Scabies

See Also

Epidemiology

Most common in children under age 2 years
Endemic to tropical areas
Incidence: Affects 100-300 million persons per year (worldwide)

Risk Factors

Young children
Crowded living conditions (Nursing Homes, shelters)
Poor hygiene
Poor nutritional status
Homelessness
Dementia
Sexually Transmitted Disease
Immunocompromised
Tropical region

Pathophysiology

Images
Caused by mite infestation: Sarcoptes scabiei
Life cycle of female mite (30 days)
1. Male and female mite mate on skin and then male mite dies
2. Female mite burrows into skin to lay eggs
  1. Lays 10 to 25 eggs over 4-6 weeks (1-3 eggs/day)
  2. Female dies after laying eggs
3. Eggs hatch within 3-4 days
  1. Scabies larvae mature into adults in 14-17 days
4. New mites cut through burrow to skin surface to begin cycle again
  1. Scabies mite can travel 2.5 cm/minute
  2. Mites cannot jump or fly
5. Mites can live up to 3 days without a human host
6. Mites can complete life cycle without host symptoms
Transmission with any contact including with fomites
1. Prolonged skin contact for at least 15-20 minutes (usually not a handshake)
  1. Exception: Hyperkeratotic Crusted Scabies can be transmitted with brief contact
  2. Hospitals and Nursing Homes
  3. Day cares
2. Household contact
3. Sexual contact
4. Shared clothing or bedding
5. Fomite transmission is rare
No significant transmission from pets with Scabies (mange)
1. Mite may be passed from pet (esp. dogs) but they do not survive
2. Short-term itching may occur but resolves in days

Symptoms

Severe, intense diffuse itch at incubation (Hypersensitivity Reaction to mite feces)
Symptoms worse at night, interfering with sleep
1. Few other pruritic dermatoses cause such intense night Pruritus
Failure to Thrive in Infants
1. Infants have worse course with diffuse involvement

Signs

Characteristics
1. Onset or incubation
  1. Initial infestation: Symptoms occur 6-8 weeks after exposure (delayed Hypersensitivity Reaction)
  2. Subsequent infestation: Symptoms may occur within 2 days of exposure
2. Initial: Tiny to small erythematous Papules (<5 mm)
3. Next: Vesicles or Pustules may form
4. Pathognomonic: Burrow (present in 10-20% of cases)
  1. May appear as short, Scaling, 1-10 mm long, wavy gray lines on surface of skin
    1. Marks the course of mite tunneling through Epidermis (typically Stratum Corneum)
    2. Most easily seen on web spaces, wrists and elbows
  2. Burrow Ink Test (BIT Test)
    1. Color burrows with magic marker and then washing the area
    2. Marker will infiltrate the burrows, and the burrows will be more evident
5. Secondary to scratching
  1. Excoriations, crusts
  2. Secondary Skin Infections
6. More intense inflammatory response in some cases
  1. Background erythema
  2. Deep Nodules
Distribution
1. Infants
  1. Vesicles, Pustules, bullae, erosions and ulcerations are more common in infants than Papules
  2. Pustules and vessicles on palms and soles
  3. Face and scalp involvement (not seen at other ages)
  4. Diffuse involvement may occur
  5. Pruritus may be absent
2. Children and adults (especially flexor surfaces)
  1. Spares face and scalp
  2. Hands and wrists
    1. Digital web spaces
    2. Sides of fingers
    3. Volar wrist
    4. Lateral palm
  3. Axillae
  4. Elbow and Antecubital fossa
  5. Trunk (may appear as a diffuse erythematous rash as Hypersensitivity Reaction)
3. Adults (includes sites for children above)
  1. Genitalia (including Scrotum and penis)
  2. Female Breast (especially areola)
  3. Gluteal crease
  4. Waistband
Variant: Hyperkeratotic Crusted Scabies (Norwegian Scabies)
1. Pathophysiology
  1. Very contagious
  2. Occurs in older, debilitated, Immunocompromised patients living in close quarters (e.g. Nursing Home)
  3. Infestations involve hundreds to more than a thousand mites (contrast with 10-15 typically)
2. Symptoms
  1. Pruritus is paradoxically mild or absent despite the severity of the infestation
  2. Lack of inflammatory response
3. Signs
  1. Thick (hyperkeratotic) crusted Plaques on hands and feet (especially palmar and plantar surfaces)
  2. Thick, dystrophic Fingernails and Toenails
  3. Red, Scaling lesions with generalized distribution may appear as Xerotic Eczema (Dry Skin)
  4. Scalp is often involved

Diagnosis

Precautions
1. Scabies is misdiagnosed in 45% of cases (often when made without microscopy or Dermoscopy)
2. Preferred diagnosis is based on identifying a mite, eggs or scybala
3. Alternative diagnostic criteria (empiric treatment criteria)
  1. Pruritus AND
  2. Lesions typical of Scabies on at least two body sites OR other household members have Pruritus
  3. Page (2007) J Fam Pract 56(7): 570-2 [PubMed]
Search for burrows holding gravid female mite
1. Consider Burrow Ink Test (BIT Test) in signs as above
2. Typical infestations involve 10-15 mites
3. Often found on nipples, axillae, hands and genitalia
4. Find newest lesions and least disturbed skin
5. Check beneath finger nail edge
Scrape Burrow or other lesions
1. Technique
  1. Apply drop of Mineral Oil to skin burrow
  2. Scrape burrow longitudinally with #15 blade along length of burrow (avoid causing bleeding)
  3. Transfer the skin scraping with oil to a glass slide
2. Microscopy under low power in oil for:
  1. Mite (female is 0.4 mm, male is 0.2 mm)
  2. Eggs
  3. Scybala (Mite feces)
3. Potassium Hydroxide (KOH)
  1. Dissolves scybala
Dermoscopy
1. Fewer False Negatives than microscopy
Skin Biopsy
1. Consider in refractory cases in which the diagnosis is elusive

Differential Diagnosis

Management

General
1. Treat all household contacts who sleep in same room
2. Treat sexual partners for the last 2 months
3. Environmental control measures (start on the morning after the treatment application)
  1. Wash in hot water (122-140 F or 50-60 C) and dry all clothing and bedding used in the last 48 hours
  2. Items that cannot be washed can be placed in a plastic bag for at least 1 week
  3. Vacuuming may be helpful
4. Itching will persist up to 2-6 weeks after treatment
  1. Pruritus clears as skin sloughs mite debris
  2. See Pruritus Management for general measures
  3. Consider Pruritus Management after treatment
    1. Topical Corticosteroids (low to medium potency such as Triamcinolone)
    2. Oral Antihistamine (e.g. Benadryl, Atarax or Zyrtec)
    3. Oral Corticosteroids for 5-7 days (severe diffuse Pruritus)
Body or Genital Scabies (follow links to agents below regarding usage)
1. First Line
  1. Permethrin (Elimite) 5% cream
    1. Apply to all areas of body from neck down
      1. Apply to all cracks and crevices (including perineum)
      2. Infants (age<1 year old) and Immunocompromised should also apply to face and head
        Healthy adults and children >1 year old need not apply to face and head
      3. Leave on overnight for 8-14 hours
      4. Wash off in morning
    2. Reapply in one week
    3. Supply adults with 60 grams (30 grams per application)
    4. Two doses typically cost $25 in 2019
    5. May be used in infants over age 2 months
    6. Pregnancy Category B
2. Second Line
  1. Ivermectin (Stremectol)
    1. Avoid in pregnancy and children under 15 kg (33 lb)
    2. May be used during Lactation
    3. Give 200 mcg/kg orally now and repeat in 7-14 days
      1. Adults will typically require 4-6 tablets per dose
      2. Total cost of 2 doses in 2019 is approximately $50
    4. Indications
      1. Patients unable to apply the cream (bed-ridden, institutionalized)
      2. Scabies refractory to Permethrin
      3. Generalized crusted Scabies
3. Other agents
  1. Benzyl Benzoate 25%
    1. Available OTC
    2. In studies, applied daily for 3 days
    3. Adverse effects include mild stinging and burning Sensation
    4. Efficacy
      1. In one study, was found significantly more effective than 5% Permethrin
      2. Meyersburg (2024) Br J Dermatol 190(4): 486-91 [PubMed]
  2. Precipitated Sulfur in petrolatum or other ointment at 6% (compounded)
    1. Has been used in newborns, pregnancy, Lactation
    2. No safety or efficacy data available
    3. Applied head to toe
      1. Leave on 24 hours
      2. Repeat application daily for 3 days total
    4. Change bed linen as treatment is completed
    5. Stings!
  3. Crotamiton (Eurax) 10% cream
    1. Used in nodular Scabies or as an alternative to other agents
    2. Apply and leave on for 24 hours, then wash and reapply for up to 3-5 days
    3. Safe in pregnancy, Lactation and infants
    4. Efficacy: 50-70%
4. Agents not recommended
  1. Permethrin 1% (OTC Nix Creme Rinse)
    1. Too low a concentration to be effective (use the 5% Permethrin instead)
  2. Lindane (Kwell, Gamma Benzene Hexachloride) 1% Lotion
    1. Avoid due to neurotoxicity (systemic absorption is 10 fold more than Permethrin)
    2. Higher risk with broken skin and young children
    3. Higher resistance rates than other agents
Crusted Scabies (Norwegian Scabies)
1. Environmental control measures
  1. See general measures as above
  2. Use barrier protection (gloves)
  3. Treat exposures
2. Dual medication therapy
  1. Permethrin 5% cream daily to full body for 7 days, then twice weekly until cure AND
  2. Ivermectin 200 mcg/kg orally on days 1, 2, 8, 9, and 15

References

DeClerck and Swadron in Herbert (2015) EM:Rap 15(10):18-19
Marco (2013) Crit Dec in Emerg Med 27(1): 2-7
Angel (2000) Pediatr Clin North Am 47(4):921-35 [PubMed]
Finders (2004) Am Fam Physician 69(2):341-50 [PubMed]
Gunning (2012) Am Fam Physician 86(6): 535-41 [PubMed]
Potts (2001) Postgrad Med 110(1):57-64 [PubMed]
Roos (2001) Drugs 61(8):1067-88 [PubMed]