Scabies
Scabies is a highly contagious skin infestation caused by the mite Sarcoptes scabiei, characterised by intense pruritus and a papular rash due to hypersensitivity reactions to the mite and its products.
This updated UKMLA guide to scabies is based on PCDS guidance, which covers causes, transmission, risk factors, symptoms, diagnosis, and management.
Causes and Pathophysiology
Scabies is caused by infestation with the mite: Sarcoptes scabiei (a human parasite)
The female mite burrows into the stratum corneum of the skin and lays eggs → delayed type IV hypersensitivity reaction to the mites, eggs, and faeces.
Transmission and Risk Factors
Scabies is typically transmitted via close / prolonged skin contact with an infected person:
- Often transmitted via sexual contact
- Transmission via shared clothing or bedding can occur
- Transmission via casual contact (e.g. handshake) is unlikely
Risk factors:
- Close contact with an infested person
- Tropical and developing countries
- High levels of poverty and social deprivation
- Crowded living conditions and institutionalisation (e.g. nursing homes, long-term care facilities, prisons, military barracks)
- Winter months
Clinical Features
| Symptoms | Primary feature: generalised itching
The absence of itching does NOT exclude scabies (e.g. young babies and in people with neurological conditions with decreased / loss of sensation) |
| Signs |
Scabies is infectious before the rash develops Location and distribution:
Appearance:
Nodular lesions may also be seen, especially on the penis and scrotum in men, buttocks, groin, and the axillary regions |
| Relevant history |
|
Crusted scabies (Norwegian scabies)
- Caused by hyperinfestation with thousands or millions of mites
- It develops mainly in immunocompromised patients (e.g. HIV)
- Pruritus is mild or absent due to impaired immune response
- Skin lesions consist of generalised, poorly defined, erythematous, fissured plaques covered by scales and crusts
Complications
- Secondary bacterial infection (e.g. impetigo, folliculitis)
- Nodular scabies (pruritic nodules of the axillae, groin, and male genitalia can persist for weeks or months after treatment)
- Secondary eczematisation (due to scratching and/or irritant effects of topical medications)
Investigation and Diagnosis
Scabies is primarily a clinical diagnosis.
The following tests can provide a definitive diagnosis (if equipment and expertise allow, and used mainly if there is diagnostic uncertainty, atypical presentation, failed treatment etc.):
- Ink burrow test: +ve test would show dark zigzagged line running across and away from the lesion (due to ink tracking down the mite burrow)
- Skin scrapings microscopy – presence of mites / eggs / mite faecal material confirms the diagnosis
Management
Referral Criteria
Seek specialist advice if:
- Scabies in <2 m/o (scabies is rare in this age group and permethrin cream is only licensed for ≥2 months)
- Crusted scabies (hospital admission may be required)
- Isolation is necessary
- Treatment would likely involve topical insecticide + oral ivermectin
- It may be necessary to investigate for underlying immunodeficiency
General / Conservative Management
Advise that bedding, clothing, and towels (and those of all potentially infested contacts) should be decontaminated by washing at a high temperature (at least 60°C) and drying in a hot dryer, or dry-cleaning, or by sealing in a plastic bag for at least 72 hours
Anti-Scabies (Pharmacological) Treatment
There are 2 treatment regimens with similar effectiveness for classic scabies (i.e. non-crusted scabies):
- Permethrin 5% cream, OR
- Oral ivermectin
For both, give 1 dose / application initially, then repeat 1 more dose after 7 days
Both have similar effectiveness; choose the regimen based on the following factors:
| Consider avoiding permethrin cream if | Consider avoiding oral ivermectin if |
|
|
Advise that itching may continue for up to 4 weeks after successful treatment of scabies.
- Re-treatment should be advised if itching persists for>2-4 weeks after the last treatment application and/or new burrows have appeared
- Resistant cases can be treated with topical permethrin PLUS oral ivermectin OR refer for specialist advise.
Contact Management
All the following should be treated with anti-scabies treatment, even if asymptomatic:
- Household members
- Sexual partners within the past month
- Any other close personal contacts
Post-Scabetic Itch
Post-scabetic itch refers to persistent itching that continues after successful treatment of scabies, despite eradication of the mites. Management:
- Crotamiton 10% cream
- If scabies mites have definitely been eradicated: topical hydrocortisone 1%
Also consider nighttime sedating antihistamine (e.g. chlorphenamine) to improve sleep and reduce scratching