The Tunnel Window Sign Identifies Active Scabies Infestation: A Validation Study

Background/Objectives: Scabies, caused by Sarcoptes scabiei, is a recognised neglected disease of rising global burden, and increasing resistance to first-line scabicides is making treatment failure more frequent. No dermoscopic marker of cure has been validated for monitoring treatment response, so a reliable marker of active infestation is crucial. The tunnel window sign (TWS)—rounded perforations along the burrow, visible under polarised dermoscopy and ultraviolet-induced fluorescence dermoscopy (UVFD)—is a recently introduced candidate. We aimed to determine whether TWS represents a dermoscopic marker of active scabies infestation by evaluating its specificity, treatment dynamics and diagnostic performance under polarised dermoscopy and UVFD. Methods: In this single-centre observational study conducted in 2026, 33 patients with active scabies (Cohort 1; 106 burrows in 30 permethrin–ivermectin-adherent and 11 in 3 non-adherent) and 30 controls with excoriated pruritic dermatoses (Cohort 2; 150 lesions) were examined. All Cohort 1 patients were re-evaluated every 4 weeks until resolution. Paired polarised/UVFD images were re-scored by four raters (three blinded). Results: TWS was undetectable in all 30 controls with pruritic dermatoses and in all 30 post-treatment adherent patients (0/30 and 0/30, respectively). It resolved with effective treatment (undetectable from week 4; McNemar p < 0.0001), preceding pruritus resolution, and persisted in non-adherent patients until retreatment. At the burrow level, UVFD detected TWS more sensitively than polarised dermoscopy (68/106 vs. 60/106; p = 0.0078; adjusted OR 1.38, 95% CI 1.07–1.79). Inter-rater agreement was almost-perfect (Fleiss’ κ = 0.872). Conclusions: TWS behaves as a dermoscopic marker of active scabies infestation: it disappears after successful treatment, persists following treatment failure, and distinguishes active infestation from both post-treatment skin and non-scabietic pruritic dermatoses. UVFD improves its detection compared with polarised dermoscopy.