Why does treatment for scabies fail?

Scabies is one of the commonest infectious dermatoses worldwide, with an estimated 400 million incident cases occurring globally each year.1 The burden of disease is highest in low- and middle-income countries with children particularly affected. Reflecting the significant burden of disease, the World Health Organization classified scabies as a Neglected Tropical Disease in 2017 and established global targets for control. Worldwide topical treatments such as benzyl benzoate and permethrin represent the most commonly used scabicides, but there is increasing evidence and interest in the use of ivermectin particularly in the context of community wide treatment2 (mass drug administration) where the ease of administration of an oral therapy is particularly valuable. For individual-level treatment the relative efficacy of both topical and oral therapies has been a subject of much debate with individually randomized trials inconclusive, and concerns in some contexts about increasing rates of treatment failure with permethrin and other topical therapies. Whether these failures represent 'true' resistance of pseudo-resistance due to poor compliance with topical therapies is a particularly pressing question facing the field. Despite the importance of this question, most data are derived from high-income settings with relatively limited data on factors associated with treatment failure from highly endemic settings. In this context, the study by Blaizot et al.3 is particularly welcome. Importantly, the study took a mixed-methods approach to provide a comprehensive assessment of factors associated with treatment completion and treatment failure. Perhaps unsurprisingly, adherence to a second dose of a scabicide was associated with successful treatment. In keeping with previous studies that suggest poor adherence to topical therapies, use of permethrin monotherapy was also associated with a higher risk of failure. More significantly the study by Blaizot and colleagues examined wider factors that influence treatment outcomes. Their data clearly show the impact of poverty, not only on the risk of scabies, but on treatment outcomes. Patients with access to health insurance and proxy markers of socio-economic status such as 'Western' clothing and 'modern' housing were also strongly associated with treatment success. These data reinforce that social determinants of health can influence the real-work impact of efficacious biomedical interventions and that failure to address underlying drivers will undermine our control efforts. While the World Health Organization's definition of a Neglected Tropical Disease emphasizes that NTDs are diseases that disproportionately affect individuals living in poverty, our public health responses have focused heavily on biomedical interventions. The new WHO NTD Roadmap emphasizes the need for a shift in perspective and the development of cross-cutting, multi-sectoral strategies.4 This shift is a call-to-arms for the scabies research community. If we are serious about reducing the suffering scabies causes, we must begin to move beyond biomedical interventions to broader health systems and structural interventions. The authors declare no conflicts of interest. Data sharing is not applicable to this article as no new data were created or analyzed in this study.