In addition to vascular disorders, another factor that might play a role in fostering rosacea is a microscopic mite called Demodex folliculorum. This mite, which is a normal resident in human skin, lives in hair follicles, where it lives on cast-off skin cells. Mites have been retrieved from almost every area of human skin, but they seem to prefer the skin on the face.
Two recent studies revealed that the mites were significantly more numerous in facial skin samples of people with rosacea than of people without the condition. In addition, the mite population peaked on the skin samples of these patients in the spring, when rosacea tends to flare up.
Other studies show that patients with steroid-induced rosacea also had a higher mite population on their faces. This population dropped when the rosacea subsided after treatment with an ointment that kills the mites (see Steroid Rosacea).
Although these findings do not prove that the skin mite causes rosacea, they do suggest that Demodex folliculorum might play a role in fostering the condition. The mites may provoke rosacea by clogging skin follicles, which, in turn, might trigger an inflammatory response.
Rosacea may also be triggered by an allergic-like reaction to these skin mites or to the bacteria that the mites harbor. These mites could then multiply excessively or penetrate more deeply into the skin, triggering an inflammatory response in the form of pimples.