Scabies

What is scabies?

Scabies is a transmissible skin disease caused by the ectoparasiticmiteSarcoptes scabiei var. hominis. This variant infests humans only; scabies cannot be caught or transferred to other animals.

Scabies is a highly contagiousinfestationof the humanepidermis. Scabies was described by Aristotle who likened the disease to ‘lice of the flesh’. Scabies presents as arashwith intense itching; it may have a characteristic appearance anddistribution.

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谁疥疮?

Scabies is a global disease, and anyone can be affected. It is estimated that about 204 million people are affected by scabies worldwide, with an annual发病率of 455 million cases. Scabies is more common in the elderly, children, and adolescents.

The burden of disease is higher in low-income areas, in the tropics, and in poorly resourced communities and countries.

Risk factors for scabies, particularly indevelopingcountries, include:

• Crowded conditions
• Poor hygiene
• Poverty
• Malnutrition
• Homelessness
• Immunodeficiency.

In developed countries, scabies occurs in family outbreaks and children may acquire it from close play contact in school or sleepovers. In situations where there may be close physical contact for the delivery of personal care, such as incare homes, large outbreaks are not uncommon. In young adults, sexual spread is frequent.

What causes scabies?

Scabiesinfectionis usually transmitted through close bodily skin contact such as holding hands for prolonged periods; spread amongst sexual partners is common. A brief handshake or hug does not usually allow for transmission unless the patient hascrustedscabies.

Spread via fomites (clothing, towels, etc.) is very uncommon as the mite perishes within hours of leaving thehost; it is relevant incrusted scabies.

Scabies cycle

• The smaller male mite fertilises the female on the surface of the skin and dies shortly after mating.
• The female miteburrowsinto thestratum corneumof the host where they lay 2 to 3 eggs per day.
• The female continues toburrowat a rate of 0.5 to 5mm per day for the duration of its 4 to 8-week lifespan.
• Eggs are deposited in the burrow and hatch intolarvaeafter approximately 2 to 5 days.
• Hatched larvae mature into adult scabiesmitesin about 3 weeks. They migrate to the skin surface, and the cycle continues.
• The itchy rash is the result of a TH1-mediatedhypersensitivityreaction to various mite-relatedantigensand is thus delayed.

What are the clinical features of scabies?

Infection typically presents with a classical itchy rash.Lesionsaresymmetrical, and mainly affect the hands, wrists,axillae, thighs, buttocks, waist, soles of the feet,areolaandvulvain females and penis and scrotum in males. The neck and above are usually spared, except in cases of crusted scabies and ininfectionsoccurring in infants, the elderly, and theimmunocompromised.

However, the rash may begeneralisedandeczematousfeaturesmay dominate and mask the classical signs. Clinical suspicion is paramount.

Itch

• Generalised
• Occurs in 4 to 6 weeks following initial infection (during which time the host is infectious)
• Occurs within hours of subsequent re-infection
• Worse at night-time
• Maypersistfor several weeks after completion of treatment.

Rash

• Erythematouspapules
• Excoriations
• Linearscratch marks
• Dermatitis
• Noduleswhich may be skin coloured, red-brown, orviolaceous
• 结硬皮(hyperkeratosisas seen in crusted scabies)
• Vesicleswhich may also be secondary to a superimposedbacterialinfection.

特殊的features which can assist in diagnosis

Burrows

• Curvilinear orserpiginousthread-like tracks measuring around 5–10 mm — these can be subtle
• Typically identified in the web spaces, palms, soles, fingers, toes, inner wrists, elbows,umbilicus, and beltline.

Nodules

• Varying in size between 3 and 15 mm
• Maydevelopon the penis and scrotum in men
• On the upper thighs andglutealfolds
• Around theareolaein women
• In the axillae

疥疮洞穴的脚趾

A burrow on the palm - the mite can just be seen adjacent to the vesicle

Numerous palmar scabies burrows in an elderly lady

Scabies burrows on the palm

Scabies burrows on a babies foot

Interdigital scaling in the first web space of the hand

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Secondaryeruption

This can be awidespreadnonspecific eczematous eruption on the torso and limbs and may partially obliterate theprimarypathognomonic features.

How do clinical features vary in differing types of skin?

Scabies may present asgranulomatousnodules in an infected person with a darker skin type.

Inflammatorychanges appear as redness in white skin and greyness in black skin.

What are the complications of scabies?

Crusted scabies

• Previously known as Norwegian Scabies.
• Associated with dense hyperkeratosis of the skin.
• Caused by an altered host immune response.
• Seen particularly in patients with:
  • ReducedT-cellimmunity e.g.,HIV
  • Reducedperipheralsensation e.g.,leprosy
  • Patients unable todebridemites mechanically e.g.,Downsyndromeandsubungualhyperkeratosis.
• Results in uncontrolledproliferationof mites (thousands to millions of mites, compared to 10–15 seen in classical scabies).

Bullousscabies

• Usually seen in elderly males.
• Bullaemay be tense orflaccid.
• Thought to be due to either secondary infection of a scabeticlesionwithStaphylococcus aureusor due to anautoantibodyresponse.

Nodularscabies

• May be due to a local hypersensitivity reaction to dead mites or may be secondary topersistentinfection.
• Commonly affects malegenitalia(5月be a widespread dominant feature in children) and breasts.
• Can be seen around the axillae or on the buttocks.
• May persist for months despite otherwise successful scabies treatment.

Other complications of classical scabies include:

Scabies incognito

• 莫迪fication of the clinical picture secondary to inappropriate use of steroids after misdiagnosis.
• Results in delayed diagnosis, sometimes for months or even years.

Secondary bacterial infection

• Staphylococcus aureusorStreptococcus pyogenesbacteraemia.
• Impetigo.
• Pustularlesions on the palms, soles, fingers, and toes.
• Toxin-mediated diseases such astoxicshock syndrome, glomerulonephritis, andacuterheumaticfever.

Psychological distress

• Scabies often afflicts young adults living away from home for the first time and thedevelopmentof a contagious rash can be embarrassing. Stigma can lead to an inability to ensure household or sexual contacts are treated.
• Sleep disturbance and an unsightly rash compound the emotional impact of the illness, especially where diagnosis and comprehensive treatment is delayed, when the symptoms can persist for many months.

Scabies with superadded streptococcal infection

Scabies nodules in the axilla

Axillary scabies nodules in an infant

How is scabies diagnosed?

A high index of suspicion should be used when assessing a patient with a new widespread itchy rash, especially one who reports itchy close contacts. The diagnosis is typically clinical and most easily confirmed usingdermoscopy.

Severalinvasiveand non-invasive tests exist to aid in confirming a diagnosis of scabies:

• Dermoscopy may reveal the ‘delta wing jet’ or ‘mini triangle’sign. In crusted scabies, dermoscopy may show hyperkeratosis.
  • For information on trichoscopic findings in scabies of the scalp, seeTrichoscopyof scalp infestations.
• Skin scrapings using either a blade or needle allow direct visualisation of mites eggs or mite faeces onmicroscopy. Visualisation of a live mite by the patient galvanises treatment concordance.
• A burrow ink test may reveal a classical zig-zag line where the ink has tracked into a burrow.
• Adhesive tape test is used to transfer a sample from a suspicious lesion directly to amicroscopeslide.
• Skinbiopsy, although rarely necessary, will show eggs, larvae, faeces, and mites.

It is useful to examine close contacts in suspicious cases as pathognomonic signs are often seen in those with minimal symptoms.

A scabies mite on microscopy after extraction from a burrow

Scanning electron microscopy of Sarcoptes scabei

The International Alliance for theControlof Scabies (IACS) updated consensus criteria for the diagnosis of scabies in 2020, copied below:

Notes

1. These criteria should be used in conjunction with the full explanatory notes and definitions (in preparation by IACS).
2. Diagnosis can be made at one of the three levels (A, B or C).
3. A diagnosis of clinical and suspected scabies should only be made if other differential diagnoses (such aseczemaand impetigo) are considered less likely than scabies.

What is thedifferential diagnosisfor scabies?

• Bullous skin disorders
• Dermatitis
• Folliculitis
• Papularurticaria
• Prurigo(subacute)
• Psoriasis
• Infantile acropustulosis

What is the treatment for scabies?

General measures

• Launder sheets, towels, and clothes worn recently. Outer garments, duvets, and blankets should be aired for 72 hours.
• Vacuum soft furnishings where possible, although this may be of limited benefit.
• Clipnailsand clean any subungual debris.

Specific measures

It is important to note that all close contacts of a confirmed case of scabies should complete eradication therapy, whether they are symptomatic or not. Contacts may be infected butasymptomaticfor several weeks, therefore they may continue to infect others and even reinfect the index case.

Topicaltherapies

• Topical 5%permethrin(acaricidal and ovicidal)creamorlotionremains first-line therapy.
• Applied from the jawline downwards, and left overnight for 8–12 hours. Reapplication is needed if hands are washed during the treatment period. Infants, the elderly, and theimmunosuppressedshould also treat the face and scalp.
• Application should include under the nails and between the toes.
• 治疗后应重复7 - 10天。
• If the case is permethrin-resistant, or permethrin is not available, alternativeinsecticides, include 25% benzyl benzoate or 0.5% malathion aqueous lotion.

Oral therapies

• Oralivermectinis indicated in cases of topical failure, inability to comply with topical therapy, non-adherence to topical therapy, institutional outbreaks, mass treatment of populations, and crusted scabies.
• Repeat dose after 2 weeks.
• Latest data suggests oral ivermectin is equally as effective as topical 5% permethrin cream or lotion, however may be slower-acting.
• Oral moxidectin is as yet unlicensed, but an effective agent that has good skin retention — it may be effective as a single treatment and prevent re-infestation.

Practical advice to patients

• 提供书面指示。
• Explain the necessity for complete, contemporaneous compliance with topical therapy.
• Household members should elect an early, convenient evening on which to be treated, assemble bedding and clothing for laundry, and apply the topical treatment before dressing and remaking beds.
• Itching may last for several weeks, and this does not represent persistent infection or treatment failure.

Post-scabetic itch

• May persist for weeks
• Moderate potencytopical steroidsmay be used and required for 3–4 weeks, particularly if the patient has a widespread eczematous reaction
• Antihistaminesare helpful in some cases
• Emollientuse and soap avoidance should be advised
• Persistent nodules may need potenttopicalorintralesional steroids.

Scabies and pregnancy

• 5% permethrin cream or lotion is considered safe in pregnancy and while lactating.
• Alternative topical preparations include sulphur 5–10% cream,ointment, or lotion.
• Oral ivermectin is not advised during pregnancy.

What is the outcome for scabies?

Classical scabies is associated with a goodprognosisprovided compliance is satisfactory and all close contacts (symptomatic or not) are simultaneously treated. Crusted scabies may need prolonged and repeated treatment and patients may have significant underlying conditions that will influence the prognosis. Pyococcal infection of scabies is one of the most common causes of acute glomerulonephritis globally.

Scabies reinfection is common and is usually secondary to spread from untreated close contacts.Repetitiveuse of disinfectants can lead toirritantdermatitis, and in the mistaken belief of continued scabies infestation, yet more disinfectant is used.

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