Scabies

Risk Factors

• Personal skin-to-skin contact (e.g., sexual promiscuity, crowding, nosocomial infection)
• Poor nutritional status, poverty, homelessness, and poor hygiene
• Seasonal variation: Incidence may be higher in the winter than in the summer (may be due to overcrowding).
• Immunocompromised patients including those with HIV/AIDS are at increased risk of developing severe (crusted/Norwegian) scabies.

History

• Generalized itching is often severe and worse at night.
• Determine any contact with infected individuals.
• Initial infection may be asymptomatic.
• Symptoms may develop after 3–6 weeks.

Physical Exam

• Lesions (inflammatory, erythematous, pruritic papules) most commonly located in the finger webs, flexor surfaces of the wrists, elbows, axillae, buttocks, genitalia, feet, and ankles
• Burrows (thin, curvy, elevated lines in the upper epidermis that measure 1–10 mm) may be seen in involved areas; these are considered a pathognomic sign of scabies.
• Secondary erosions or excoriations
• Pustules (if secondarily infected)
• Nodules in covered areas (buttocks, groin, axillae)
• Crusted scabies (Norwegian scabies) is a psoriasiform dermatosis occurring with hyperinfestation with thousands of mites (more common in immunosuppressed patients).

Geriatric Considerations

• The elderly often itch more severely despite fewer cutaneous lesions and are at risk for extensive infestations, perhaps related to a decline in cell-mediated immunity. There may be back involvement in those who are bedridden.

Pediatric Considerations

• Infants and very young children often present with vesicles, papules, and pustules and have more widespread involvement, including the hands, palms, feet, soles, body folds, and head (rare for adults).

Diagnostic Tests & Interpretation

• Definitive diagnosis requires microscopic identification of mite, eggs, or feces.
• Failure to find mite does not rule out scabies

Initial lab tests

• Complete blood count (CBC) is rarely needed but may show eosinophilia.

Diagnostic Procedures/Surgery

• Examination of skin with magnifying lens:
  • Look for typical burrows in finger webs and on flexor aspects of the wrists and penis.
  • Look for a dark point at the end of the burrow (the mite).
  • Presumptive diagnosis is based on clinical presentation, skin lesions, and identification of burrow
  • The mite can be extracted with a 25-gauge needle and examined microscopically.
• Mineral oil mounts
  • Place a drop of mineral oil over a suspected lesion. Nonexcoriated papules or vesicles also may be sampled.
  • Scrape the lesion with a no. 15 surgical blade.
  • Examine under a microscope for mites, eggs, egg casings, or feces.
  • Scraping from under fingernails often may be positive.
• Potassium hydroxide (KOH) wet mount not recommended because it can dissolve mite pellets (1
• Burrow ink test
  • If burrows are not obvious, apply blue–black ink to an area of rash. Wash off the ink with alcohol. A burrow should remain stained and become more evident.
  • Then apply mineral oil, scrape, and observe microscopically, as noted previously.
• Epiluminescence microscopy and high-resolution video dermatoscopy are expensive and have not been proven to be more sensitive than skin scraping (2,3)[C].

Treatment

• Medication

First Line

• Permethrin is the most effective topical agent for scabies (5)[A]. 5% cream (Elimite, Acticin):
  • After bathing or showering, apply cream from the neck to the soles of the feet; then wash off after 8–14 h. A 2nd application 1 week later is recommended if new lesions develop.
  • 30 g is usually adequate for an adult.
  • Side effects include itching and stinging (minimal absorption).

Pediatric Considerations

• Permethrin may be used on infants. In children <5 years of age, the cream should be applied to the head and neck as well as to the entire body.

P.1175

Second Line

• Crotamiton (Eurax) 10% cream:
  • Apply from the neck down for 24 h, rinse off, then reapply for an additional 24 h, and then thoroughly wash off.
  • Nodular scabies: Apply to nodules for 24 h, rinse off, then reapply for an additional 24 h, and then thoroughly wash off.
• Ivermectin (Stromectol):
  • Not Food and Drug Administration (FDA) approved for scabies; 200–250 µg/kg as single dose; repeated in 1 week
  • May need higher doses or many need to use in combination with topical scabicide for HIV-positive patients
• Precipitated sulfur 5–10% in petrolatum: Apply to the entire body from the neck down for 24 h, rinse by bathing, then repeat for 2 more days (3 days total). It is malodorous and messy but is thought to be safer than lindane, especially in infants <6 months of age, and safer than permethrin in infants <2 months of age.
• Lindane (Kwell) 1% lotion:
  • Apply to all skin surfaces from the neck down and wash off 6–8 h later.
  • 2 applications 1 week apart are recommended but may increase the risk of toxicity.
  • 2 oz is usually adequate for an adult.
    • Side effects: Neurotoxicity (seizures, muscle spasms), aplastic anemia
    • Contraindications: Uncontrolled seizure disorder, premature infants
    • Precautions: Use on excoriated skin, immunocompromised patients, conditions that may increase risk of seizures, or medications that decrease seizure threshold
    • Possible interactions: Concomitant use with medications that lower the seizure threshold

Alert

Lindane: FDA black box warning of severe neurologic toxicity; use only when 1st-line agents have failed.

Pediatric Considerations

The FDA recommends caution when using lindane in patients who weigh <50 kg. It is not recommended for infants and is contraindicated in premature infants.

Pregnancy Considerations

• Permethrin is category B and lindane and crotamiton are category C drugs.
• Permethrin is considered compatible with lactation, but if permethrin is used while breast-feeding, the infant should be bottle fed until the cream has been thoroughly washed off.

Additional Treatment

General Measures

• Treat all intimate contacts and close household and family members.
• Wash all clothing, bed linens, and towels in hot (60°C) water or dry clean.
• Personal items that cannot be washed or dry cleaned should be sealed in a plastic bag for 3–5 days.
• Some itching and dermatitis commonly persists for 10–14 days and can be treated with antihistamines and/or topical or oral corticosteroids.

• Patient Education

• Patients should be instructed on proper application and cautioned not to overuse the medication when applying it to the skin.
• Patient fact sheet is available from the Centers for Disease Control and Prevention (CDC): http://www.cdc.gov.

Prognosis

• Lesions begin to regress in 1–2 days, along with the worst itching, but eczema and itching may persist for up to 1 month after treatment.
• Nodular lesions may persist for several weeks, perhaps necessitating intralesional or systemic steroids.
• Some instances of lindane-resistant scabies have now been reported. These do respond to permethrin.

Complications

• Poor sleep owing to pruritus
• Social stigma
• Secondary bacterial infection
• Sepsis
• Glomerulonephritis
• Eczema
• Pyoderma
• Postscabetic pruritus
• Nodules (nodular scabies) may persist for weeks to months after treatment.

References

1. Chosidow O. Scabies. N Engl J Med. 2006;354:1718–27.

2. Heukelbach J, Feldmeier H. Scabies. Lancet. 2006;367:1767–74.

3. Leone PA. Scabies and pediculosis pubis: An update of treatment regimens and general review. Clin Infect Dis. 2007;44(Suppl 3):S153–59.

4. Hengge UR, Currie BJ, Jäger G et al. Scabies: a ubiquitous neglected skin disease. Lancet Infect Dis. 2006;6:769–79.

5. Stong M, Johnstone PW. Interventions for treating scabies. Cochrane Database Sys Rev. 2007:3.