There are two species of demodex mites found on the human eye; demodex folliculorum, found at the base of the lashes; and demodex brevis, usually found in the meibomian glands. Demodex brevis are usually found alone; whereas demodex folliculorum usually live in colonies.
Demodex folliculorum tend to live and lay eggs at the base of the eye lashes. They eat the roots of the lashes. Demodex brevis are usually found in the meibomian glands where they feed on meibum. Their dead bodies clog the ducts; the bacteria they release cause inflammation of the cells that turn into meibum. The claws of these mites also cause epithelial hyperplasia, an overgrowth of cells, in the meibomian glands. The result is plugged or blocked meibomian glands, and poor quality meibum. Furthermore, as the dead bacteria and demodex carcasses break down, and their fatty acids mix with the lipid layer of the tear film. The result can be frothy or soapy tears, saponification. Some individuals may be highly sensitive, or even allergic, to the mites.
Demodex mites are usually not harmful. They are considered to be commensals, rather than parasites, because they do not harm or benefit the host. However, if there is a large population of demodex mites, or a sensitivity to the mite or its bacteria, the result may be demodicosis, producing the symptoms associated with posterior blepharitis or meibomian gland dysfunction MGD).
Demodex mites can cause inflammation. Sensitivities or allergies to demodex mites, their carcasses, feces, or the bacteria they harbor can lead to inflammation. There is evidence that the claws of demodex mites tear the lining of the meibomian glands, leading to inflammation.
Some studies show an associate between demodex and various kinds of acne, including acne rosacea.
For more information see Medscape.