Scabies: diagnosis and management

Scabies – a highly contagious and pruritic skin infestation

Scabies is caused by the microscopic female scabies mite (Sarcoptes scabiei var. hominis) burrowing beneath the skin.^{1, 2} Scabies affects people of all ages, however, children, older people and people who are immunocompromised are most vulnerable to infestation, particularly those living in low socioeconomic areas (see: “Person-to-person contact is usually required for transmission”).¹

The characteristic pruritic skin rash associated with scabies often does not develop until weeks after the initial infestation, as it is caused by a delayed type IV hypersensitivity reaction to the mites eggs, faeces and saliva (see: “Classic scabies is usually diagnosed clinically based on the characteristic features of burrows, rash and pruritus”).^{1, 2} Usually by the time the patient develops symptoms, scabies transmission will have already occurred.¹ Prompt treatment is therefore important to prevent ongoing transmission. Following treatment for scabies, some people experience persistent pruritus, a secondary rash, e.g. dermatitis, or develop a secondary bacterial infection from excessive excoriation (see: “Manage any ongoing symptoms and secondary complications”).^{3, 4}

There are three main presentations of scabies:

• Classic scabies is the most common form, involving infestation with a low number of mites (approximately 5 – 15) and usually accompanied by a pruritic skin rash that typically starts in an acral distribution, e.g. palms, soles, fingers, toes, and spreads to the trunk and limbs¹
• Nodular scabies is a feature of chronic infestation of classic scabies, characterised by clusters of pruritic inflammatory nodules and papules on axillary folds and genital areas, e.g. the shaft of the penis, the scrotum and buttocks.⁵ Nodules can persist even after successful treatment for classic scabies.⁵
• Crusted scabies (uncommon) results from hyper-infestation with thousands to millions of mites causing thick crusted plaques on the skin and hyperkeratosis.¹ Crusted scabies is more contagious than classic scabies and most commonly affects people with underlying immunodeficiency, neurological diseases that cause reduced sensation or immobility, older people and those living in institutional facilities (see: “Crusted scabies is uncommon, but more contagious than classic scabies”).^{4, 6}

Person-to-person contact is usually required for transmission

Scabies transmission most often occurs through prolonged (e.g. 20 minutes) skin-to-skin contact with someone who has scabies.¹ More fleeting contact with someone who has crusted scabies may also result in transmission due to the extensive number of mites.⁷ Scabies mites spread easily between household members, sexual partners and through other close contact behaviours, e.g. hand holding.^{1, 2} Less commonly, mites transfer from the sharing of materials or fomites, such as clothing, towels, bedding and other furnishings; this form of indirect transmission is most often seen in cases of crusted scabies.⁴

Scabies is not the result of poor hygiene. The risk of developing a scabies infestation and its transmission is influenced by factors such as:^{1, 3}

• Living in hotter, more humid environments
• Living in areas of high population density, e.g. poverty and overcrowding
• Living or working in high-contact environments or institutional care facilities, e.g. hospitals, educational or residential care facilities, prisons
• Underlying immunodeficiency or immunosuppression^*, e.g. due to HIV, long-term oral corticosteroid use (or rarely, prolonged topical corticosteroid use)
• Contact with contaminated fomites, e.g. through shared clothing and materials (typically only in crusted scabies)
• Travelling to endemic areas overseas, e.g. developing countries with hot, humid environments

^*Particularly increases the risk of crusted scabies¹

Classic scabies is usually diagnosed clinically based on the characteristic features of burrows, rash and pruritus

Classic scabies is usually identified with the presence of characteristic features, e.g. mite burrows, a papular rash and pruritus (Figure 1), supported by risk factors, e.g. known contact with a person or location with active infestation (see: “Crusted scabies is uncommon, but more contagious than classic scabies” for details on the characteristic features of crusted scabies).^{1, 4} Some people, however, may present with atypical or subtle features which can make the diagnosis more challenging. A type IV hypersensitivity reaction to the mites’ antigens often results in a delay of up to six weeks after the initial infestation, before the pruritic rash develops.³ Conversely, people who have been exposed to scabies previously can experience symptoms within a few hours or days following exposure.^{1, 3}

Consider the possibility of scabies in any situation where multiple patients or members of the same household report pruritus, particularly on the trunk and limbs, even if burrows or rash are minimal or absent.³

Ask about pruritus

For most patients with scabies, pruritus begins in an acral distribution, e.g. palms, soles, fingers, toes, and spreads to the trunk and limbs, and in infants it may spread to the head and neck.^{1, 3} Patients often report that itching is worse after a hot bath or shower, and at night and disturbs their sleep.^{1, 3} Severe pruritus and sleep disturbance can impact on school or work attendance and excessive excoriation can lead to a secondary bacterial infection (see: “Manage any ongoing symptoms and secondary complications”).¹

Examine for rash

The generalised skin rash associated with a scabies infestation can be variable or polymorphic in appearance and is often characterised by:^{1, 3, 4}

• Small erythematous papules, generally capped with haemorrhagic crusts on the trunk and limbs, wrists, webbing between fingers and toes, buttocks and genitalia, axillary folds, waist, breasts and periumbilical area. In young children and older people, papules may spread to the scalp, palms and soles (Figure 2).
• Urticaria
• Dermatitis, typically diffuse or discoid
• Vesicles on the palms and soles. Young children may also get pustules that can mimic impetigo. In infants, vesicles or pustules can arise on any body site.
• Nodules usually 0.5 – 1 cm in diameter often clustered in the groin, genitalia, buttocks or axillary folds in persistent cases (Figure 3)

Rarely, other inflammatory conditions such as a morbilliform rash, folliculitis, vasculitis or bullous pemphigoid may follow untreated scabies.

Figure 2. Scabies infestation on the sole of a young child (image supplied by DermNet).

Figure 3. Scabies infestation with nodular skin lesions (image supplied by DermNet).

Search skin for burrows

Scabies mite burrows generally appear as thin, irregular, brown-grey lines within superficial epidermal layers, approximately 0.5 – 1.5 cm in length.^{1, 3, 4} Burrows are most commonly found on webbing between fingers and on the wrists or palms.^{1, 3, 4} Other less commonly affected sites include the periumbilical area, genitalia, buttocks, breasts, axillary folds, soles of feet and between the toes and rarely, on the scalp.^{1, 3, 4}

Excoriation, secondary bacterial infection or concurrent skin conditions, e.g. eczema, can make burrows difficult to identify.^{1, 4} For easier identification, an ink test may be used to check for burrows and dermatoscopy used to visualise the mite.¹

Microscopy to confirm a diagnosis of scabies is rarely required

Laboratory-based microscopy of burrow content and skin scrapings can be used to identify mites, their eggs or faeces to confirm scabies in patients where the diagnosis is uncertain or in atypical cases, e.g. a major outbreak in a residential care facility. A negative result does not always exclude scabies due to the possibility of sampling error.

In practice, however, if scabies is suspected, treatment is usually commenced on the basis of a clinical diagnosis without the need for confirmation via microscopy. Response to empiric treatment supports a diagnosis of classic scabies.¹

If crusted scabies is suspected, a diagnosis should always be confirmed by dermatoscopy (preferably digital) or from skin scrapings (see: “Crusted scabies is uncommon, but more contagious than classic scabies”).¹

Consider differential diagnoses

Potential differential diagnoses for scabies include insect bites, bacterial infections, e.g. impetigo^* or folliculitis, atopic eczema or other forms of dermatitis and immune-mediated conditions, e.g. pityriasis rosea, papular urticaria.¹ Also consider seborrhoeic dermatitis and psoriasis in the differential diagnosis of crusted scabies.¹

^*Scabies can also mimic impetigo in young children

Managing scabies: treatment and reducing transmission

Permethrin is the recommended first-line topical treatment

Permethrin 5% cream or lotion (funded, also available over-the-counter [OTC]) is the recommended first-line treatment for classic scabies (Table 1).² Treatment may not be sufficiently effective against eggs so it should be repeated seven to ten days after the initial application to cover any newly hatched larvae. Household members should also be treated. Advise patients that a transient increase in pruritus is common in the first few days following permethrin application and can be managed with other treatments, e.g. topical crotamiton (Table 2).¹ Crotamiton is also a weak scabicide but is primarily used for reducing pruritus in people with classic scabies.³ Both lindane and malathion lotions are no longer available for the treatment of scabies in New Zealand.⁹

Reducing the chain of transmission

Reducing scabies transmission is crucial for successful management and the prevention of re-infestation.^{1, 3} The following strategies can reduce the chain of transmission:^{1, 3}

• Advise the patient to avoid direct contact with other people for at least eight hours following treatment. In crowded settings, e.g. hospitals, residential care facilities, prisons, ideally the person with scabies should be kept apart from other people. This is particularly important for those with crusted scabies.
• Recommend that all household members are treated. Patients should also advise known close contacts (within the last 30 days) to seek assessment and treatment from their general practitioner even if pruritus is mild or absent
• Recommend basic cleaning practices to eliminate mites from fomites or other materials, e.g. hot laundering (or hot ironing) of bedding, clothing and towels and vacuuming of carpets and upholstery. This is most important in institutional settings or in cases of crusted scabies as crusted plaques are often shed. Excessive cleaning is unnecessary as scabies mites can only survive for three days outside of a human host; there is also little evidence that cleaning practices are effective outside of institutional settings.
• Provide patient information on the management of scabies, e.g. www.healthinfo.org.nz/patientinfo/519865.pdf

N.B. Oral ivermectin is occasionally used for prophylaxis when there is a scabies outbreak in an institutional facility.^{3, 4}

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Table 1. Treatment options for classic and crusted scabies.^{3 ,6, 9}

^* Manufacturer recommends exclusion of head and neck, however, NZF recommends application should extend to the scalp, neck, face and ears.⁹ This is particularly important in infants.

^† Funded with Special Authority approval following discussion with a dermatologist, infectious disease specialist or clinical microbiologist for crusted scabies, large scale outbreaks, e.g. in institutions, or for patients who cannot use or are unresponsive to topical treatment.⁹ Up to 100 tablets are available on Practitioners Supply Order for outbreaks within institutions.⁹ N.B. The safety of oral ivermectin in children weighing < 15 kg is not established.¹⁰

Follow-up after initial treatment

Even after treatment has successfully eliminated all of the scabies mites, a nodular rash and pruritus may persist for several weeks.^{1, 3, 4} Reassure patients that ongoing symptoms do not necessarily indicate a lack of treatment response as these symptoms are likely due to the hypersensitivity reaction to mite antigens retained within the skin or a secondary rash.^{1, 3, 4} Symptoms that worsen or do not improve within four-to-six weeks may indicate inadequate treatment response which could be due to:³

• Inadequate treatment (e.g. application), treatment resistance or over-treatment resulting in contact dermatitis
• Scabies re-infestation
• Secondary bacterial infection
• Incorrect diagnosis

Consider discussion with or referral to a dermatologist or infectious disease specialist if symptoms persist after six weeks despite treatment adherence and appropriate cleaning practices.

Manage any ongoing symptoms and secondary complications

For patients with persistent pruritus, emollients, calamine or crotamiton, tar oil, short-term oral antihistamines or mild potency topical corticosteroids can be used to manage their symptoms (Table 2).^{1, 3}

Treat secondary bacterial infection, e.g. Streptococcus pyogenes, Staphylococcus aureus, from a broken skin barrier due to excoriation, with oral antibiotics (Table 2).¹ A secondary bacterial infection can cause local soft tissue complications, e.g. impetigo, cellulitis, abscess, and rarely post-streptococcal glomerulonephritis.¹ Observational studies in New Zealand have also shown a strong association between scabies infestation, Group A streptococcal infection, impetigo and acute rheumatic fever.¹¹

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Table 2. Treatment options for persistent symptoms and secondary complications following treatment for scabies.^{1, 3, 9}