chitinous exoskeletons has been shown to induce an inflammatory immune response.¹Demodex hosts several types of bacteria, including Staphylococcus and Bacillus oleronius, which may perpetuate inflammatory processes or responses on their own accord.¹

HIDING IN PLAIN SIGHT

Undigested waste, lipids, keratin, Demodex eggs, and dead Demodex mites migrate out of the eyelash follicle and accumulate around the base of the eyelashes to form the pathognomonic sign of Demodex blepharitis, described as collarettes or cylindrical dandruff (Figure).^1,2 One study found the prevalence of Demodex blepharitis to be 57.7% among all patients visiting eye care clinics, using the presence of collarettes as diagnostic criteria.² In a study of eyelashes with and without collarettes, 100% of the lashes with collarettes were found to have Demodex mites.²