133.0 – Scabies

Scabies is a parasitic infestation resulting when the mite, Sarcoptes scabiei var. hominis, burrows into and just below the stratum corneum in the epidermis. It is transmitted most often via direct person-to-person contact and less frequently by fomites. Human scabies is extremely contagious, spreading between individuals who share close contact or living spaces. Prevalence rates are higher in children, residents of long-term care facilities, and sexually active persons, although scabies can appear in individuals of all ages, ethnicities, and socioeconomic groups. Factors that contribute to the persistence and spread of scabies are overcrowding, delays in diagnosis, and poor public health awareness. Outbreaks in health care facilities, such as nursing homes, can result in dozens of patients and staff becoming infected.  

A typical infestation has 10–20 mites, but most patients mount an intense hypersensitivity reaction resulting in a widespread and intensely pruritic skin eruption. The hypersensitivity reaction usually develops 2–6 weeks after becoming colonized. The condition persists without medical treatment because the mite lays eggs that cause continued infestation. Canine scabies can be spread from dogs to humans but not between humans. Scabies can cause a generalized eruption resembling erythroderma in the elderly, the institutionalized, and patients with immunosuppression or neurologic dysfunction (so-called Norwegian scabies or, more appropriately, crusted scabies). In these patients, the mite burden is much higher, with thousands to millions of mites present on affected skin.

The tell-tale diagnostic sign is the burrow. The burrow of the female scabies mite is a fine, thread-like, serpiginous line with a terminal tiny (smaller than a pin head), black speck representing the mite itself. The burrow, along with small erythematous papules and vesicles, is seen mostly on the flexor wrists, elbows, and areolae, in the interdigital web spaces, axillae, and umbilicus, along the lower abdomen, and in the genital and buttocks regions. Scabies classically spares the head and neck areas. Note, however, that in many infested patients, obvious burrows may not be identified.

Secondary lesions are due to scratching and include excoriations, impetiginized lesions with crusts, and prurigo-like nodules.

Crusted scabies, as the name implies, consists of scaling and crusting of the skin, with ill-defined, widespread areas of erythema. Any site can be involved, but the most common are the arms and hands. Patients may present with generalized erythema and scaling. Nail dystrophy may be present, and the head and neck may be involved.

In adults, pruritic lesions on the areola in women and penis and scrotum in men are highly suggestive of scabies.

Look closely for the burrow; mites are almost never found by scraping papules or excoriated lesions. The tiny black dot present at the edge of an intact linear papule represents a mite. Look for and/or inquire about lesions and symptoms in family members and caretakers.

Pruritus tends to be more intense at night and when the patient is warm.

A negative scabies prep does not rule out this diagnosis; mites can be infrequent and difficult to isolate in patients with normal immune function.

Scabies presents a diagnostic challenge, as it is a great clinical imitator. Some of the conditions scabies is mistaken for include the following:
Insect bites
Papular urticaria
Canine scabies – This mite can transiently infect humans but does not have the classical distribution of human scabies and cannot subsequently be passed from person to person.
Atopic dermatitis
Nummular dermatitis
Folliculitis
Impetigo
Bedbug bites
Varicella
Contact dermatitis
Dermatitis herpetiformis
Seabather's eruption
Dyshidrotic dermatitis
Lichen planus
Id reaction
Neurotic excoriations
Prurigo nodularis
Bullous pemphigoid – Urticarial phase

Perform a scabies prep. Put a small amount of mineral oil on the skin area to be tested, and take a #15 blade and gently remove the terminal end of the burrow where you see the tiny black speck. Apply this scraping to a glass slide, cover with oil and a cover slip, and examine under the microscope for the presence of the mite or its ova or fecal pellets, known as scybala.

Burrows may be more easily identified by covering a suspected burrow with the ink from a fountain or marking pen, then wipe away with an alcohol pad after a minute or two. The ink will penetrate the burrow, making it more visible.

In cases of crusted scabies, add a few drops of 10% potassium hydroxide (KOH) solution to the skin scraping to break down the excess keratin. Scales will typically contain many mites.

Treat the entire family and all close contacts. Close contacts may be infected but not yet symptomatic and will, therefore, be unaware of the infection. If untreated, such individuals will pass the mite back to others. 

Patients should be instructed to launder bed linens, towels, and clothing used in the last 72 hours prior to treatment in hot water and dry on high heat. Items that cannot be laundered can be sealed in air-tight plastic bags for 10–14 days. All carpets and upholstered furniture should be thoroughly vacuumed and the vacuum bags or canisters disposed of. It is important that such control measures coincide with the pharmacologic treatment of household members.

Make sure to tell the patient that lesions can take a week or more to clear, as the immune reaction will continue despite killing the mite with the treatment. Itching may persist for up to 4 weeks despite complete eradication of live mites.

Lesions may become secondarily infected with Staphylococcus aureus.

Scabies is very difficult to eradicate from hospital settings once an outbreak has occurred. Prompt identification of affected patients and appropriate isolation is essential. Hospital infection control teams should be involved at the outset of case identification to help control spread.

Precautions: Standard and Contact (Isolate patient, wear gloves and a gown, limit patient transport, and avoid sharing patient-care equipment.)

In the US, outbreaks of scabies should be reported in AZ, ND, and OH.

CDC-Recommended Regimens
Permethrin cream (5%) applied to all areas of the body from the neck down and washed off after 8–14 hours. The treatment is repeated again a week later.
OR
Ivermectin 200 μg/kg p.o. once. Repeat in 1 to 2 weeks.

Note that resistance to topical permethrin appears to be on the rise.

CDC-Recommended Alternative Regimen
Lindane (1%) – 1 oz. of lotion or 30 g of cream applied in a thin layer to all areas of the body from the neck down and thoroughly washed off after 8 hours. Lindane is not recommended as first-line therapy because of toxicity. It should only be used as an alternative if the patient cannot tolerate other therapies or if other therapies have failed.

Lindane should not be used immediately after a bath or shower, and it should not be used by persons who have extensive dermatitis, women who are pregnant or lactating, or children aged younger than 2 years. Lindane resistance has been reported in some areas of the world, including parts of the United States. Seizures have occurred when lindane was applied after a bath or used by patients who had extensive dermatitis. Aplastic anemia after lindane use has also been reported.

Permethrin is effective and safe, and it is less expensive than ivermectin. One study demonstrated increased mortality among elderly, debilitated persons who received ivermectin, but this observation has not been confirmed in subsequent reports.

For patients with crusted scabies, an attempt should be made to remove as much of the crusted scale as possible prior to initiating therapy with a topical agent. Mechanical debridement can be facilitated with warm soaks followed by the application of a keratolytic agent (eg, Lac-Hydrin cream). Such patients may require repeated applications of a topical antiscabietic.

Antihistamines for pruritus is an important adjunctive treatment:

• Diphenhydramine hydrochloride (Benadryl®) (25, 50 mg tablets or capsules): 25–50 mg nightly or every 6 hours as needed
• Hydroxyzine (Atarax®) (10, 25 mg tablets): 10–25 mg every 6 hours as needed
• Cetirizine hydrochloride (Zyrtec®) (5,10 mg tablets): 5–10 mg per day
• Loratadine (Claritin®) (10 mg tablets and RediTabs®): 10 mg tablet or RediTab once daily

Consider a short course of topical corticosteroids for patients in whom the reaction is severe. Intralesional corticosteroids have been used in the treatment of scabetic nodules.

Meinking DL, Burkhart CN, Burkhart CG, Elgart G. Infestations. In: Bolognia J, Jorizzo JL, Rapini RP, eds. Dermatology. 2nd ed. St. Louis, MO: Mosby/Elsevier; 2008:1291-1295.

Stone SP, Goldfarb JN, Bacelieri RE. Scabies, other mites, and Pediculosis. In: Fitzpatrick TB, Wolff K, eds. Fitzpatrick's Dermatology in General Medicine. 7th ed. New York, NY: McGraw-Hill; 2008:2029-2037.

Strong M, Johnstone PW. Interventions for treating scabies. Cochrane Database Syst Rev. 2007;(3):CD000320. PubMed Id: 17636630

Hengge UR, Currie BJ, Jäger G, Lupi O, Schwartz RA. Scabies: a ubiquitous neglected skin disease. Lancet Infect Dis. 2006 Dec;6(12):769-79. PubMed Id: 17123897

Chosidow O. Clinical practices. Scabies. N Engl J Med. 2006 Apr 20;354(16):1718-27. PubMed Id: 16625010

Johnston G, Sladden M. Scabies: diagnosis and treatment. BMJ. 2005 Sep 17;331(7517):619-22. PubMed Id: 16166133

Scheinfeld N. Controlling scabies in institutional settings: a review of medications, treatment models, and implementation. Am J Clin Dermatol. 2004;5(1):31-7. PubMed Id: 14979741

Chouela E, Abeldaño A, Pellerano G, Hernández MI. Diagnosis and treatment of scabies: a practical guide. Am J Clin Dermatol. 2002;3(1):9-18. PubMed Id: 11817965

Usha V, Gopalakrishnan Nair TV. A comparative study of oral ivermectin and topical permethrin cream in the treatment of scabies. J Am Acad Dermatol. 2000 Feb;42(2 Pt 1):236-40. PubMed Id: 10642678

Appearance
No Acute Distress

Body Location
Ankle
Arm
Corona
Crural or Inguinal Fold
Female Genital
Foreskin
Glans of Penis
Hand or Fingers
Labia Majora
Male Genital
Penis
Scrotum
Shaft of Penis
Umbilicus
Web Spaces of Fingers
Web Spaces of Toes
Wrist

Configuration
Linear
Serpiginous

Distribution
Diaper Area
Genitals, Buttocks, Perineum
Hands and/or Feet
Scattered Few
Scattered Haphazard
Widespread
Widespread Male Genital

Lesion
Crust
Excoriated
Excoriation
Fine Scaly Papule
Linear Burrow
Nodule
Papule
Scale Thick
Scaly Papule
Vesicle

Occupations
Military
Sex Worker (Prostitute)
Veterinarian

Signs and Symptoms
No Fever (Afebrile, Apyrexial)
Pruritus (Itching)

Social History
Elementary School (K-5)
Homeless
Institutionalized Population
Middle/High School (6-12)
Multiple Sexual Partners
Nursing Home Resident
Overcrowding

Temporal
Developed Acutely Over Days to Weeks
Developed Steadily Over Weeks to Months

Medical History
Acquired Immune Deficiency Syndrome (AIDS)
Human Immunodeficiency Virus (HIV) Disease