Increased density of Demodex folliculorum and evidence of delayed hypersensitivity reaction in subjects with papulopustular rosacea

Background Rosacea is a common chronic dermatosis that evolves in stages. The mite Demodex folliculorum has been implicated in its obscure aetiopathogenesis. Aim To evaluate the importance of D. folliculorum in the aetiology and course of rosacea. Methods We studied 92 consecutive cases of papulopustular rosacea and 92 age- and sex-matched controls. Prevalence and density of D. folliculorum were estimated by microscopic examination of the expressed follicular content. Histological examination and immunohistochemical study of the inflammatory infiltrate were performed in 10 subjects (five with positive D. folliculorum finding and five with negative finding). Results D. folliculorum was detected in 83 (90.2%) of the 92 rosacea subjects but only 11 (11.9%). of the controls. The mean mite density was 2.03 mites/visual field in the rosacea group (range 0-5, SD = 1.2) and 0.16 mites/visual field (range 0-2, SID = 0.52) in the control group. The difference was statistically significant (P < 0.0001) for both mite prevalence and density. Hair follicle infestation was associated with intense perifollicular infiltrate of predominantly (90-95%) CD4 helper/inducer T cells. We observed an increased number of macrophages and Langerhans cells only in those subjects with a positive D. folliculorum finding. Conclusions Although Demodex mites do not seem to be the cause of rosacea, they may represent an important cofactor, especially in papulopustular rosacea. Immunohistochemical findings suggest that a delayed hypersensitivity reaction, possibly triggered by antigens of follicular origin, probably related to D. folliculorum, may occur, stimulating progression of the affection to the papulopustular stage.