Which factors influence Demodex proliferation? A retrospective pilot study highlighting a possible role of subtle immune variations and sebaceous gland status.

Demodex folliculorum and brevis are commensal mites that live in low densities in human pilosebaceous follicles as part of the normal adult microbiota, but that give rise to demodicosis and, possibly, rosacea, when they proliferate excessively. This proliferation is favored by various factors, including age, marked immunosuppression, sebaceous gland hyperplasia, and hypervascularization-related factors. To study possible factors influencing mite proliferation, we explored the effects of different variables on Demodex densities (Dd) in a retrospective study of two groups of subjects selected on the basis of their clinical diagnosis: Demodex+, consisting of subjects with demodicosis or with centro-facial papulopustules suggesting rosacea (n = 844, mean Dd 263.5 ± 8.9 D/cm2 ), and Demodex-, consisting of subjects with other facial dermatoses or healthy facial skin (n = 200, mean Dd 2.3 ± 0.4 D/cm2 ). Demodex densities were measured using two consecutive standardized skin surface biopsies (SSSB1 [superficial] and SSSB2 [deep]) taken from the same facial site on each subject. In the Demodex+ group: the SSSB1 decreased with age in women (p = 0.004), and the SSSB2 increased with age in men (p = 0.001) (the pattern was similar for SSSB1 + 2, but not statistically significant); Dds were lower in those who had received cortisone (either topically or systemically); 13 subjects (1.5%) had known immunosuppression, 62 (7.3%) had hypothyroidism, and in 20 (3.6% of the women) there was a reported link with pregnancy; 78 of the subjects (9.2%) were part of a pair from the same family or household; when associated bacterial infection was suspected, Staphylococcus epidermidis was often isolated. Our results suggest close interactions between the mite, sebaceous gland size and function, and subtle variations of immune status. Potential factors influencing Demodex proliferation should be further investigated, including hypothyroidism, pregnancy, corticosteroid administration, Staphylococcus epidermidis, contagiousity, and genetic background.