Scabies

Original article by Tom Leach | Last updated on 4/6/2014
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Scabies aka “The Itch”
Caused by the arachnid (mite) sarcotopes scabei.
 

Aetiology

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

Presentation

  • Papular rash - commonon the: abdomen, inner thigh, digital web spaces and flexor surface of the wrist.
  • Typically the itch is worse at night
 

Spread

  • Highly contagious
  • Spread within an individual by scratching an affected papule, and then scratching another patch of skin
  • Spread between individuals via inanimate objects, e.g. PC peripherals, toilets clothing, towels, bedding
  • Mites can survive days with no host
  • Mites live 3-4 weeks in the hosts skin
  • The females lay eggs in the stratum corneum which hatch in 3-10 days. These spend the larval stage on the skin surface, before burrowing under the skin again as adults.
    • Adult males may burrow under the skin, but tend to live on the skin surface. Females burrow to lay eggs.
  • Preventing spread
    • Wash clothes , bedding and towels on a high heat
    • Tumble dry clothes, bedding and towels on a high heat
 

Pathology

The mites burrow under the skin, resulting in papules.
 
In the image below you can see the original papule, and then in the top right, you can see the route where the mite burrowed. The mite itself is visible as a dark spot at the end of the burrow.
 
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
 

Diagnosis

  • 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.
 

Treatment

  • Treat all members of the household!
  • 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.
  • To prevent re-infection:
    • Wash clothes , bedding and towels on a high heat
    • Tumble dry clothes, bedding and towels on a high heat
 
Scabies 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.
This is sometimes called Norweigen scabies