What is scabies?

Scabies is a very itchy rash caused by a parasitic mite that burrows in the skin surface. The human scabies mite's scientific name is Sarcoptes scabiei var. hominis.

Scabies

Who gets scabies?

Scabies affects families and communities worldwide. It is most common in children, young adults, and older persons. Factors leading to the spread of scabies include:

• Poverty and overcrowding
• Institutional care, such as rest homes, hospitals, prisons
• Refugee camps
• Individuals with immune deficiency or that are immune suppressed
• Low rates of identification and proper treatment of the disease.

What causes scabies?

Scabies is nearly always acquired by skin-to-skin contact with someone else with scabies.

• The contact may be quite brief such as holding hands with an infested child.
• It is sometimes sexually transmitted.
• Occasionally scabies is acquired via bedding or furnishings.

Typically, several scabies mites infest an affected host. After mating, the male mite dies. The female scabies mite burrows into the outside layers of the skin, where she lays up to 3 eggs each day for her lifetime of one to two months. The development from egg to adult scabies mite takes 10–14 days.

What are the clinical features of scabies?

Scabies causes a very itchy rash. It’s essential to search for burrows carefully in a patient with a severe itch, especially if the rash is mild. Contacts should be examined for burrows, whether or not they are itchy.

Itch

• If it is the first episode of scabies, itch arises 4–6 weeks after transmission of a mite
• It may occur within a few hours of subsequent infestation.
• Itch is characteristically more severe at night, disturbing sleep
• It affects the trunk and limbs, sparing the scalp.
• Itch is mild or absent in some patients with crusted scabies.
• The itch can persist for several weeks after successful treatment to kill the mites.

Burrows

Scabies burrows appear as 0.5–1.5 cm grey irregular tracks in the web spaces between the fingers, on the palms and wrists. They may also be found on or in elbows, nipples, armpits, buttocks, penis, insteps and heels. Dermatoscopic or microscopic examination of the contents of a burrow may reveal mites, eggs or mite faeces (scybala).

Generalised rash

Scabies rash is a hypersensitivity reaction that arises several weeks after the initial infestation. It has a varied appearance.

• Erythematous papules on the trunk and limbs, often follicular
• Diffuse or nummular dermatitis
• Urticated erythema
• Vesicles on palms and soles
• Acropustulosis (sterile pustules on palms and soles) in infants
• Papules or nodules in the armpits, groins, buttocks, scrotum and along the shaft of the penis
• Rare involvement of face and scalp.

Scabies

See more images of scabies.

What are the complications of scabies?

Secondary infection

Secondary infection is due to scratching and the effect of the mite on the skin's ability to fight bacteria.

• Staphylococcal and streptococcal infection results in crusted plaques and pustules (impetigo).
• Streptococcal cellulitis results in painful swelling and redness, and fever.
• Systemic sepsis with staphylococci and streptococci is potentially very serious.
• Scabies outbreaks lead to cases of post-streptococcal glomerulonephritis and acute rheumatic fever.

Crusted scabies

Crusted scabies (previously called Norwegian scabies ) is a very contagious variant of scabies in which an individual is infested by thousands or millions of mites living in the surface of the skin.

• The patient presents with a generalised scaly rash. This is often misdiagnosed as psoriasis or seborrhoeic dermatitis.
• The scale is often prominent in the finger webs, on wrists, elbows, breasts and scrotum.
• The itch may be absent or minimal.
• Crusted scabies may affect the scalp.

Risk factors for crusted scabies include:

• Very old age
• Malnutrition
• Immune deficiency
• Intellectual deficit
• Neurological disease
• A specific inherited immune defect in some otherwise healthy people.

A case of crusted scabies is the usual reason for an outbreak of scabies in an institution. Patients and staff in the institution may present with:

• Usual scabies
• Crusted scabies
• Hypersensitivity rash but no burrows, ie, not infested.

How is scabies diagnosed?

The clinical suspicion of scabies in a patient with an itchy rash, especially when reporting itchy household members, can be confirmed by:

• Dermatoscopy: the mite at the end of a burrow has characteristic jet-plane or hang-glider appearance
• Microscopic examination of the contents of a burrow
• Skin biopsy: this may reveal characteristic scabies histopathology, but this is often negative or nonspecific, eg if taken from the inflammatory rash rather than the surface of a burrow.

Scabies dermoscopy

Scabies dermoscopy

Scabies dermoscopy

Crusted scabies reveals numerous mites on dermatoscopy or microscopy, raised immunoglobulin E (IgE) and eosinophilia.

Differential diagnosis of scabies includes insect bites or papular urticaria, skin infections, dermatitis, urticaria, and bullous pemphigoid.

Consensus criteria for the diagnosis of scabies were published by the International Alliance for the Control of Scabies (IACS) in 2018 [1]. 

What is the treatment for scabies?

Management of a scabies outbreak involves the identification and treatment of individual patients and household contacts with insecticides. Oral antibiotics are required for secondary infection.

Careful attention to instructions is essential if scabies is to be cured.

The chemical insecticides used to treat scabies are called scabicides. The scabicide is applied to the whole body from the scalp to soles. The usual topical treatment is 5% permethrin cream, left on the entire skin for 8–10 hours. It should be applied under fingernails using a soft brush.

Oral ivermectin 200 mcg/kg is convenient but more expensive than topical permethrin. It may be slightly less effective. It is mainly used for mass treatments in institutions, or in patients unable to use topical therapy.

Gamma benzene hexachloride cream is no longer recommended or available due to neurotoxicity; it is also a suspected carcinogen and is no longer marketed in New Zealand. Other proven treatments include:

• 0.5% aqueous malathion lotion, left on for 24 hours
• 25% benzyl benzoate lotion, applied daily for 3 days. This is irritant, and should not be used in children.
• 2–10% precipitated sulphur ointment

Treatment should be repeated after 8–10 days after the first application to catch mites that have newly hatched. Crotamiton cream can be used to reduce itch; it is a weak scabicide.

Patients with crusted scabies may need repeated oral and topical treatments over several weeks or longer. 

Additional management

Contacts must be identified and treated. In addition:

• Bed linen, towels and clothing should be laundered after treatment.
• Non-washable items should be sealed in a plastic bag and stored for one week.
• Rooms should be thoroughly cleaned with normal household products. Fumigation or specialised cleaning is not required.
• Carpeted floors and upholstered furniture should be vacuumed.

What is the outlook for scabies?

Scabies itch and rash are expected to improve within a few days of successful treatment and to completely clear within a month.

A rash may persist after scabies treatment. Reasons for this include:

• Persistent infestation due to incorrectly applied treatment, treatment resistance, or re-infestation due to an untreated contact.
• The hypersensitivity reaction can be slow to settle, despite the complete cure of parasitic infestation.
• On-going dermatitis can be due to the mite, the scratching, irritation of topical treatment, or other factors. Persistently itchy papules, nodules and eczematous plaques should be treated with frequent applications of emollients and mild topical corticosteroids once or twice daily.
• Incorrect diagnosis.