Scabies

This is a pruritic eruption due to cutaneous invasion by the mite Sarcoptes scabiei variety hominis (1).

• after mating the adult males remain on the skin surface and the adult females burrow into the stratum corneum within 30 minutes
• female scabies mites live for 4-6 weeks; they lay 1-3 eggs per day and burrow at a rate of 2-3 mm per day. In classic scabies 5 to 15 mites live in an infected host but in crusted scabies the number of mites can be as high as hundreds or even millions (1)
• each egg hatches within 3-4 days - when the egg hatches the larva escapes from the burrow (via cutting through the roof and then digging short burrows (moulting pockets) on the skin surface)
• the hatched larva remains on the skin surface and remains there until maturity; it takes 10-13 days from the laying of the egg to maturation of adult scabies mite

Transmission occurs by skin-to-skin contact. It is a common condition and is pandemic.

• direct (skin-to-skin contact) - the main route of transmission
• indirect (via infested clothing, bedding) - indirect transmission can be seen in crusted scabies but it rarely occurs in classic scabies (1)
  
  
• transmission in institutional settings and within family members is common (1)
  
  
• sexual transmission is possible (1)
  
  
• scabies is common and is pandemic
  • scabies is more prevalent in children, young adults, and in urban more than rural areas. Scabies is more common in women than in men and more common in winter than summer
  • there is little risk of transfer of infestation via towels, bedding, clothing, upholstery used by patients with typical symptoms of scabies - this is despite the ability of infectious mites to survive for up to 36 hours of the host at standard room conditions; however such transfer is a risk if a patient presents with Norwegian scabies (crusted scabies)

The main symptoms of scabies are due to immune reaction to burrowed mites and their products (e.g. faeces, saliva or eggs) (2)

Key points (3):

• First choice permethrin:
  • treat whole body from ear/chin downwards and under nails
  • if using permethrin and patient is under 2 years, elderly or immunosuppressed, or if treating with malathion: also treat face and scalp
• Home/sexual contacts:
  • treat within 24 hours

Reference:

• 1. Oliver Chosidow. Scabies. N Engl J Med 2006;354:1718-1727
• 2. Engelman D, Yoshizumi J, Hay RJ, et al. The 2020 International Alliance for the Control of Scabies Consensus Criteria for the Diagnosis of Scabies. Br J Dermatol. 2020 Nov;183(5):808-820.
• 3. Public Health England (June 2021). Managing common infections: guidance for primary care