Human Scabies

 

Salient features

·        Human scabies is caused by infestation with the host-specific mite Sarcoptes scabiei var. hominis, that lives its entire life cycle within the epidermis.

·        Causes a diffuse, pruritic eruption after an incubation period of 4 to 6 weeks.

·        Is transmitted by direct close physical contact or by fomites.

·        Topical therapy with permethrin 5% cream is most effective, but oral ivermectin, although off-label is also effective.

·        Because of the common occurrence of asymptomatic mite carriers in the household, all family members and close contacts should be treated simultaneously.

 

Introduction

Scabies is a superficial epidermal infestation caused in humans by the mite Sarcoptes scabiei var. hominis. Pruritus associated with this infestation is usually severe, especially at night, and treatment requires prescription scabicidal therapy.

 

Epidemiology

Incidence and prevalence

Scabies affects around 100–300 million people worldwide.

Age

Scabies occurs in all age groups.

Sex ratio

The overall sex incidence is probably equal.

 

Pathogenesis

The highly host-specific, eight-legged mite Sarcoptes scabiei var. hominis causes human scabies. The adult female measures approximately 0.4 mm long by 0.3 mm broad, and the smaller male 0.2 mm long by 0.15 mm broad and are too small to be seen by the naked eye. The body is creamy white and is marked by transverse corrugations and on its dorsal surface by bristles and spines. There are four pairs of short legs; the anterior two pairs end in elongated peduncles tipped with small suckers. In the female, the rear two pairs of legs end in long bristles (setae), whereas in the male bristles are present on the third pair and peduncles with suckers on the fourth.

 

 

The entire 30 day life cycle of mites is completed within the epidermis. The female mite by a combination of chewing and body motions is able to excavate a sloping burrow in the stratum corneum to the boundary of the stratum granulosum. Along this path, which can be 1 cm long, she lays two to three eggs a day during her life span of 30 days, which require approximately 10 days to adults. Eggs hatch after 3–4 days into larvae, which leave the burrow to mature into adult mites in about 4 days on the skin surface. The adults may then either stay in that host or be scratched off and transmitted to a new host. The male mite lives on the surface of the skin and enters burrows to procreate. Adult females can live in the host for up to a month. The mites show a preference for certain sites in which to burrow, and appear to avoid areas with a high density of pilosebaceous follicles. The average number of adult female mites on an individual suffering from the common form of scabies is about 12. Only in crusted scabies are large numbers of mites present. Scabies mites usually live 3 days or fewer off a human host, but those from patients with crusted scabies may live up to 7 days by feeding on sloughed skin.

Scabies is usually transmitted directly by close physical contact, such as prolonged hand‐holding or the sharing of a bed, or indirectly via fomites. Prevalence is higher in children and people who are sexually active, and spread of the infestation among family members and other close contacts is common. It is often suggested that fertilized female mites are responsible for transmission.

Hypersensitivity of both immediate and delayed types to the mite or its products appears to play an important role in determining the development of lesions other than burrows, and in producing pruritus.

The incubation period before symptoms develop can range from days to months. In first-time infestations, it usually takes 2–6 weeks before the host’s immune system becomes sensitized to the mite or its by-products, resulting in pruritus and cutaneous lesions. In contrast, a subsequent infestation often becomes symptomatic within 24–48 hours. Asymptomatic scabies-infested individuals are not uncommon, and they can be considered “carriers”.

Environmental factors that promote its spread include overcrowding, delayed treatment, and lack of public awareness of the condition.

 

Clinical features

Classical scabies occurs in patients with normal immune response. The epidemiologic history (e.g. pruritus in household members or close personal contacts), the distribution and types of lesions, and pruritus form the basis of the clinical diagnosis. The intense pruritus is classically accentuated at night and by a hot bath or shower. Itching is the most obvious manifestation of scabies and may present before any overt physical signs appear. Some individuals experience pruritus for many months with no rash. Similar to the human response to other insects such as fleas, yellow jackets, and mosquitoes, there is a wide range of clinical responses to an infestation with scabies.

Cutaneous lesions are symmetrical. On physical examination, patients display disseminated erythematous papules that favor the interdigital web spaces of the hands, sides of fingers, volar aspects of the wrists and lateral palms, elbows, axillae, posterior auricular area, waist (including the umbilicus), ankles, feet and buttocks. The papules are small, often excoriated with hemorrhagic crusts on top. In men, penile and scrotal lesions are common, while in women, the areolae, nipples and vulvar area are often affected. The head, palms and soles are usually spared in healthy adults; but in infants, elderly, and immunocompromised, all skin surfaces are susceptible. Indurated, crusted nodules can be seen in children on intertriginous areas. Irritant or allergic contact eczema can be induced following topical treatment.

The pathognomonic lesions of scabies are the burrows, representing the tunnels that female mites excavates while laying eggs. Clinically, the burrow is a thin, wavy, thread-like, slightly raised brownish tortuous lesion 0.5–1 cm in length. The burrow is best seen in the areas with few or no hair follicles, usually where stratum corneum is thin and soft, i.e., the interdigital webs of the hands, wrists, palms and soles in infants, shaft of penis, elbows, feet, buttocks or axillae; however, it can be difficult to find in early stages of the condition, or after the patient has extensively excoriated the lesions or due to secondary bacterial infection. Identification of a burrow can be facilitated by rubbing a black felt-tip marker across an affected area.  After the excess ink is wiped away with an alcohol pad, the burrow appears darker than the surrounding skin because of ink accumulation in the burrow. Inflammatory pruritic firm nodules, sometimes surmounted by burrows, are usually found on the male genitalia, groin and buttocks. The genitalia of males should be therefore systematically examined once scabies is suspected as these lesions may provide an important diagnostic clue if burrows are absent or difficult to find. Nodules are intensely itchy, and may persist for weeks or months after the scabies has been effectively treated.

 

Scabies: Predilection sites

Burrows are most easy to Identify on the webs pace of the hands, wrists, and lateral aspects of the palms. Scabietic nodules occur uncommonly, arising on the genitalia, especially the penis and scrotum, waist, axillae, and areolae.

 

 

Clinical variants

Atypical forms of scabies may occur and be very difficult to diagnose. The clinical features of scabies in infants and young children differ in certain respects from those in older children and adults. In addition to the more extensive distribution of burrows mentioned above, vesicular and vesiculopustular lesions on the hands and feet are frequent (acral vesiculopustules), extensive eczematization is often present, and there may be multiple crusted nodules on the trunk and limbs.

In the elderly, burrows commonly occur on the palms and soles, and may be very numerous. Truncal papulosquamous lesions, often surmounted by burrows, are common. Secondary eczematization is often troublesome.

Crusted scabies (the term ‘Norwegian scabies’ should no longer be used) occurs in patients with severe immune deficiency due to disease (e.g. AIDS, HTLV1-infection, malignancy and leprosy) or therapy (e.g. immunosuppressant drugs and biologicals, neurological disease causing reduced sensation, immobility with reduced ability to scratch or in genetically susceptible patients. Pruritus is mild or absent. Skin lesions consist of generalized, poorly defined, erythematous, fissured plaques covered by scales and crusts. On bony prominences (e.g. finger articulations, elbows and iliac crest), the plaques have a yellow-to-brown, thick, verrucous surface. Hyperkeratotic plaques also develop diffusely on the palmar and plantar regions, with thickening and dystrophy of the toenails and fingernails. Diffuse non-crusted scabies with involvement of the back may also occur. Secondary bacterial infection can result in malodorous skin lesions.

Special forms of scabies

Involved subpopulation
Infants and young children	Lesions are vesicles, pustules and nodules, but their distribution may be atypical. Eczematization and impetigo are common; scabies may be confused with atopic eczema or acropustulosis. Pruritus may be so severe that infants can be irritable and eat poorly.
The elderly	Atypical presentation is common. Scabies epidemics are reported frequently in nursing homes, where a single patient with crusted scabies may be the index patient leading to infection of other residents, as well as health care workers and their families.
Immunocompromised patients	Severe scabies (i.e. atypical papular scabies or crusted scabies) develops predominantly in patients receiving topical or systemic corticosteroids, those with HIV infection, organ‐transplant recipients and patients of advanced age. Pruritus may be mild or absent (i.e. scabies incognito)
Atypical presentation
Scabies of the scalp	Scabies may accompany or simulate seborrheic dermatitis or dermatomyositis on the scalp; infants, children, the elderly, patients with AIDS and patients with crusted scabies may be affected
Nodular scabies	A few violaceous, pruritic nodules are often localized on the groin, axillae and male genitalia; they represent a hypersensitivity reaction to mite antigens and persist weeks or months after treatment.

 

Complications

In addition to these primary manifestations, secondary features may occur, and can confuse the clinical picture. Eczematous changes are common, and may be widespread and severe. The inappropriate use of topical steroids may further modify the clinical picture to mimic other dermatoses – so‐called ‘scabies incognito’. Secondary bacterial infections with Staphylococcus aureus or Streptococcus pyogenes, manifest as folliculitis or impetigo, may also be severe and extensive.  Where nephritogenic strains of β‐haemolytic streptococci is responsible for secondary impetigo, hematuria‐related glomerulonephritis occurs as a complication of scabies.

 

Diagnosis

The typical history of generalized pruritus with nocturnal exacerbations, the presence of contact cases within the family and the distribution of the eruption of inflammatory papules, should suggest the diagnosis. The presence of genital lesions in men or breast nodules in women is strongly suggestive. Absolute confirmation can only be made by the discovery of burrows and/or microscopic examination. Highest yield in identifying a mite is in typical burrows on the finger webs, flexor aspects of wrists, and penis. This is accomplished by placing a drop of mineral oil over a burrow and then scraping longitudinally with a number 15 scalpel blade along the length of the burrow, being careful not to cause bleeding. The scrapings are then applied to a glass slide and examined under low power. Three findings are diagnostic of scabies: S. scabiei mites, eggs, and fecal pellets (scybala). Failure to find mites is common and does not rule out scabies. Parasitological confirmation of the diagnosis should be made in all cases if possible. It is essential in cases of crusted scabies or scabies in health care settings.

Dermoscopy and confocal microscopy can prove useful for direct in vivo visualization of mites and eggs. Dermoscopy is useful for detecting burrows and visualizing their contents, the mite in its burrow resembling a ‘jet‐with‐contrail’ (40× magnification). More recently, it is shown that low‐magnification (10×) standard handheld dermoscopy could be a valuable tool for diagnosing common scabies. At low magnification, the circumflex accent‐like image (as the French letter ‘ô’) represents the head and the two pairs of front legs of the mite. It has been shown that sensitivity of this technique is 91% and specificity 86%. Dermoscopy is less time‐consuming than skin scraping procedure because it allows a quick screening of a large number of sites. It causes less discomfort for the patient, so it is also better accepted.

Transparent adhesive tape test may be useful in particular situations. After firmly applying the adhesive side of the tape onto an appropriate skin lesion of patients, the tape is pulled off and transferred directly onto a slide for microscopy, affixing the adhered separated part of the corneal skin. This tape method is simple and may be useful for diagnosis of severe scabies infestation in long‐term nursing units.

A skin biopsy may confirm the diagnosis of scabies if a mite or parts can be identified. However, in most cases, the histology shows non‐specific features, with epidermal spongiosis, papillary edema, and superficial and deep perivascular lymphohistiocytic inflammatory cell infiltrates with numerous eosinophils. On transection, a scabies mite may occasionally be seen within the epidermis. Pink “pigtail”-like structures attached to the stratum corneum, which represent fragments of the adult mite exoskeleton, can serve as a clue to the diagnosis of scabies when entire mites, scybala and eggs are not identified.

 

Treatment

Indication for therapy

Treatment should be prescribed to the patient and to close physical contacts, even without pruritus or cutaneous lesions.

Patient education

Patients should be advised to avoid close physical contact until they and their household members and sexual partners have been treated. Detailed verbal and written information about scabies infestation should be given to the patient.

Treatment options

Topical permethrin and oral ivermectin are commonly used for the treatment of classical scabies. If permethrin is not available, benzyl benzoate may be used. Oral ivermectin may be preferred for patients who cannot tolerate topical therapy or are unlikely to adhere to a therapeutic regimen.

 

TOPICAL AND ORAL TREATMENTS FOR SCABIES
Therapy	Administration	Concerns	Efficacy & resistance	Use in infants	FDA pregnancy category
Permethrin cream (5%)	Topically overnight on days 1 and 8	Allergic contact dermatitis in individuals with sensitivity to formaldehyde	Good, but some signs of tolerance developing	FDA approved for infants ≥2 months of age	B
Crotamiton lotion or cream (10%)	Topically overnight for 3–5 days	Irritant contact dermatitis, especially in areas of denuded skin	Very poor; has antipruritic properties and may be used for postscabetic pruritus	Not established, but considered safe	C
Sulfur ointment (5–10%)	Topically overnight for 3 successive days	Toxicity studies not performed	Good	Not established, but considered safe	Not rated, but considered safe
Ivermectin (available as 3 mg tablets)	Oral dose of 200–400 mcg/kg on day 1 and repeat on day 8 or 14*,**	Potential CNS toxicity in infants and young children	Excellent	Safety not established for children weighing <33 pounds (15 kg) or breastfeeding mothers	C (but generally not recommended for scabies in pregnant women)

* Administration of second dose at 2 weeks recommended by CDC.

** For patients with crusted scabies who fail treatment despite appropriate environmental measures, consider both topical permethrin 5% every 2–3 days for 1 to 2 weeks and oral ivermectin (200 mcg/kg/dose) administered as three doses (days 1, 2 and 8) or as five doses (days 1, 2, 8, 9, and 15), depending upon the severity of the infection

 

Permethrin

Permethrin is a synthetic pyrethroid formulated in a 5% cream that is currently the most effective and standard topical scabicide. Like other pyrethroids, it inhibits sodium transport in arthropod neurons, thereby causing paralysis. Adverse reactions are rare and are usually related to brief stinging on application. Allergic contact dermatitis in individuals with sensitivity to formaldehyde may occur.  Signs of tolerance to permethrin have been noted, but resistance has not been documented. Pregnant females, breast-feeding mothers, and children under 2 years should limit their two applications (1 week apart) to 2 hours only when using permethrin.

 

Crotamiton

An alternative scabicide is crotamiton, which is formulated in a 10% lotion and cream. It is well tolerated and safe for infants and is often used in scabies nodules in children. Of note, this topical medication is less effective than all of the other prescription options. Although it has an antipruritic effect, crotamiton can cause irritation of denuded skin.

 

Sulfur ointment

Another alternative therapy for scabies is three consecutive overnight applications of 5–10% sulfur in a petrolatum base. A pharmacist must compound the medication, and toxicity studies have not been performed. Sulfur is messy, malodorous, and irritating to the skin and can stain clothing. However, its efficacy has been reported to be as high as 60–96% with three consecutive night applications.

 

Ivermectin

Ivermectin is a macrocyclic lactone produced by Streptomyces avermitilis. Although not FDA-approved for scabies, ivermectin represents an effective treatment for this and other ectoparasitic infestations. By blocking transmission across nerve synapses that utilize glutamate or γ-aminobutyric acid (GABA), ivermectin causes paralysis of peripheral motor function in insects and acarines. Although GABA and glutamate are neurotransmitters within the human cerebral cortex, after early infancy, the blood–brain barrier prevents CNS penetration of the drug. Ivermectin is not recommended for children who weigh <33 pounds (15 kg), pregnant women, or breastfeeding mothers because of the lack of safety data in these groups.

In clinical studies, ivermectin has proven to be extremely safe. Oral ivermectin can cure scabies. It is given at 200 μg/kg as a single dose in patients >2 years of age and >15 kg. A second dose is necessary 7–14 days later due to the lack of ovicidal action of the drug. Because ingestion of food increases the bioavailability of ivermectin by a factor of 2, taking it with food might enhance the penetration of the drug into the epidermis. Success rates approach 100% in studies where entire households and close contacts of infested individuals are treated while maintaining strict fomite controls. Topical ivermectin in a 1% concentration (recently FDA-approved for the treatment of rosacea) also appears to be effective for scabies, but further study is warranted before recommending its routine use for this indication. In patients with subungual disease, addition of a topical scabicide is recommended because the oral medication will not penetrate into thickened keratotic debris.

 

European guideline for the management of scabies,2017

Scabies is caused by Sarcoptes scabiei var. hominis. The disease can be sexually transmitted. Patients’ main complaint is nocturnal itch. Disseminated, excoriated, erythematous papules are usually seen on the anterior trunk and limbs. Crusted scabies occurs in immunocompromised hosts and may be associated with reduced or absent pruritus. Recommended treatments are permethrin 5% cream, oral ivermectin and benzyl benzoate 25% lotion. Alternative treatments are malathion 0.5% aqueous lotion, ivermectin 1% lotion and sulphur 6–33% cream, ointment or lotion. Crusted scabies therapy requires a topical scabicide and oral ivermectin. Mass treatment of large populations with endemic disease can be performed with a single dose of ivermectin (200 micrograms/kg of bodyweight). Partner management needs a lookback period of 2 months. Screening for other STI is recommended. Patients and close contacts should avoid sexual contact until completion of treatment and should strictly observe personal hygiene rules when living in crowded spaces.

 

General principles of treatment

Two topical treatments (1 week apart) with a prescription antiscabietic medication should be recommended. The topical preparation is applied at night and left in place for 8–12 h. The skin should be cool and dry. The topical preparation is applied to the entire body surface, from head to toe, in infants and the elderly. In other age groups, the face and scalp can be excluded from treatment. Special attention should be paid to the finger and toe web spaces, the skin beneath the ends of the nails, intergluteal cleft, all folds, groin, external genitalia, and umbilicus. Treating the face of babies is essential because transmission may occur by breastfeeding. This may be not necessary in adults with classical scabies. Hands should not be washed during therapy, otherwise the treatment should be reapplied. If topical treatment is applied by another person, it is recommended that this person wears protective gloves. Patients should be given a detailed explanation of their infestation together with clear written information. The infestation is considered cleared if 1 week after the end of treatment there are no manifestations of active scabies (no active lesions, no nocturnal pruritus). Post-treatment itch may persist for up to 2-4 weeks.

 

Recommended treatments

• Permethrin 5% cream applied head to toe and washed off after 8–12 h. The treatment must be repeated after 7– 14 days.

• Oral ivermectin (taken with food) 200 micrograms/kg as two doses 1 week apart.

• Benzyl benzoate lotion 10–25% applied once daily at night on 2 consecutive days with re-application at 7 days.

 

Alternative treatments

• Malathion 0.5% aqueous lotion.

• Ivermectin 1% lotion is reported to be as effective as permethrin cream 5%.

• Sulphur 6-33% as cream, ointment or lotion is the oldest antiscabetic in use. It is effective and requires application on three successive days.

• Synergized pyrethrins are available as a foam preparation and are as effective as permethrin cream 5%.

• Lindane is no longer recommended because of its potential to cause neurotoxicity.

 

Crusted scabies

• A topical scabicide (permethrin 5% cream or benzyl benzoate lotion 25%) repeated daily for 7 days then 2x weekly until cure AND

• Oral ivermectin 200 micrograms/kg on days 1, 2 and 8. For severe cases, based on persistent live mites on skin scrapings at follow-up visit, additional ivermectin treatment might be required on days 9 and 15 or on days 9, 15, 22 and 29.

 

Special situations

• Permethrin is safe in pregnancy and lactation, and is licensed for use in children from age 2 months onwards.

• Benzyl benzoate and sulphur are considered safe in pregnancy 

• Ivermectin should not be used during pregnancy or in children weighing less than 15 kg.

 

Mass population treatment

• Mass population treatment is recommended for the control of scabies in endemic areas, for example remote communities or mass population displacements, and in the management of epidemics in closed communities such as nursing homes or jails.

• All individuals should be treated irrespective of symptoms.

• Oral ivermectin is easier to administer than traditional topical scabicides, thus facilitating treatment of large populations.

• A single dose of oral ivermectin 200 micrograms/kg of bodyweight is effective.

• Ivermectin may not sterilize scabies eggs, and a second dose given after one week has been shown to increase the response. The administration of a second dose of ivermectin is recommended, although the importance of this second dose for scabies control need to be further evaluated.

• Drug resistance to scabicides including permethrin and ivermectin is an emerging concern, and the impact of mass treatment programmes on development of drug resistance requires future study.

 

Prevention

Several measures should be considered to reduce the potential of re infestation by fomite transmission. Because of the common occurrence of asymptomatic mite carriers in the household, all family members and close contacts should be treated simultaneously, even if they have not developed any pruritus or clinical signs. After each treatment, treated individuals should wear clean clothing, and all clothing, pillow cases, towels and bedding used during the previous week should be washed in hot water and dried at high heat at 50 °C or higher. Non washables should be dry-cleaned, ironed, put in the clothes dryer without washing, or stored in a sealed plastic bag in a warm area for 1 week. Floors, carpets, upholstery (in both home and car), play areas, and furniture should be carefully vacuumed. Fumigation of living spaces is not recommended. Pets also do not need to be treated because they do not harbor the human scabies mite. Secondary bacterial infections need to be treated with appropriate antibiotics. The risk of scabies can be reduced by limiting the number of sexual partners. Transmission is not prevented by condom use.

 

Follow‐up

Following successful treatment, pruritus and skin lesions can persist for 2–4 weeks or occasionally longer, especially for acral vesiculopustules in infants or nodular lesions. This is referred to as “postscabietic” pruritus or dermatitis. Patients should be informed that such reactions do not imply treatment failure, but rather represent the body’s response to dead mites that are eventually sloughed off (within 2 weeks) along with natural epidermal exfoliation. Many patients, however, experience relief from pruritus within 3 days. The second application of topical medication is performed in order to reduce the potential for reinfestation from fomites as well as to ensure killing of any nymphs that may have survived within the semi-protective environment of the egg and subsequently hatched. The persistence of itching after 4 weeks should be reinvestigated.

 

Causes of persistent itching after scabicide therapy and management

	Causes	Management
Cutaneous irritation	Overtreatment Eczematization Contact dermatitis	Intensive use of emollient Intensive use of emollient Topical steroid
Treatment failure	Poor compliance: inappropriate or insufficient treatment Resistance to scabicide Reinfestation or relapse	Further scabicide application Change scabicide Further scabicide application
Psychogenic pruritus	Delusions of parasitosis Non‐parasitic dermatosis	Antipsychotic drugs (prescribed by dermatologists and/or psychiatrists) Treat the underlying cause

 

Post-treatment itch

Post-treatment itch should be treated with repeated application of emollients. Oral antihistamines and mild topical corticosteroids may also be useful.

 

Partner management

Patients should be advised to avoid close contact until they and their sexual partners have completed treatment and should strictly observe personal hygiene rules when living in crowded spaces.

Infestation in children due to sexual abuse is rare and is more usually associated with close non-sexual contact.

Assessment and epidemiological treatment is recommended for sexual partners over the past 2 months.