Beyond classic vs. crusted: the high-load non-crusted scabies phenotype in a prospective multicenter cohort

Abstract

In classic scabies, mite burden is typically low, whereas crusted scabies exhibits massive infestation. During recent surges in presentations, we observed immunocompetent patients with non-crusted scabies referred for persistent or relapsing symptoms after standard topical therapy, who appeared to carry higher burdens. We aimed to estimate the frequency of a pragmatic high-load scabies phenotype among patients presenting with apparent topical treatment failure and to summarize its clinical features and outcomes. In a prospective, multicenter cohort across 10 dermatology centers in Türkiye, we enrolled patients with suspected scabies who had persistent or relapsing symptoms despite standard therapy and fulfilled a predefined high-load phenotype (≥50 mite-confirmed burrows on dermoscopy). The primary endpoint was Day-28 dermoscopic clearance, defined as no mites/ova and no new burrows at index sites. At baseline, all burrows were counted under a shared standard operating procedure (SOP). High-load scabies was defined a priori as ≥50 burrows. Of 3,316 patients screened, 76 (2.3%) met the high-load definition. The cohort comprised 42 men (56.1%) and 34 women (43.9%) with a mean age of 30.24 years. Baseline burrow counts ranged 50–400 (mean 86). Age showed a moderate positive association with burrow count (Spearman’s ρ=0.365, p<0.01), whereas sex and household high-load history were not associated (both p>0.05). In routine care, combined oral ivermectin plus topical therapy was associated with clinical clearance in 89% (n=68/76) of these high-load cases. This observation reflects outcomes among patients presenting after apparent topical treatment failure and does not establish a causal role of mite burden in treatment failure or transmission. This multicenter cohort defines a pragmatic high-load scabies phenotype (≥50 dermoscopically counted burrows) that lies between classic and crusted disease. Routine dermoscopy allowed pragmatic burden stratification in this cohort; in patients meeting the ≥50-burrow definition, combined oral and topical therapy was associated with high clearance in routine care.