Scabies

Introduction

Scabies is a highly contagious skin infestation caused by a mite. It is spread by close contact and is more common among disadvantaged populations. It is readily treatable, and a broad approach to treatment is important to ensure eradication.

Estimates of prevalence are highly variable, but scabies is thought to affect up to 100 million people every year. Children, the elderly, and people residing in long-term care facilities are at the highest risk.

Prevalence can vary among ethnic groups depending on exposure to risk factors. Scabies is more prevalent in indigenous and refugee communities, due to socioeconomic disadvantage and overcrowding. It is considered a neglected tropical disease.²

Aetiology

Scabies is caused by a parasitic mite, Sarcoptes scabiei var. hominis. It is transmitted through direct skin-to-skin contact, or less commonly indirectly via fomites.¹

The clinical features of scabies infection primarily result from a local allergic reaction to the presence of the scabies mite, rather than being directly caused by the mite itself.

Risk factors

Key risk factors for contracting and transmitting scabies include:

• Direct contact: skin-to-skin contact with a person with scabies, which can be quite brief (such as through holding hands or sexual contact).
• Fomites: indirect contact through bedding, clothes, or towels, although this is far less common than direct contact.
• Living conditions: poverty and overcrowding are key risk factors. This includes institutional care facilities, such as residential aged care homes, hospitals, and prisons.
• Environment: transmission is more common in warm, tropical, humid environments.
• Immunosuppression: HIV infection or immunosuppressive medication increases the risk of infection with scabies and may also lead to more severe and persistent infection.³

Clinical features

The clinical features of scabies infection primarily result from a local allergic reaction to the presence of the scabies mite, rather than being directly caused by the mite itself.

History

In the first infection, scabies may be asymptomatic for up to 6 weeks after exposure, but the asymptomatic period will shorten significantly if subsequent infections occur. Once symptomatic, patients report severe pruritus worse at night, severe enough to cause sleep disturbance.⁴

On further history taking, patients may have other family members or close contacts with similar symptoms. It is important to take a detailed social history for potential risk factors (such as poor living conditions and overcrowding), to determine potential contacts and if additional holistic support may be required.

Clinical examination

Scabies is typically found in the webbing and sides of fingers, wrists, elbows, axillae, feet and genitals. The rash of scabies is due to a local hypersensitivity reaction to the mite.

The appearance of the rash is quite variable but can include erythematous papules or vesicles, and surrounding dermatitis.

Burrows are characteristic and appear as small irregular tracks ~1cm in length, classically found in the webbed spaces between the fingers. On dermatoscopy, mites or mite eggs or faeces, may be visible in burrows (known as the delta sign).

Clinical presentations

Crusted scabies (sometimes called Norwegian scabies) is a severe variant of scabies where an individual is infected with thousands or millions of mites (compared with 5-20 in a typical infection). It is very contagious and causes a crusted, scaly, keratotic rash, often in the finger webs, wrists, and elbows. Itch may be less prominent, or even absent.⁶

Differential diagnoses

Because the scabies rash can be quite variable in appearance, differential diagnosis is broad. It can include insect bites, skin infections, urticaria and more. It is important to always consider scabies when evaluating itchy rashes.

Investigations

Scabies can be diagnosed clinically, but investigations can assist with the diagnosis.

Dermatoscopy is a useful adjunct to clinical examination, to assist with visualising burrows that may be too small to see macroscopically.

Skin scraping and biopsy can be useful for histopathology, but it is important to sample the burrow itself, rather than the surrounding rash.

Management

General advice

Scabies can be difficult to eradicate as it is highly contagious and has a long incubation period. 

Good hygiene is important. Washing all bed linen, clothes, toys, and other items is recommended to eradicate scabies from the household. Close contacts of a person with scabies will need to be examined and treated concurrently.  

Topical therapies

Permethrin 5% cream is the treatment of choice for scabies, applied to the entire body (excluding the face) and left on for 8 hours, with treatment repeated in 7 days. Some guidelines do not recommend permethrin for use in children younger than 2 months of age, so it is important to check local guidelines.

Benzyl benzoate 25% emulsion is used as a second-line treatment in cases of allergy or treatment failure but can cause skin irritation.⁸

Systemic therapies

Ivermectin is an effective oral scabicide and is the mainstay of systemic treatment of scabies for adults and older children. It is usually given at a dose of 200µg/kg.

Ivermectin is effective but more expensive than permethrin. It can be useful in situations where compliance is difficult or to contain widespread outbreaks (e.g. in care facilities).

Complications

Secondary bacterial infection of scabies rashes is common, due to patients scratching the highly pruritic rash, most commonly with Streptococcus pyogenes (a group A streptococcus) or Staphylococcal aureus.

It is important to recognise and treat secondary infections early with appropriate antibiotics, because of the risk of sequelae from group A streptococcus including glomerulonephritis and rheumatic fever.

Due to the underlying social determinants of health, many patients who are at highest risk of scabies are also at risk of these significant post-infectious sequelae.³

Key points

• Scabies is a highly contagious skin infestation caused by the Sarcoptes scabiei mite and typically spreads through direct skin-to-skin contact.
• Poverty, overcrowding, and immunosuppression are key risk factors for scabies.
• The clinical presentation of the scabies rash is variable, but the webbing of the hands and feet are commonly affected, and severe itch is common. In a patient’s first presentation of scabies, there may be a delay of up to 6 weeks before symptoms appear.
• Scabies can be treated with permethrin or benzyl benzoate topical therapy, or with ivermectin orally in severe cases (including crusted scabies). Treatment of secondary bacterial infection may also be required.

Reviewer

Consultant Dermatologist

Editor

Dr Chris Jefferies

References

1. DermNet NZ. Scabies. 2015. Available from: [LINK]
2. WHO. Scabies and other ectoparasites. 2020. Available from: [LINK]
3. Australian Family Physician. Scabies. 2017. Available from: [LINK]
4. British Association of Dermatologists. Handbook for Medical Students and Junior Doctors. 2020. Available from: [LINK]
5. Wikimedia Commons. Scabies affecting the hand and fingers of a Caucasian child. Licence: [CC BY-SA]. Available from: [LINK]
6. DermNet NZ. Scabies. 2015. Available from: [LINK]
7. DermNet NZ. Severe crusted scabies. Licence: [CC NC BY-SA]. Available from: [LINK]
8. eTG Complete. Scabies. 2020. Available from: [LINK]