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Quantification of Demodex folliculorum by PCR in rosacea and its relationship to skin innate immune activation

The aim of this study is to quantify D. folliculorum colonisation in rosacea subtypes and age‐matched controls and to determine the relationship between D. folliculorum load, rosacea subtype and skin innate immune system activation markers. We set up a multicentre, cross‐sectional, prospective study in which 98 adults were included: 50 with facial rosacea, including 18 with erythematotelangiectatic rosacea (ETR), and 32 with papulopustular rosacea (PPR) and 48 age‐ and sex‐matched healthy volunteers. Non‐invasive facial samples were taken to quantify D. folliculorum infestation by quantitative PCR and evaluate inflammatory and immune markers. Analysis of the skin samples show that D. folliculorum was detected more frequently in rosacea patients than age‐matched controls (96% vs 74%, P < 0.01). D. folliculorum density was 5.7 times higher in rosacea patients than in healthy volunteers. Skin sample analysis showed a higher expression of genes encoding pro‐inflammatory cytokines (Il‐8, Il‐1b, TNF‐a) and inflammasome‐related genes (NALP‐3 and CASP‐1) in rosacea, especially PPR. Overexpression of LL‐37 and VEGF, as well as CD45RO, MPO and CD163, was observed, indicating broad immune system activation in patients with rosacea. In conclusion, D. folliculorum density is highly increased in patients with rosacea, irrespective of rosacea subtype. There appears to be an inverse relationship between D. folliculorum density and inflammation markers in the skin of rosacea patients, with clear differences between rosacea subtypes.