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Scabies

Key Points

  • Scabies is a highly contagious infestation due to the skin mite, Sarcoptes scabiei.
  • Typical sites of infestation are: wrists, ankles, interdigital web spaces, umbilicus, intertriginous sites (axilla, inguinal folds), and genitalia. The face and scalp are usually spared.
  • Patients commonly report intense pruritus. However, affected patients with neurologic disease, immunocompromised status, infants, or elderly individuals may not report pruritus.
  • The diagnosis of scabies can be made by presence of mites, eggs, or mite feces (scyballa) in skin scrapings, and can be visualized by skin biopsy.
  • The mainstay of scabies treatment in the United States includes permethrin 5% cream (such as Elimite) applied overnight in combination with decontamination of potentially infested clothing, bedding, and linens. Treatment of infants and pregnant women is distinct, and many different therapies are utilized around the world.
  • All close contacts or household members of infested individuals should be treated at the same time as the patient.
  • Systemic anti-parasitic treatment, such as ivermectin (such as Stromectol), may be necessary in severe cases such as generalized, crusted scabies infestation.

Initial Evaluation

Erythematous, crusted, excoriated papules, pustules, and vesicles. Linear burrows representing the path of the mite through the skin may also be seen. Wrists, interdigital web spaces, axilla, genitalia are typically affected sites.

Infants may present with typical features seen in adults but also with a vesiculo-pustular eruption of the palms and soles in association with linear burrows.

Hyperkeratotic lesions of crusted scabies.

Infants may present with typical features seen in adults but also with a vesiculo-pustular eruption of the palms and soles in association with linear burrows.

Diagnostic scraping shows scabies mites and eggs*.

*Reproduced from http://www.dpd.cdc.gov/dpdx/Default.htm.

Differential diagnosis

Atopic dermatitis

Principles of Management of Scabies

Initial therapy

First line: Topical 5% permethrin cream, applied overnight to entire body except face, hair-bearing scalp. Rinse thoroughly and follow decontamination protocol.

  • Treat patient and all household members and close contacts at the same time.
  • For children over 2 months of age: Topical 5% permethrin cream, applied overnight to entire body except areas around the eyes. Rinse thoroughly and follow decontamination protocol. Repeat in one week.
  • For infants under the age of 2 months: Precipitated sulfur 6% in petrolatum to be left on for 24 hours a day for 3 days.
  • Benzyl benzoate (10%) solution is a common alternative in Europe and in Australia; it is applied for 24 hours in most individuals. It may be an alternative treatment for infants < 2 years of age and pregnant women; it is applied for 12 hours in these two patient subsets.
  • For cases of crusted scabies, a combination of topical permethrin and ivermectin is used in children over the age of 5 years of age (>15 kg).
    • Week 1: Topical permethrin, also ivermectin 200 mcg/kg (first dose)
    • Week 2: Topical permethrin
    • Week 3: Topical permethrin, also ivermectin 200 mcg/kg (second dose)
  • Note that single-dose ivermectin is not effective in sterilizing eggs, and a second dose is required at least one week later to eradicate newly hatched mites.
  • Ivermectin should not be given to children under the age of 5 years of age.
  • Decontamination protocol: All clothing and bedding used prior to treatment should be washed in hot water with detergent and dried in a dryer. Non-washable garments, bedding, and other items should be sealed in a plastic bag for 2 weeks or dry-cleaned if possible.
  • Contact precautions: Patients should remain in contact precautions for 24 hours following treatment (e.g., 24 hours after the treatment has been washed off).
  • Patients often need a combination of topical steroids (for the dermatitis; start 24 hours after the anti-parasitic treatment) and systemic antihistamines (for itch).
    • For patients with dermatitis, prescribe a medium-potency topical steroid (such as triamcinolone acetonide 0.1% lotion, cream, or ointment applied b.i.d.).
    • For patients with pruritus, prescribe oral antihistamines.
  • All effective anti-pruritic agents can result in drowsiness as a side effect.
    • Start by prescribing an evening dose, taken approximately 1 hour prior to bedtime. Give oral antihistamines (hydroxyzine, such as Atarax or Vistaril)
    • Hydroxyzine suspension (10 mg/5 cc)
    • Doxepin suspension (10 mg/cc)
    • Diphenhydramine (such as Benadryl, 5 mg/kg/day maximum) may be substituted for hydroxyzine as it has less-sedating effect.
    • Loratadine (Claritin®) and cetirizine hydrochloride (Zyrtec®) suspension can be also used.
  • If the diagnosis of scabies is not confirmed by skin scraping, yet the clinical findings are highly suspicious, it is reasonable to treat with a single application of permethrin 5% cream.
  • Persons who have thick or crusted lesions frequently require a repeat course of treatment, or require a keratolytic agent (such as topical urea or salicylic acid cream) as an adjunctive treatment.

Pitfalls

  • Most therapeutic failures are related to improper use of the topical agents. The topical scabicides must be applied to the whole cutaneous surface (below the neck, except in young children and those with head and neck lesions). Antiscabetic medications are never applied to the lesions only. Care must be taken to apply the topicals carefully between the fingers and toes and under the fingernails, which should be trimmed short. If hands are washed during the treatment period, the medication must be reapplied.
  • Another major reason treatment fails is that all affected persons are not treated at the same time. All family members, even if they do not itch, must be treated. In households where there are babies, this includes babysitters and occasional visitors who have held the baby. Infested persons may not itch for 4-6 weeks after acquiring infection, yet may infect others.
  • Patients often report persistent pruritus despite adequate treatment for scabies. This may be due to persistence of inflammatory cells or mediators in the areas of previously affected skin. It is important to evaluate patients with this common complaint to determine whether they have new lesions suggestive of persistent infestation. Pruritus should be managed with combination therapy of topical corticosteroids (started 24 hours after the initial treatment), gentle skin care, and systemic antihistamines.
  • Bacterial superinfection with Staphylococcus aureus infection is very common. The presence or development of pustules, folliculitis, or cellulitis should be evaluated with high suspicion of bacterial superinfection. A bacterial culture should be performed. For those with secondary infection, give oral antistaphylococcal antibiotics for 1 week.
  • Lesions of scabies may develop into non-infectious prurigo nodules. Involvement of the genitalia is common and may require intralesional corticosteroid treatment.

When to refer to a dermatologist

  • If the diagnosis of scabies is not clear.
  • If the skin eruptions persist despite repeated treatments.
  • For management of nodular scabies or lesions requiring intralesional corticosteroids.

Clinical Cases

Case 1

  • 6-year-old male
  • Attends elementary school; develops intensely pruritic rash on wrists, umbilicus, hands and feet
  • No sick contacts
  • Excoriated scaly papules clustered on the wrists/hands, umbilicus, and scattered across the trunk; genitalia, face and scalp are spared

Initial evaluation

  • A skin scraping taken from the feet reveals mites and eggs
  • The patient is prescribed:
    • Permethrin 5% cream, to be applied overnight to the whole body except face/ scalp and repeated in 1 week
    • Triamcinolone 0.1% ointment b.i.d. (started 24 hours after first anti-parasitic treatment)
    • Hydroxyzine for nocturnal pruritus
  • Simultaneous treatment of entire family and babysitter is mandatory
  • Instructions for decontamination protocol are given
  • Report to school infection control
  • Follow-up in 2 weeks

Follow-up evaluation

  • Patient’s eruption has resolved, but pruritus persists
  • Patient has several follicular-based pustules with surrounding erythema on the legs; a bacterial culture is taken, oral cephalexin is empirically started for presumed staphylococcal pyoderma, the culture reveals methicillin-sensitive Staphylococcus aureus
  • No additional sick contacts
  • Continue oral antihistamines and topical steroids for 1-2 more weeks
  • Follow-up in 1 month (lesions, pruritus resolved)

Case 2

Crusted Scabies

  • 37-year-old HIV+ male
  • Recently discharged from hospital; was hospitalized for 2 weeks
  • 9-week history of non-pruritic eruption, almost generalized; medium-potency topical steroids alleviate scale, but lesions are persistent
  • No sick contacts

Initial evaluation

  • Patient presents with almost generalized, thickly crusted papules and plaques, including on the scalp, ears, palms, and soles
  • Skin scraping from the palms reveals numerous scabetic mites
  • The patient is prescribed:
    • Ivermectin, two doses of 200 mcg/kg given 14 days apart
    • Permethrin 5% cream, 3 applications applied overnight, 7 days apart
    • Triamcinolone 0.1% ointment b.i.d. (started 24 hours after first anti-parasitic treatment)
    • Hydroxyzine 50 mg at bedtime
  • Close contacts, including sexual partners and household members, should also all be treated at the same time as the patient
  • Instructions for decontamination protocol are given
  • Reporting to hospital infection control
  • Follow-up in 3 weeks

Follow-up evaluation

  • Patient’s eruption has resolved
  • Two pruritic nodules on the scrotum persist and are treated with intralesional triamcinolone 0.1% (5 mg/cc)
  • No additional sick contacts
  • Continue oral antihistamines and topical steroids for 2-4 more weeks
  • Follow-up in 6 weeks (lesions, pruritus resolved)

Suggested Reading

Currie BJ and McCarthy JS (2010) Permethrin and Ivermectin for scabies, NEJM, 362: 717-725.