Published on 18/03/2015 by admin
Filed under Dermatology
Last modified 18/03/2015
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William F.G. Tucker and James B. Powell
Evidence Levels: A Double-blind study B Clinical trial ≥ 20 subjects C Clinical trial < 20 subjects D Series ≥ 5 subjects E Anecdotal case reports
Scabies is a characteristically pruritic skin condition due to infestation by the itch mite Sarcoptes scabiei. Patients can be of any age or social strata, and personal hygiene is no guarantor of freedom from infection. It is endemic in many impoverished communities worldwide. Heavy parasitization results in crusted or ‘Norwegian scabies,’ and tends to occur in the elderly, mentally disadvantaged, immunocompromised, and those unable to scratch.
Lassa S, Campbell MJ, Bennett CE. Br J Dermat 2011; 164: 1329–34.
Those aged 10–19 years showed highest infestation rates with prevalence in females greater than in males. Prevalence dropped steadily from the 20–29 years age group but rose again in the 80+ group.
Infection with the scabies mite results from close personal contact with an infested individual, and generally requires prolonged skin-to-skin contact. Where heavily colonized, as in crusted scabies, clothing and surroundings can become sufficiently contaminated with live mites as to cause a significant hazard to their attendants and companions. The development of itching in close contacts/family members is almost pathognomonic.
Because newly infected patients do not begin to itch until 4 to 8 weeks after being infested, and may have little or no visible rash, the infestation is passed on easily and unintentionally. Once sensitized to the mite, re-infestation results in immediate symptoms. Equally, itching can continue for some weeks after mite cure. Secondary impetiginization is common in children.
Itching at night and when warm is a hallmark feature. Inflammatory papules and nodules often develop in genital areas, axillae, and around the nipples. In crusted scabies hyperkeratotic and crusted plaques can be seen.
Suspicion is the prerequisite for disease control and scabies infestation should come to mind in any patient with (unexplained) pruritus.
Total isolation from other people is clearly unnecessary with a causative organism that cannot jump or fly, and can survive for only approximately 72 hours away from the skin.
Topical application of antiscabetic agents is standard practice. Sulfur was the first effective agent and is still used in many countries, and when nothing else is suitable, as in very young infants. Benzoyl benzoate has been the mainstay of therapy, and is cheap and effective if used properly. It has an unfortunate tendency to cause eczematous rashes after repeat applications. Lindane (γ-benzene hexachloride) was in very widespread use until recently, and is very effective. Unfortunately, it accumulates in body fat stores and has been implicated in causing neurologic damage in infants; it has been withdrawn in the UK. Malathion is well tolerated and has the theoretic advantage of prolonged retention in the epidermis, so that re-infestation may be reduced. Permethrin as a 5% cream is easy to use, and has proven to be well tolerated and effective.
When deciding upon which antiscabetic to use, efficacy, cost, tolerability, resistance, and safety are important. Treatment regimens vary between products but the key to success is to ensure that adequate concentrations of the scabicide are in contact with the whole body for a sufficiently long period. It is generally accepted that the face and scalp do not need treatment except in infants and the immunocompromised individual. Reservoirs of infection include the subungual areas which must therefore be addressed when applying topical therapy. What is essential is that all intimates of the patient, whether apparently infected or not, are treated at about the same time. Many clinicians find an explanatory leaflet, detailing exactly how to apply a prescribed antiscabetic and also the need to launder clothes and bedding, helpful. The skills and patience of the physician are often rigorously tested throughout this process!
Ivermectin in single doses of 200 µg/kg has been shown to be very effective in a number of studies, although it is no more effective than properly applied permethrin. It is unlicensed for this indication in humans, but is used in animal ‘mange’ and is widely and safely used in onchocerciasis. Ivermectin should therefore be reserved for the treatment of crusted scabies and when appropriate application of topical antiscabetics is not possible, e.g., compliance, severe eczematization.
When treating crusted scabies combined topical and oral therapy may have to be considered to achieve a cure. Additional keratolytic therapy such as emollients, salicylic acid, and bathing may be needed to remove hyperkeratotic plaques.
Visualization of mites and burrows
The one specific investigation for scabies is to isolate the mite; nothing is more guaranteed to ensure compliance than to show a patient their co-dweller under the bench microscope. Dermatologists take great delight in capturing their quarry, and each will be an advocate of one particular method. With standard scabies, burrows are generally easiest to find around the hands, and the mite can often be seen as a dark dot at one end. A dermatoscope can help visualize the mite in situ and even reveal the characteristic triangle or ‘delta wing’ appearance at the mite’s head.
Argenziano G, Fabbrocini G, Delfino M. Arch Dermatol 1997; 133: 751–73.
With the aid of a blunt needle or sewing pin, the mite can be winkled out and placed on a glass slide. Note that hypodermic needles are useless for this purpose because they slice and shred the mite! Alternatively, the burrow is carefully scraped or shaved with a 15 Bard–Parker blade and the slice of stratum corneum placed on a glass slide with some immersion oil. Scraping/shaving is the test of choice in crusted scabies. Skin biopsy is often illuminating, particularly because the mite may be a surprise finding.
Treatment of Skin Disease Comprehensive Therapeutic Strategies 4e
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