Scabies

• Scabies:

  • Highly contagious skin infestation caused by a mite
  • Spread by close contact and is more common among disadvantaged populations
  • Children, the elderly, and people residing in long-term care facilities are at highest risk
• Aetiology:

  • Caused by parasitic mite - Sarcoptes scabiei var. hominis
  • Transmitted through direct skin-to-skin contact, or less commonly indirectly via fomites (contaminated objects)
  • Clinical features of scabies infection primarily result from a local allergic reaction to the presence of the scabies mite, rather than being directly caused by the mite itself
• Risk factors:

  • Direct contact - can be brief
  • Fomites - indirect contact through bedding, clothes, or towels
  • Living conditions - poverty and overcrowding - institutional care facilities
  • Environment - transmission more common in warm, tropical, humid environments
  • Immunosuppression - increases risk of infection with scabies and may also lead to more severe and persistent infection
• History:

  • In the first infection scabies may be asymptomatic for up to 6 weeks after exposure - but asymptomatic period will shorted significantly if subsequent infection occur
  • Once symptomatic patients report severe pruritus worse at night, severe enough to cause sleep disturbance
  • Patients may have other family members or close contacts with similar symptoms
  • Important to take a detailed social history for potential risk factors and identify contacts
• Clinical exam:

  • Scabies is typically found in the webbing and sides of fingers, wrists, elbows, axillae, feet and genitals
  • The rash is due to a local hypersensitivity reaction to the mite
  • The appearance of the rash is quite variable but can include erythematous papules of vesicles, and surrounding dermatitis
  • Burrows are characteristic and appear as small irregular tracks around 1cm in length, classically found in the webbed spaces between fingers
  • On dermatoscopy, mites or mite eggs/faeces, may be visible in borrows - known as delta sign
• Crusted (Norwegian) scabies:

  • Severe variant of scabies where an individual is infected with thousands or millions of mites (5-20 in typical infection)
  • Very infectious
  • Causes a crusted, scaly, keratotic rash
  • Other in the finger webs, wrists, and elbows
  • Itch may be less prominent, or even absent
• Investigations:

  • Scabies can be diagnosed clinically but investigations can assist with the diagnosis:
  • Dermatoscopy - visualise burrows that may be too small to see with naked eye
  • Skin scraping a biopsy for histopathology - sample the burrow itself not surrounding skin
• General management advice:

  • Can be difficult to eradicate as it is highly contagious and has a long incubation period
  • Good hygiene is important - washing all bed linen, clothes, toys etc
  • Close contacts of a person with scabies will need to be treated
• Topical therapies:

  • First line = permethrin 5% cream - apply to entire body (excluding face) and leave for 8 hours. Treatment repeated in 7 days
  • Benzyl benzoate 25% emulsion - second line in cases of allergy or treatment failure but can cause skin irritation
• Night-time use of a sedating anti-histamine (e.g. chlorphenamine) may help with sleep and reduce itching
• Systemic therapies:

  • Ivermectin - effective oral scabicide
  • Consider use if resistant scabies after 2 courses of topical therapy
  • Useful where compliance is difficult or to contain widespread outbreaks
  • A period of 4 weeks following treatment with ivermectin should elapse before full recovery can be considered
• Seek specialist advice/refer if:

  • Treatment is required in a child under 2 months old
  • Crusted scabies is suspected - hospital admission may be required, investigate for underlying immunodeficiency
  • Persistent scabies
• Complications:

  • Secondary bacterial infection - common due to patients scratching. Most commonly Streptococcus pyogenes or Staphylococcal aureus
  • Important to recognise and treat secondary infections early with antibiotics - risk of sequelae from group A strep including glomerulonephritis and rheumatic fever