Introduction

Scabies (aka “The Itch”) is a common parasitic skin disorder caused by the arachnid (mite) sarcotopes scabei. It is a very common disorder, and its morbidity is probably underrated. It is particularly a problem in the developing world, but is also often seen in general practice in the developed world. 

In some areas, resistance to standard treatments is starting to be seen.

It was one of the first diseases to have its cause identified, when the mite was observed under the microscope in the 17th Century.

Scabies mite - sarcotopes scabei

Scabies mite – sarcotopes scabei

Aetiology

  • Pets – particularly dogs
  • Other family members affected – highly contagious within families.
  • Sexually transmitted

Pathology and spread

  • The mites range from about 0.2 – 0.4mm in length, females are bigger than males
  • After they mate on the skin surface, the male dies, and female burrows in to the epidermis and lays her eggs. It takes about 10-15 days for the eggs to develop into mature adults
  • Adults then return to the skin surface to reproduce once again
  • They can live on the skin surface for about 4-6 weeks
  • Mites can move about 2.5cm / minute
    • They can’t jump or fly!
  • There is an incubation period of about 6 weeks, during which time, the affected individual will become sensitised to the mites faeces and saliva. Thus in the intial 2-6 weeks there are rarely signs and symptoms, but after this period, the papules appear, as the result of an immune response against the mites faeces (and possibly eggs).
    • In those with previous scabies the incubation period is only around 1-4 days
    • Even on those with a typical infection, there are a small number of mites at any one time – usually about 10
  • Highly contagious
  • Transmission can occur by direct skin contact, or transmitted via contaminated furniture, clothes, bedding and towels
    • Transfer via an inanimate object only occurs in crusted scabies (see below)
  • Mites can survive days with no host
  • Preventing spread
    • Wash clothes , bedding and towels on a high heat
    • Tumble dry clothes, bedding and towels on a high heat

Presentation

  • Papular rashcommonon the: abdomen, inner thigh, digital web spaces and flexor surface of the wrist.
    • The textbook definition often describes burrows being visible in the webbed spaces between the fingers. In my experience this does happen fairly frequently, but the presentation is quite variable.
  • Typically the itch is worse at night
Multiple skin lesions cause by scabies

Multiple skin lesions cause by scabies

Close-up of a scabies ‘burrow’ skin lesion

Close-up of a scabies ‘burrow’ skin lesion

Complications

Signs of complication may be visible at the initial presentation. These include:

  • Bacterial infection
  • Flare-up of underlying skin disorders
    • Eczema
    • Psoriasis

Diagnosis

The diagnosis is usually clinical, and no investigations are typology required.

In cases where there is doubt over the diagnosis, then skin scrapings may be sent for MC+S.

  • Microscopy – attempt to find one of the burrows, and tease out a mite for microscopy. It you can’t manage this, then scrapings from a burrow with a scalpel may yield eggs or faeces which can also be sent for microscopy.

Crusted scabies

  • This is sometimes called Norweigen scabies and may be seen in the immunosuppressed patient
  • In immunosuppressed individuals, particularly those with AIDS, cytotoxic T cells are unable to attack the mites, and the infection can get out of control. The whole body, except the face may become affected.
  • Patients with peripheral neuropathy, or the frail and elderly may also be affected
  • There may be thousands of mites as part of the infection, and they can be easily shed in flakes of skin that are lost.
  • The skin rash that results is hyperkeratototic and may resemble the plaques seen in psoriasis.
  • The lesions often suffer from secondary infection
  • Crusted scabies is very difficult to treat
  • There may also be an associated generalised lymphadenopathy

Differentials

  • Insect bites
  • Atopic dermatitis (eczema), and other forms of dermatitis
  • Tinea (fungal skin infection)
  • Impetigo
  • Psoriasis (crusted scabies)
  • Folliculitis
  • Lichen planus
  • Almost any other type of red rash!

Treatment

Scabies will continue indefinitely unless treated. Treatment is usually with a topical parasitical agent – such as permethrin.

  • Treat all members of the household and any sexual contact simultaneously
  • Permethrin – 5% topical agent. Treatment of choice. Apply to the affected areas, and wash off 8-12 hours later (usually applied before bed and left on overnight). Avoid on the eyes. If hands washed before 8h elapsed, re-apply. Repeat every 7 days until infection has gone, usually only 1-2 applications required.
    • Some places recommend “apply to affected areas” others recommend apply to “whole body from head to toe”
      • Use a brush to apply under the nails
    • Alternative agents include:
      • Malathion (topical)
      • Crotamiton (topical)
      • Ivermectin (oral) – may be used in conjunction with topical agents ons specialist advice in cases of crusted scabies
      • Permethrin has been shown to be the most effective
      • Those with crusted scabies should be treated for 2-3 consecutive days for each treatment to ensure that enough of the active ingredient penetrates the skin
  • Children can return to school the day after the first treatment
  • To prevent re-infection:
    • Wash clothes , bedding and towels on a high heat (>50°C)
    • Tumble dry clothes, bedding and towels on a high heat (>50°C)
    • An alternative is to leave items of clothing, bedding and towels in plastic bags for at least 72 hours to kill the mites. Particularly effective if you leave the bags in the sun on a hot day – the temperature in the bag will likely exceed >50°C.
  • Treating itch:
    • Anti-histamines – e.g. phenergan 10-25mg
    • Steroid cream
    • Emollients – especially if kept in the fridge before application

Flashcard

References

  • Scabies - patient.info
  • Beers, MH., Porter RS., Jones, TV., Kaplan JL., Berkwits, M. The Merck Manual of Diagnosis and Therapy
  • Murtagh’s General Practice. 6th Ed. (2015) John Murtagh, Jill Rosenblatt
  • Oxford Handbook of General Practice. 3rd Ed. (2010) Simon, C., Everitt, H., van Drop, F.

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