Scabies is a contagious skin infestation caused by the microscopic mite Sarcoptes scabiei var. hominis. It burrows into the upper layer of the skin (stratum corneum), where the female mite lays eggs, deposits faecal pellets, and stimulates a host immune response. The disease is recognized by the World Health Organization as a neglected tropical disease because of its high global burden, especially in resource-limited settings.
How Does the Scabies Mite Live and Spread?
The life cycle of Sarcoptes scabiei includes egg, larva, nymph, and adult stages. After mating on the host skin, the fertilized female tunnels into the stratum corneum, laying eggs over her lifespan. The eggs hatch into larvae, which continue development through nymph to adult. This whole cycle takes about two to three weeks.
Transmission occurs primarily by prolonged direct skin-to-skin contact. The mite can survive off the human host for a limited time (hours to a few days), depending on environmental conditions such as temperature and humidity. Overcrowded living conditions, frequent contact among individuals (e.g., in schools, nursing homes, prisons), poor access to treatment, and low socio-economic status vector higher rates of transmission.
Where and in Whom Does Scabies Occur?
Scabies is found worldwide. However, its prevalence is much higher in tropical and subtropical regions, and in communities with limited access to healthcare. Children and the elderly are particularly vulnerable.
In high-income countries, scabies occurs often in outbreaks (e.g. in institutions), rather than as a constant burden of disease. In low- and middle-income countries, it may be endemic, affecting a large proportion of communities.
What Are the Clinical Signs and Symptoms?
Scabies typically causes intense itching, often worse at night. The rash appears as small papules (bumps), sometimes vesicles, excoriations (from scratching), and may include burrows—fine, serpentine tracks in the skin where the mite tunnels. Common sites include finger webs, wrists, elbows, buttocks, genitalia, and in infants, also the scalp, palms and soles. The appearance can vary with age and immune status.
Crusted scabies (also called Norwegian scabies) is a more severe form seen in patients who are immunocompromised or have reduced ability to scratch or mount an immune response. In crusted scabies, thick crusts containing many mites form, and there is greater risk of spread.
Symptoms often develop within a few days in people previously exposed, but for first-time infestations, may take up to 4-8 weeks before symptoms appear.
What Does Scabies Look Like Under a Dermatoscope?
Dermoscopy has emerged as a valuable non-invasive tool for diagnosing scabies, particularly in settings where traditional skin scraping is challenging. Characteristic dermoscopic features include:
·Serpentine Burrows: Curvilinear, white to yellowish structures resembling scabies tunnels.
·"Delta Glider" Sign: A dark triangular shape at the burrow's end, representing the mite's head and anterior legs.
·"Jet with Contrail" Pattern: A dark triangular structure (the mite) followed by a lighter, linear trail (the burrow), resembling a jet contrail.
·Punctate or Globular Vessels: Focally distributed blood vessels, often observed around burrows.
How Is Scabies Treated?
Treatment options include topical agents and oral medications. Topical treatments are first-line in many guidelines; among them are creams (5 % permethrin cream) or lotions applied to the whole body (not just symptomatic areas). When topical therapy is not feasible or in certain severe cases like crusted scabies, systemic treatment (e.g., oral ivermectin) is used.
Treatment of close contacts is essential even if they show no symptoms, to prevent reinfestation. Environmental decontamination of bedding, clothing, towels (washing or isolating for days) is part of comprehensive management. Follow-up is needed to ensure symptoms resolve and that new mites are not present.
What Should Be Expected After Treatment?
After an adequate treatment course, itching and rash typically begin to improve within days, though itch may persist for some time as old lesions recover. Full resolution may take 2-4 weeks. Scratching may linger (“post-scabetic itch”) even when mites are eradicated.
In cases of crusted scabies or in immunocompromised patients, multiple treatments or combined topical plus systemic treatments may be required. Close monitoring for reinfestation (from contacts or environment) is necessary. Preventing relapse depends heavily on treating all contacts and dealing with environmental reservoirs.
What Are the Challenges in Diagnosing and Treating Scabies?
Several challenges affect control of scabies. Misdiagnosis is common because early lesions may mimic other skin conditions (eczema, insect bites, dermatitis). Access to diagnostic tools (dermoscopy, microscopy) is limited in many settings. Treatment resistance or suboptimal response has been reported in some mites.
Another challenge is the sustainability of mass drug administration campaigns: while these can reduce prevalence, maintaining that reduction is hard. Reinfection from untreated or partially treated close contacts, household members, or community settings causes relapse. Environmental and social determinants—poverty, overcrowding—remain barriers.