History
A 20-year-old man presents to his GP with an intensely pruritic eruption. He is finding that the itching is keeping him awake at night, and consequently it is hard to concentrate in his college exams. He has come to the GP to ask for a sick note. One of his housemates has also recently become itchy and they had begun to suspect their student flat may have an infestation of some sort. He was previously well and takes no regular medication. The patient had applied calamine lotion on the rash as recommended by his local chemist, with little relief.
Examination
There are multiple erythematous papules and excoriations over his limbs and trunk, which are particularly severe over his hands (Fig. 83.1) and on his genital area. There are some very subtle, palpable linear marks running along the sides of his fingers. His scalp, nails and mouth are normal.
Questions
• What is the most likely diagnosis?
• Should the pest control team be requested to visit their flat?
• Who should be treated and with what?
Medicine
There are few pruritic skin eruptions that keep patients awake at night and usually this is highly suggestive of a scabies infestation. The human mite Sarcoptes scabiei is passed from one individual to another, usually by direct skin contact but also through contact with fomites such as bedding and towels. The female parasitic mite that burrows into the skin can only live away from the human host for 1–2 days in the environment. Once the infestation has been transmitted there is a delay of about six weeks before the onset of pruritus, which is a type IV delayed hypersensitivity reaction to the proteins in the mites/eggs/faeces. However, symptoms of pruritus in subsequent infestations can start within days as the individual is already ‘primed’ to the protein. Several individuals in the same household may be infested simultaneously, especially if living in overcrowded or poor quality accommodation.
Approximately 10 adult females in burrows are present in the human skin during a sca-bies infestation, and consequently the burrows themselves may be difficult to identify.
They are linear wandering lesions, usually most easily seen in the web-spaces of the fingers and on the genitalia. If you look closely with a magnifying lens/dermatoscope you may just see a small black ‘speck’ (the mite) at the advancing edge. The remainder of the skin rash consists mainly of erythematous papules and excoriation marks, which reflects a pruritic allergic-type hypersensitivity eruption. Babies have a slightly different clinical presentation in that lesions may look more like blisters and they are particularly common on the soles of the feet (Fig. 83.2) and in the axillae, which should help mto distinguish scabies from atopic dermatitis. Crusted scabies occurs in individuals who have thousands of mites in their infestation such that the skin looks as if it has crust or fine scale on the surface. These individuals may be in a poor state of health with a suboptimally functioning immune system. Infested individuals and all close contacts should be treated with topical permethrin 5% lotion applied to all the skin from the neck downwards (babies should have the face/neck and scalp areas also treated). The treatment should be left on overnight, washed off in the morning and repeated after 7 days as the treatment is not ovicidal. Patients who are immunosuppressed or who have crusted scabies (avoid in children < 5 years old and in pregnancy) can be treated with oral ivermectin 200 μg/kg body weight in 2 doses two weeks apart. Both topical and oral treatments are highly effective. Apparent treatment failures usually result from not all close contacts being treated simultaneously.
The pruritus will continue for approximately 4–6 weeks after successful treatment of the infestation, as this is the time needed for the body to degrade and remove all the mite pro-tein in the skin. Post-scabies treatment therefore consists of soothing anti-itch emollients such as menthol in aqueous cream, and topical steroids can help to alleviate symptoms.
KEY POINTS
• The pruritus caused by a scabies infestation keeps patients awake at night.
• A detailed history concerning similarly affected household members is helpful in making a diagnosis.
• All close contacts should be treated simultaneously to ensure successful eradication of the mite.
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