Dermatology

Scabies

Are You Confident of the Diagnosis?

  • What to be alert for in the history

The sine qua non for the diagnosis of scabies is pruritus. As always, there is an exception: the elderly, the immunosuppressed, and certain individuals with physical or mental limitations may manifest exaggerated scabies without obvious itching (Exaggerated, Crusted or Norwegian Scabies.) The combination of severe pruritus, often worse at night, and the itching—and rash—present in other members of the household is strong evidence for scabies infestation.

  • Characteristic findings on physical examination

Characteristic findings of scabies on physical examination include nonspecific erythematous papules, excoriations, and where they can be located, burrows (Figure 1). These winding, short tunnels in the epidermis are pathognomonic, but because of the severe itching and scratching, may not easily be found.

Figure 1.

Acral scabies with burrow in a 4 year old (Photo courtesy Lucinda Buescher, MD)

The distribution of lesions is often the best evidence of the diagnosis: interdigital webs (Figure 2), periumbilical and periareolar areas, the anterior and posterior axillary fold areas, wrists (especially volar) (Figure 3), elbows, intergluteal areas, genitalia (particularly male) and buttocks. Although these areas should be examined for burrows, excoriation of these areas is strongly suggestive. The scalp is rarely involved, but intense itching associated with extensive scaling may be a clue to the rare case of scalp infestation.

Figure 2.

Scabies on the hand

Figure 3.

A burrow on the wrist of the mother of the 4-year-old patient (Photo courtesy of Lucinda Buescher, MD)

Older individuals, particularly the institutionalized (such as nursing home residents), may present with atypical lesions—including papulonodular lesions—and on the trunk and buttocks of bedridden patients. Scabies should be considered in any nursing home resident with an unexplained generalized eruption (Figure 4).

Figure 4.

Scabies, elderly patient

Infants and small children may have papulopustular lesions, especially in palmoplantar locations, suggestive of impetigo or infantile acropustulosis. They are also susceptible to nodular scabies, thought to be a delayed hypersensitivity reaction to the mites, and manifested as persistent nodules, erythematous and pruritic. Nodular scabies is also seen in adults—usually adult males—and primarily on the penis and scrotum.

Crusted, or Norwegian scabies, is reviewed in a separate chapter. It differs from classical scabies in that the grossly crusted areas may occur anywhere on the body, including face and scalp.

  • Expected results of diagnostic studies

The primary diagnostic study is the microscopic examination of scrapings of skin for direct viewing of the mite itself. The scraping may not reveal the mite, but discovery of ova or even fecal particles (scybala) is also confirmatory of the diagnosis (Figure 5). Scrapings may be obtained with a scalpel blade, although the late Walter Shelley and others (including this author) prefer to use a razor blade to obtain a superficial shave of a larger area.

Figure 5.

Ova and scybala (circled) of scabies (Photo courtesy of Joseph Conlon, MD)

A drop of mineral oil may be applied to the target area, which is then scraped or shaved and examined under low power. Mineral oil is preferable to KOH, such as is used for fungal examination, as the oil preserves the scybala, while they may dissolve in the KOH.

Skin biopsy is less helpful, sometimes showing only an inflammatory reaction suggestive of insect bite, a pseudolymphomatous reaction (especially with nodular scabies) or occasionally a portion of the mite may be found in a superficial location within the stratum corneum. Current opinion holds that the patient with a classical scabies infestation may only have 5 to 15 (average 11) mites on the entire body surface, although that number has recently come into question.

Recently, a number of authors have proposed using dermoscopy, videodermoscopy and even digital photography to demonstrate the mite within the burrow, describing it as having the shape of a "jet plane with contrails" or an "accent circumflex" (such as over this letter: ê) Polymerase chain reaction (PCR) has also been reported as used to prove the diagnosis, with reversion to a negative PCR test 2 weeks after treatment. Although the ELISA serologic test has been reported in the diagnosis of canine sarcoptic mange, it has not yet been reported in human scabies diagnosis. Clear package wrapping tape may also be helpful, applying it to a suspicious area (such as the periumbilical area of a squirming infant) then after leaving several seconds to adhere, stripping it off and examining the sticky side with the microscope.

  • Diagnosis confirmation

The differential diagnosis of scabies includes any widespread pruritic eruption with excoriation. Common conditions confused with scabies may include atopic dermatitis, atypical folliculitis or impetigo (particularly impetiginized scabies), arthropod bites, prurigo nodularis and dermatitis herpetiformis. In adults—especially older adults—bullous scabies may mimic bullous pemphigoid. Immunofluorescence is usually—but not always—negative in such cases, and occasionally the mites may be found within the bullae. In children, acropustulosis of infancy must also be ruled out.

Who is at Risk for Developing this Disease?

An estimated 300 million persons worldwide are afflicted with scabies (although the accuracy of that number has been questioned). Data collected by the Royal Infirmary of Edinburgh from 1815 to 2000 indicate that scabies accounted for 5% of their recorded cases of skin disease, with up to 30% during World Wars I and II (supporting the idea that there is an increase in such infestations in wartime).

Traditionally, scabies has been known as the 7-year itch, with alleged cyclic increases in incidence from 7 to 30 years apart, but the Edinburgh data and a recent 20-year Israeli army study shows a seasonal variation, with more cases reported in the winter months. It is likely that the 7-year itch term refers more to the chronicity of the infection when untreated than to any cyclical epidemics.

Infants and small children appear to be more susceptible to the disease (suggesting a lack of immunity) while healthcare workers also have a high incidence (suggesting increased exposure).

There do not appear to be any sex differences, but poverty, overcrowding, malnutrition, poor hygiene and homelessness appear to be risk factors.

What is the Cause of the Disease?

  • Etiology

  • Pathophysiology

The disease is caused by the human mite, Sarcoptes scabiei var hominis. The generally accepted taxonomy of this organism (per CDC) is listed in Table I.

Table I.

Taxonomy of Sarcoptes scabiei var hominis
Kingdom Animalia
Phylum Arthropoda
Class Arachnida
Subclass Acari
Family Sarcoptidae
Genus/Species Sarcoptes scabiei var hominis

Transmission of scabies between humans and other species (sarcoptic mange of dogs, for example, with S scabiei var canis) can occur, but the infestation is short term and self-limited, as the mites are unable to reproduce on a host of another species. The distribution of the lesions is also not the characteristic distribution of human scabies.

Transmission of human scabies is most often through direct skin-to-skin contact; hence in certain age groups it may be considered a sexually transmitted disease. The mite may remain viable for 2 or 3 days away from its host, but under the right conditions (high humidity, moderate temperatures) a mite might be capable of survival—and thence infection of the host—for up to 5 days.

Because the life cycle takes place entirely on and in the skin of the host, infection from fomites is unusual (one would have to be infested by a gravid female or a mating pair). In the case of extensive infestation (such as crusted scabies), the large number of shed mites might permit acquiring the infestation from fomites.

Once infestation is acquired, it may take weeks to months before the delayed hypersensitivity reaction appears, and thus the symptom of pruritus with the associated itching and excoriations. During this period, the condition is contagious but undiagnosed. Reinfestation begins to itch more quickly, as the patient is already sensitized to the mite.

Systemic Implications and Complications

Although generally considered a benign (albeit severely distressing) condition of pruritus alone, there have been a number of serious complications. Most recently reported was a necrotizing soft tissue infection secondary to a Staphylococcus aureus infection of excoriations on the vulva. Post-streptococcal glomerulonephritis has also been reported to occur after secondary infection of excoriations with Streptococcus pyogenes.

More common, however, is simple impetigenization of excoriated areas. In the Third World in particular, cutaneous bacterial infection has been a significant complication of scabies infestation. According to a recent report out of Australia, for example, scabies, which is endemic among many aboriginals, is responsible for some 50% to 70% of streptococcal pyodermas.

Treatment Options

Topical therapies for scabies are listed in Table II.

Table II.

TOPICAL THERAPIES FOR SCABIES
Permethrin 5% cream
Malathion 5% solution
2% to 10% Sulfur in petrolatum
10% Benzyl benzoate
5% gamma benzene hexachloride (lindane)
Crotamiton 10% cream
Australian tea tree oil
Ivermectin topical

SYSTEMIC THERAPIES

-Ivermectin 200 ug/kg (and 250 ug/kg)

The treatment of choice in scabies remains permethrin 5% cream, applied topically. Although a single treatment is said to be curative, it is recommended that it be repeated in one week--perhaps to capture recently hatched organisms, or perhaps to reach areas of the skin that might have been missed with the first application. It is the treatment of choice in pregnancy and is approved down to 2 months of age.

Unfortunately, the elimination of the scabies organism with a single application of permethrin 5% cream does not eliminate the pruritus nor the eczematous eruption, which are both probably related to the immunologic response to the presence of the organism (and its detritus) in the stratum corneum burrows.

The administration of topical or systemic steroids may provide temporary relief of the pruritus, after the definitive therapy has terminated the infestation. Oral antihistamines or other antipruritics are not particularly helpful in such cases. Hence, I routinely administer an injection of approximately 1mg/kg triamcinolone acetonide to patients suffering from severe pruritus after treating the infestation.

In the situation of a patient who does not accept injections (or a physician who does not care to administer parenteral steroids), I would recommend oral prednisone or its equivalent in a dose of 1mg/kg up to 60 mg per day, in a 10- to 14-day course, with the option of reducing the dose by 50% at day 7. Any mid-strength topical steroid may be used, although my favorite recipe is clobetasol topical solution 0.05% 50 mL added to a 220 mL bottle of Sarna Lotion and applied BID to TID.

Lindane (also GBHC or gamma benzene hexachloride) application has been used for many years to manage scabies successfully. Because of environmental toxicity and neurotoxicity, it has been banned in California and a number of countries. It is effective, and continues to be available in many nations. There are no head-to-head studies of GBHC vs permethrin. It is labeled as a second-line treatment, and should only be used in the event of failure of treatment with one or more first-line drugs. I have not used it for years, but prefer to move to oral ivermectin immediately if treatment failure with permethrin appears to have occurred.

Sulfur, 10% in petrolatum, has been shown to be as effective as lindane and appears to be safe. It is usually administered to infants under 2 years of age. Until the acceptance of permethrin's safety record in pregnancy, sulfur in petrolatum was used in that situation; today we use permethrin. Aqueous malathion 0.5% is effective as well; note: the form of malathion available in the United States is not approved for scabies: the base contains isopropyl alcohol, which will irritate genital and other inflamed skin.

Crotamiton cream is available in the United States, but in head-to-head comparison to permethrin, crotamiton was significantly inferior. Its only advantage is that it does have a mild antipruritic effect.

Ten percent to 25% benzyl benzoate is not available in the United States. A recent Cochrane review indicated that the data do not exist to compare the relative efficacy of permethrin and benzyl benzoate. It appears, however, to be extremely effective.

Ivermectin is a member of the avermectin group of drugs, the treatment of choice for onchocerciasis and FDA approved for that condition as well as strongyloidiasis; it has been used off label for scabies with no reports of significant side effects. The recommended dose is generally 200 to 250 mcg/kg, repeated in 1 week. It is not approved for use in children under 5 years of age or weighing less than 15 kg.

Compounded topical ivermectin, prepared to1% solution in propylene glycol, has been reported as effective in several anecdotal reports. A group in Colombia reported the use of topical ivermectin 1% in propylene glycol, 400 mcg/kg in 12 adults and 20 children in the 1- to 10-year age group with 100% cured, and no side effects, after two treatments a week apart. The total dose applied being 400 mcg/kg of the 1% solution, a 50 kg patient would receive 400 mcg X50—or 20 mg topically or 2mL. To this was added an additional 10 mL of propylene glycol, which would give a volume sufficient to cover the entire body. Again, this is an off label use.

Optimal Therapeutic Approach for this Disease

The treatment of choice in scabies remains permethrin 5% cream, applied topically, although there have been scattered reports of resistance. Although a single treatment is said to be curative, it is recommended that it be repeated in 1 week—perhaps to capture recently hatched organisms, or perhaps to reach areas of the skin that might have been missed with the first application. It is the treatment of choice in pregnancy and is approved down to 2 months of age.

Patient Management

When using topical therapies for adults and children over 5 years of age, the medication should be applied from the neck to the soles of the feet, paying particular attention to skin folds (neck, axillae, inguinocrural areas, intergluteal and interdigital areas). In the pediatric age group, the face and scalp are also treated.

Ideally, all members of the household should be treated, whether itching or not. In the morning (after treatment), the bedding and all clothing worn the previous day should be laundered in the washing machine and dryer. Items that cannot be laundered can be pressed, dry cleaned, or placed dry in the clothes dryer.

Items such as stuffed animals, leather, or other heat sensitive materials may be sealed in plastic bags for up to two weeks. In summer months, I recommend that such items be air dried in the sunlight for a few days. The scabies mite only survives for 72 hours or so away from the host in the best conditions (high humidity, low temperature) and this air drying may be all that is needed. More extensive housecleaning is not recommended.

Unusual Clinical Scenarios to Consider in Patient Management

In immunosuppressed patients with exaggerated or crusted scabies, patients may have minimal (or no) pruritus. The diagnosis must be considered in any recalcitrant psoriasiform or chronic eczematous rash in these patients who have failed to respond to standard therapy for those disorders.

What is the Evidence?

Ramos-e-Silva, M. "Giovan Cosimo Bonomo (1663-1696): discoverer of the etiology of scabies". Int J Dermatol. vol. 37. 1998. pp. 8 625-30.

(Dr Ramos-e-Silva has written a delightful historic narrative of the disease and its recognition.)

Hengge, UR, Currie, BJ, Jäger, G, Lupi, O, Schwartz, RA. "Scabies: a ubiquitous neglected skin disease". Lancet Infect Dis. vol. 6. 2006. pp. 769-79.

(An excellent review of scabies to date.)

Walton, SF. "The immunology of susceptibility and resistance to scabies". Parasite Immunol. vol. 32. 2010. pp. 532-40.

(A detailed look at the immunology of scabies, and an analysis of the factors leading to crusted scabies.)

Wolf, R, Davidovici, B. "Treatment of scabies and pediculosis: facts and controversies". Clin Dermatol. vol. 28. 2010. pp. 511-8.

(An excellent review of lindane and permethrin treatment, and a challenge to the long time oft quoted average of 12 mites per infested patient.)

Micali, G, Lacarrubba, F, Verzì, AE, Chosidow, O, Schwartz, RA. "Scabies: Advances in Noninvasive Diagnosis". PLoS Negl Trop Dis. vol. 10. 2016 Jun 16.

(An updated review of new means of diagnosis, including inexpensive videodermatoscopy, and a number of fascinating downloadable photographs.)
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