Introduction
Scabies is a relatively common infestation caused by the mite Sarcoptes scabiei var hominis.
Epidemiology
Scabies affect around 200 million people per year globally and the prevalence is highest in tropical and subtropical regions, especially in low- and middle-income countries. Regions with the highest prevalence of scabies include India, the South Pacific, and Northern Australia. Scabies remain endemic in regions such as the Pacific Islands, sub-Saharan Africa, and Southeast Asia. In high-income countries, scabies is typically observed as localized outbreaks within institutional settings, including hospitals, long-term care facilities, and correctional institutions. An estimate of 1 in 50 long-term care facilities will experience a scabies outbreak in a year.
Scabies occur more often in children <15 years old, sexually active young adults, the immunocompromised, and persons living in crowded living conditions (eg nursing homes, military barracks, and prisons). Epidemiologic data indicate no significant variation in prevalence between males and females.
Etiology
Scabies is a contagious skin disorder caused by infestation of the epidermis by the female mite Sarcoptes scabiei var hominis, which burrows into the stratum corneum to lay its eggs.
Pathophysiology
The pathophysiology of scabies involves complex interactions between the host immune system and mite-derived antigens released during infestation. Sarcoptes scabiei produces multiple immunogenic proteins including proteases, pseudoproteases, and other allergen-like molecules, that disrupt the epidermal barrier and promote host sensitization. The mites also employ immune-evasive mechanisms, such as modulation of complement pathways and suppression of antimicrobial peptides, allowing them to persist within the host and increasing the risk of secondary bacterial infection, particularly with Staphylococcus aureus.
Scabies_Disease Background
Transmission typically occurs through prolonged direct skin contact with an infected person. Sexual transmission is common among adults. Transmission through fomites (eg bedding, clothing) and casual skin contact is uncommon in classic scabies. Though there is limited documentation, transmission by fomites may be possible (especially in cases of crusted scabies, where a large number of parasites are involved). The mites can live for up to 30 days on a host and remain alive for 3 days on furniture, bedding, etc.
Risk Factors
Scabies occur more commonly in young children and is associated with risk factors such as inadequate hygiene, malnutrition, outbreaks in institutional environments (eg hospitals and nursing homes), inadequate access to health facilities, overcrowding, and immunocompromised states.
Classification
Scabies have several forms such as classical scabies, crusted scabies (which is an uncommon form of scabies), and nodular scabies.
Classical Scabies
Classical scabies occur in immunocompetent patients. The mite burden is on average 5-15 mites/host during an initial episode.
Crusted Scabies
Crusted scabies are also known as Norwegian scabies, Boeck scabies, scabies crustosa, or keratotic scabies. This is a highly contagious form of scabies, with the affected person being infested with hundreds to millions of mites.
Nodular Scabies
Nodular scabies are highly pruritic reddish-brown nodules and papules that typically develop after scabies treatment. This is not necessarily an active or unresolved infestation but may persist for weeks to months after successful treatment.