Scabies
Scabies is an infectious disease caused by parasites (Sarcoptes scabiei). It is characterized by intense itching and a distinct rash.
Pathogen
The infestation begins when female mites burrow into the stratum corneum of the skin, laying eggs that hatch into larvae within 3-4 days. These larvae grow into adults within about two weeks, perpetuating the cycle. The intense itching and rash result from a hypersensitivity reaction to the mites, their eggs, and their faecal matter. These symptoms usually manifest 2 to 6 weeks after the initial infestation.
Scabies is primarily transmitted through direct skin-to-skin contact with an infected individual. Prolonged contact, typically lasting 10 to 15 minutes, is usually required, although in severe infestations, shorter exposure can suffice. Scabies mites can also spread via contact with contaminated textiles, such as bedding or clothing, where they can survive for up to 36 hours outside the human host.
Crusted scabies (or Norwegian Scabies), a severe form of the disease, involves hyperkeratosis and scaling due to a massive mite burden, often exceeding a million mites. This extreme form of scabies can be found in immunocompromised patients and is usually characterized by the lack of the typical inflammatory response, leading to minimal itching but severe skin damage.
Risk Factors
Close Contact: Living in crowded or communal environments increases transmission risk.
Compromised Immunity: Immunosuppressed individuals are more susceptible to crusted scabies.
Socioeconomic Factors: Poverty and limited access to healthcare facilitate the spread of infestation.
Age: Children and elderly individuals are disproportionately affected.
Epidemiology
Scabies affects over 200 million people worldwide at any given time, making it a significant public health concern. It is prevalent in crowded living conditions such as care homes for elderlies, refugee camps or kindergartens.
Clinical Presentation
Scabies typically presents as an itchy rash characterized by small, red papules often accompanied by track-like burrows. The most commonly affected areas are:
finger webs
wrists
axillae
groin
genitals
In infants, the palms, soles, and scalp may also be affected.
The itching tends to worsen at night, as the mites are more active and burrow into the stratum corneum of the skin to lay their eggs. The burrowing activity, along with the eggs and faecal matter produced by the mites, are the primary triggers of the immune response and associated itching. Apart from the primary itchy rash, secondary bacterial infections, may result from excoriations, caused by the continuous scratching.
Crusted Scabies: Patients present with thick, scaly plaques that can be mistaken for psoriasis. They are highly contagious and may require inpatient treatment.
Immunosuppressed patients or those undergoing glucocorticoid therapy often exhibit reduced itching and a milder rash due to the suppressed immune response. As a result, the infection may be less apparent in these cases, potentially masking its presence.
Diagnosis
Scabies is primarily a clinical diagnosis based on history and physical examination. Confirmatory methods include:
Dermatoscopy to identify burrows and mites.
Microscopy from scraped off skin, to visualize mites, eggs, or faecal pellets.
In endemic settings, clinical suspicion often suffices for initiating treatment.
Therapy
Effective management of scabies requires both pharmacological treatment and environmental decontamination
Pharmacological treatment
The pharmacological treatment consists in using Permethrin Cream (5 %), this cream needs to be applied to the entire body (including under nails and soles), and left on for 8-12 hours, it needs to be repeated after 7 days to eliminate the mites which hatched from the eggs which survived the first application of the cream.
In severe cases like crusted scabies, or when any resistance is encountered, oral Ivermectin might be necessary, it is administered as a single dose and repeated after one week.
For symptomatic relief crotamiton cream or oral antihistamines can be used to alleviate itching, which may persist for up to 4 weeks post-treatment due to a lingering hypersensitivity reaction.
Environmental Decontamination
Environmental Decontamination includes washing all clothing, bedding, and towels used by the patient on a hot cycle (> 50 °C). vacuum carpets and furniture thoroughly.
Remember that all household and close contacts need to be treated simultaneously, regardless of symptom presence, to prevent reinfestation.
References
Salavastru CM, Chosidow O, Boffa MJ, Janier M, Tiplica GS. European guideline for the management of scabies. Journal of the European Academy of Dermatology and Venereology : JEADV. 2017 Aug;31(8):1248-1253. DOI: 10.1111/jdv.14351. PMID: 28639722.