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With pruritus being the main symptom, scabies, derived from the Latin verb scabere, meaning to scratch, is an appropriate name for the human infestation caused by the mite Sarcoptes scabiei.1 The infestation is acquired by skin-to-skin contact and becomes established when adult female mites burrow into the skin and lay eggs. After an incubation period of 4–6 weeks, pruritus develops together with variable signs including small papules, excoriations, burrows and nodules. The incubation period is shorter in case of reinfection.2 Consensus criteria for the diagnosis of scabies were published in 2020,2 although they may be inconsistently applied and diagnosis may still remain challenging. Several treatment options exist and the recommended treatment for uncomplicated cases is topical permethrin 5% cream, benzyl benzoate 10%–25% lotion or oral ivermectin, with the latter preferred in mass drug administration (MDA) campaigns.3 Malathion and ivermectin lotions and sulphur preparations are alternative topical treatments.
This year marks the tenth anniversary from when, in October 2013, scabies was added to the WHO list of neglected tropical diseases. While the exact global prevalence is unknown, it appears to exceed that of psoriasis; however, a cursory PubMed search for "psoriasis" and "prevalence" at the time of writing this commentary yielded 2715 results compared to less than 500 when searching for "scabies" and "prevalence". Understanding disease prevalence helps in understanding disease burden at a point in time as well as guiding service planning.
The systematic literature review on the worldwide scabies prevalence by Schneider et al.4 provides a comprehensive and welcome update on the only previous systematic review on the subject published in The Lancet in 2015.5 At the same time, limitations and shortcomings of such data are evident. The latest review includes 43 articles published about this topic between October 2014 and March 2022. Of these, 41 studies were performed in Africa, Asia and Oceania, mainly in countries with a medium or low human development index (HDI). The prevalence of scabies is expected to be higher in these contexts because poverty and poor hygiene are known to be associated with increased transmission. The reported prevalence was however still highly variable, and for example with respect to children, the highest prevalence of 76.9% was reported in Indonesia which has a higher HDI than Uganda where the study with the lowest reported prevalence of 0.2% was performed. These results, although valuable, may reflect variable sampling methods utilized and lack of use of standardized diagnostic criteria. A high prevalence might also reflect a local outbreak while low prevalence in a sample may not necessarily reflect that of a whole population. It is hoped that future studies follow established consensus diagnostic criteria and include larger cohorts.2 Data for this study comes from only 23 countries, exposing limitations in availability of published data on the prevalence of scabies. This is even more striking in Europe and North America from where there is no data. While HDI is generally high in countries within these continents, one cannot automatically assume a low scabies prevalence. Furthermore it is known that, in such countries, poverty still exists, migration is an important phenomenon and iatrogenic immunosuppression is not uncommon. Factors like these may all increase the risk of scabies.1
A consistent finding in published reviews is that scabies is always more prevalent in the paediatric population, probably related to children getting in close contact, especially while playing. Public health efforts might thus need to particularly target the younger age groups. Another interesting finding of the latest review concerns the effect of MDA campaigns. It appears that oral ivermectin is highly effective and can drastically lower the prevalence of scabies. Topical scabicides appear to be less efficacious in these situations, unless combined with oral ivermectin. It also appears that over time, after MDA, the prevalence of scabies starts rising again and this may be averted by yearly administration of oral therapy.
This latest review provides a valuable update on the global prevalence of scabies; however, it remains clear that overall, scabies is still a neglected disease, with limited validated data available to truly estimate the worldwide prevalence. Well-designed population-based studies can estimate the true prevalence of scabies in different age-groups and help target awareness and treatment campaigns accordingly.