Rosacea is an inflammatory disease characterized by genetic predisposition and different environmental factors.

Vascular abnormal reactions are associated to a disregulation of the immune system which end up in an inflammatory state. Demodex is one of the environmental factors that may paly a role in influencing the inflammatory reaction observed in rosacea patients. Demodex folliculorum and brevis are human mites ubiquitous in normal adults pilosebaceous unit, unique in that they are obligate human ectoparasites. In most cases they do not cause host immune reaction. Their number is significantly increased in patients affected by papulopustular rosacea, situations of immunosuppression by disease or therapy.

Microscopic Demodex  mites are a natural part of the human microbiome — the ecological community of microorganisms that live within and on the body. Two species ofDemodex are found in humans. Demodex folliculorum, live in hair follicles, primarily on the face, as well as in the meibomian glands of the eyelids; Demodex brevis live in the sebaceous glands of the skin.

While Demodex folliculorum are found on the skin of all humans, they frequently occur in greater numbers in those with rosacea. There has been much debate as to whether their increased numbers are a cause or result of rosacea. However, evidence appears to be mounting that an overabundance of Demodex may possibly trigger an immune response in people with rosacea, or that the inflammation may be caused by certain bacteria associated with the mites.

Increased number of Demodex is not always inducer of inflammatory reactions. When it occurs it can be initiated by altered local immune reactivity, mechanical blockage of follicles, damage to follicular epithelium by mites, alteration of sebum composition, surface bacterial transportation, endobacterial release from degenerating mites, release of endogenous enzymes, mite-related bacteria or other mechanisms not yet elucidated. An abundance of Demodex on the facial skin of otherwise healthy adults, mainly females, possibly secondary to

an innate immune tolerance, may give rise to cutaneous lesions such as follicular scaling either or not associated with fair surrounding erythema, pruritus and stinging. This condition is diagnosed as demodicosis. Interestingly, an anti-mite therapy has demonstrated a significant effect in patients affected by rosacea.