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Scabies
Kaelen C. Dunican, PharmD
Brandi Hoag, DO
image BASICS
DESCRIPTION
  • A contagious parasitic infection of the skin caused by the mite Sarcoptes scabiei, var. hominis
  • System(s) affected: skin/exocrine
  • Synonym(s): sarcoptic mange
EPIDEMIOLOGY
Incidence
Predominant age: children and young adults
Prevalence
  • Global prevalence is estimated at 300 million cases per year.
  • May be more prevalent in urban areas and areas of overcrowding
ETIOLOGY AND PATHOPHYSIOLOGY
Itching is a delayed hypersensitivity reaction to the mite saliva, eggs, or excrement.
S. scabiei, var. hominis
  • An obligate human parasite
  • Female mite lays eggs in burrows in the stratum corneum and epidermis.
  • Primarily transmitted by prolonged human-to-human direct skin contact
  • Infrequently transmitted via fomites (e.g., bedding, clothing, or furnishings)
RISK FACTORS
  • Personal skin-to-skin contact (e.g., sexual promiscuity, crowding, nosocomial infection)
  • Poor nutritional status, poverty, and homelessness
  • Hot, tropical climates
  • 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.
GENERAL PREVENTION
Prevent outbreaks by prompt treatment and cleansing of fomites (see “Additional Therapies”).
image DIAGNOSIS
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 to 10 mm in length) may be seen in involved areas—a pathognomic sign of scabies.
  • Secondary erosions or excoriations from scratching
  • Pustules (if secondarily infected)
  • Pruritic nodules in covered areas (buttocks, groin, axillae) resulting from an exaggerated hypersensitivity reaction
  • 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).
DIFFERENTIAL DIAGNOSIS
  • Atopic dermatitis
  • Contact dermatitis
  • Folliculitis/impetigo
  • Tinea corporis
  • Dermatitis herpetiformis
  • Eczema
  • Insect bites
  • Papular urticaria
  • Pediculosis corporis
  • Pityriasis rosea
  • Prurigo
  • Psoriasis (crusted scabies)
  • Pyoderma
  • Seborrheic dermatitis
  • Syphilis
DIAGNOSITC TEST & INTERPRETATION
  • Definitive diagnosis is based on microscopic identification of mites, eggs, or fecal pellets (scybala) but diagnosis may be based on history and physical exam (1,2,3,4)[C].
  • A failure to find mites does not rule out scabies.
Initial Tests (lab, imaging)
CBC is rarely needed but may show eosinophilia.
Diagnostic Procedures/Other
  • Examination of skin with magnifying lens
    • Look for typical burrows in finger webs and on flexor aspect of the wrists and on the penis.
    • Look for a dark point at the end of the burrow (the mite).
    • Presumptive diagnosis based on clinical presentation, skin lesions, and identification of burrow (1,2,3).
  • Skin scraping
    • Place a drop of mineral oil over a nonexcoriated lesion or burrow.
    • Scrape the lesion with a surgical blade.
    • Examine under a microscope for mites, eggs, egg casings, or feces (1,2)[C].
    • Scraping from under fingernails often may be positive.
    • When mite is not found with scraping, biopsy may reveal mite, eggs, or feces (1,3).
  • Potassium hydroxide (KOH) wet mount not recommended because it can dissolve mite pellets.
  • Burrow ink test
    • If burrows are not obvious, apply gentian violet or India 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 (1,3)[C].
Test Interpretation
Skin biopsy of a nodule (although performed rarely) will reveal portions of the mite in the corneal layer.
image 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 and dry in hot dryer.
  • Personal items that cannot be washed should be sealed in a plastic bag for at least 3 to 5 days.
  • Some itching and dermatitis may persist for up to 6 weeks and can be treated with oral antihistamines and/or topical or oral corticosteroids.
MEDICATION
First Line
Permethrin 5% cream (Elimite) is generally accepted as first-line therapy (1,2,3,5)[A].
  • After bathing or showering, apply cream from the neck to the soles of the feet paying particular attention to areas that are most involved; then wash off after 8 to 14 hours and repeat in 1 week.
  • The adult dose is usually 30 g per treatment.
  • Side effects include itching and stinging (minimal absorption).
  • Crusted scabies may require more frequent application (q2-3d for 1 to 2 weeks) in combination with repeated doses of PO ivermectin on days 1, 2, 8, 9, and 15 (2,3,5)[C].
P.941

Pediatric Considerations
Permethrin may be used on infants >2 months of age. In children <5 years of age, the cream should be applied to the head and neck as well as to the entire body.
Second Line
  • Ivermectin (Stromectol)
    • Not FDA approved for scabies
    • 200 to 250 &mgr;g/kg PO as single dose; repeated in 1 week
    • Take with food to improve bioavailability and enhance penetration into the epidermis.
    • May need higher doses or may need to use in combination with topical scabicide for HIVpositive patients
  • Crotamiton (Eurax) 10% cream
    • Apply from the neck down for 24 hours, rinse off, then reapply for an additional 24 hours, and then thoroughly wash off.
    • Nodular scabies: Apply to nodules for 24 hours, rinse off, reapply for an additional 24 hours, then thoroughly wash off.
  • Precipitated sulfur 2-10% in petrolatum
    • Not FDA approved for scabies
    • Apply to the entire body from the neck down for 24 hours, 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 (&ggr;-benzene hexachloride, Kwell) 1% lotion
    • Apply to all skin surfaces from the neck down and wash off 6 to 8 hours later.
    • Two 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: Do not use on excoriated skin, on immunocompromised patients, in conditions that may increase risk of seizures, or with medications that decrease seizure threshold.
      • Possible interactions: concomitant use with medications that lower the seizure threshold
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.
  • PO ivermectin should be avoided in children <5 years and in those weighing <15 kg.
Pregnancy Considerations
  • Permethrin is pregnancy Category B, and lindane, ivermectin, and crotamiton are Category C.
  • Permethrin is considered compatible with lactation, but if permethrin is used while breastfeeding, the infant should be bottlefed until the cream has been thoroughly washed off.
ADDITIONAL THERAPIES
  • Crusted scabies may require use of keratolytics to improve penetration of permethrin.
  • Benzyl benzoate lotion (not available in the United States but used widely in developing countries)
    • Not FDA approved for scabies.
    • Dose for adults is 25-28%; dilute to 12.5% for children and 6.25% for infants
    • After bathing, apply lotion from the neck to soles of feet for 24 hours.
  • Topical ivermectin 1% lotion (investigational, not available for use in the United States) (4)
    • Not FDA approved for scabies.
    • Apply to affected sites and wash off 8 hours later.
COMPLEMENTARY & ALTERNATIVE MEDICINE
Herbal products such as tea tree oil, clove oil, neem oil, and aloe vera require additional evidence to establish efficacy (4).
image ONGOING CARE
FOLLOW-UP RECOMMENDATIONS
Patient Monitoring
Recheck patient at weekly intervals only if rash or itching persists. Scrape new lesions and retreat if mites or products are found.
PATIENT EDUCATION
  • Patients should be instructed on proper application and cautioned not to overuse the medication when applying it to the skin.
  • A patient fact sheet is available from the CDC: http://www.cdc.gov/parasites/scabies/
PROGNOSIS
  • Lesions begin to regress in 1 to 2 days, along with the worst itching, but eczema and itching may persist for up to 6 weeks 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 cases do respond to permethrin.
REFERENCES
1. Hicks MI, Elston DM. Scabies. Dermatol Ther. 2009;22(4):279-292.
2. Gunning K, Pippitt K, Kiraly B, et al. Pediculosis and scabies: treatment update. Am Fam Physician. 2012;86(6):535-541.
3. Markova A, Kam SA, Miller DD, et al. In the clinic. Common cutaneous parasites. Ann Intern Med. 2014;161(5).
4. Rosamilia LL. Scabies. Semin Cutan Med Surg. 2014;33(3):106-109.
5. Currie BJ, McCarthy JS. Permethrin and ivermectin for scabies. N Engl J Med. 2010;362(8):717-725.
See Also
&NA;
Arthropod Bites and Stings; Pediculosis (Lice)
Codes
&NA;
ICD10
B86 Scabies
Clinical Pearls
&NA;
  • Prior to diagnosis, use of a topical steroid to treat pruritic symptoms may mask symptoms and is termed scabies incognito.
  • Environmental control is recommended. All linens, towels, and clothing used in the previous 4 days should be washed in hot water or dry-cleaned. Personal items that cannot be washed or dry-cleaned should be sealed in a plastic bag for 3 to 5 days.
  • All members of the affected household may require treatment, especially close contacts (those sharing the same bed or who have intimate contact).
  • Eczema and itching may persist for up to 6 weeks after treatment, causing many patients to falsely believe that they have failed treatment or are being reinfected.
  • In patients with actual reinfection, either the patient has not applied the medication properly or, more likely, the index patient has not been identified and treated.