Scabies is caused by the mite Sarcoptes scabiei var. hominis (Fig. 15.3).
Adult mites are 0.3– 0.4 mm long and therefore just visible, although hard to see except through a lens. It is now well established that the mites are transferred from person to person by close bodily contact and not via inanimate objects.
Once on the skin, fertilized female mites burrow through the stratum corneum at the rate of about 2 mm per day, and produce two or three oval eggs each day. These turn into sexually mature mites in 2–3 weeks. The number of mites varies from case to case, from less than 10 in a clean adult to many more in an unwashed child. The generalized eruption of scabies, and its itchiness, are thought to be caused by a sensit-ization to the mites or their products.
The prevalence of scabies in many populations rises and falls cyclically, peaking every 15–20 years. The idea of ‘herd immunity’ has been put forward to explain this, spread being most easy when a new generation of susceptible individuals has arisen.
For the first 4 – 6 weeks after infestation there may be no itching, but thereafter pruritus dominates the picture, often affecting several people and being par-ticularly severe at night.
The most dramatic part of the eruptionaexcoriated, eczematized or urticarial papulesais usually on the trunk, but these changes are non-specific and a burrow has to be identified to confirm the diagnosis (Fig. 15.4).
Most burrows lie on the sides of the fingers, finger webs, sides of the hand and on the flexural aspects of the wrists. Other favourite sites include the elbows, ankles and feet (especially in infants; Fig. 15.5), nipples and genitals (Fig. 15.6). Only in infancy does scabies affect the face. Burrows are easily missed grey-white slightly scaly tortuous lines of up to 1 cm in length.
The acarus may be seen through a
lens as a small dark dot at the most recent least scaly end of the burrow. With
experience it can be removed for microsco-pic confirmation. On the genitals,
burrows are associated with erythematous rubbery nodules (Fig. 15.7).
Scabies persists indefinitely unless treated. In the chronic stage, the number of mites may be small and diagnosis is correspondingly difficult. Relapses after apparently adequate treatment are common and can be put down to reinfestation from undetected and untreated contacts.
• Secondary infection, with pustulation, is common (Fig. 15.8). Rarely, glomerulonephritis follows this.
• Repeated applications of scabicides can cause skin irritation and eczema.
Persistent itchy red nodules may remain on the gen-itals or armpits of children for some months after ade-quate treatment.
• Venereal disease may be acquired at the same time as scabies.
• Crusted (Norwegian) scabies, which may not be itchy, is a widespread crusted eruption in which vast numbers of mites are found. It affects people with learning difficulties or the immunosuppressed, and can be the unsuspected source of epidemics of ordinary scabies.
Only scabies shows characteristic burrows. Animal scabies from pets induces an itchy rash in humans but this lacks burrows. The lesions of papular urticaria are excoriated papules, in groups, mainly on the legs. Late-onset atopic eczema, cholinergic urticaria, lichen planus, neurotic exco-riations and dermatitis herpetiformis have their own distinctive features. Fibreglass can also cause epidemics of itching.
With practice an acarus can be picked neatly with a needle from the end of its burrow and identified microscopically; failing this, eggs and mites can be seen microscopically in burrow scrapings mounted in potassium hydroxide or mineral oil. Some find dermatoscopy a quick and reliable way to identify the mite.
• Use an effective scabicide; there are many on the market now (Formulary 1). In the UK, the preferred treatment is with malathion or permethrin; lindane is no longer available. Topical treatment plus ivermectin (on a named patient basis in the UK), in a single dose of 200 µg/kg by mouth, is effective for Norwegian scabies and scabies that does not respond to topical measures alone.
• For babies over 2 months, toddlers and young children we advise permethrin cream, 25% benzyl benzoate emulsion diluted with three parts of water, or 6% precipitated sulphur in white soft paraffin (petrolatum).
• It is still not clear which scabicides can be safely used to treat pregnant women or those who are breast-feeding. Despite the absence of convincing evidence that unborn children can be damaged by topical scabicides, we prefer to use the same measures that we use to treat babies (above).
• Do not just treat the patient: treat all members of the family and sexual contacts, whether they are itching or not (Fig. 15.9).
• Have a printed sheet to give to the patient and go through it with themascabies victims are notoriously confused.
• One convenient way to apply scabicides to the skin is with a 5 cm (2 inch) paintbrush. The number of applications recommended varies from dermatologist to dermatologist. There is no doubt that some pre-parations, such as malathion, disappear quickly from the skin, leaving it vulnerable to any mites which hatch out from eggs that have survived. A second application, a week after the first, is then essential. With permethrin, this may be less important. The main reason for recommending a second application is that it will cover areas left out during an inefficient first application.
• Make sure that patients grasp the fact that scabic-ides have to be applied to all areas of skin below the jaw line, including the genitals, soles of the feet, and skin under the free edge of the nails. If the hands are washed, the scabicide should be reapplied. A hot bath before treatment is no longer recommended.
• Ordinary laundering deals satisfactorily with cloth-ing and sheets. Mites die in clothing unworn for 1 week.
• Residual itching may last for several days, or even a few weeks, but this does not need further applications of the scabicide. Rely instead on calamine lotion or crotamiton.
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