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Scabies

What is scabies?

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

How does one get 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. Frequently it is acquired from children, and sometimes it is sexually transmitted. Occasionally scabies is acquired via bedding or furnishings, as the mite can survive for a few days off its human host.

Scabies is not due to poor hygiene. Nor is it due to animal mites, which do not infest humans. However, animal mites can be responsible for bites on exposed sites, usually the forearms.

Typically, an affected host is infested by about 10-12 adult mites. After mating, the male 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 requires 10 to 14 days.

Symptoms and signs

Itch
The itching appears a few days after infestation. It may occur within a few hours if the mite is caught a second time. The itch is characteristically more severe at night and affects the trunk and limbs. It does not usually affect the scalp.

Burrows
Scabies burrows appear as tiny, grey, irregular tracks between the fingers and on the wrists. They may also be found in armpits, buttocks, on the penis, insteps and backs of the heels. Microscopic examination of the contents of a burrow may reveal mites, eggs or mite faeces (scybala).

Generalised rash
Scabies rash appears as tiny red, intensely itchy, bumps on the limbs and trunk. It can easily be confused with dermatitis or hives (and may be accompanied by these). The rash of scabies is due to an allergy to the mites and their products and may take several weeks to develop after initial infestation.

Nodules
Itchy lumps or nodules in the armpits and groins or along the shaft of the penis are very suggestive of scabies. Nodules may persist for several weeks or longer after successful eradication of living mites.

Acropustulosis
Blisters and pustules on the palms and soles are characteristic of scabies in infants.

Secondary infection
Impetigo commonly complicates scabies and results in crusting patches and scratched pustules. Cellulitis may also occur, resulting in localised painful swelling and redness, associated with fever.

Scabies only rarely affects the face and scalp. This may be the case in young babies and bed-bound elderly patients.

Scabies - Images copyright DermNet (NZ)

Burrows (arrows point to mites) Affected webspace
Pustules on hand Baby with scabies rash

Crusted scabies

Crusted scabies (also called 'Norwegian scabies') is a very contagious variant of scabies in which there are thousands or even millions of mites, but very little itch. The patient presents with a generalised scaly rash. It is frequently misdiagnosed as psoriasis. Unlike the usual form of scabies, crusted scabies may affect the scalp.

Crusted scabies is most likely to affect individuals with a poor immune system, neurological diseases, the elderly or those with mental incompetence. It is the usual cause of severe outbreaks of scabies in an institution such as a hospital, rest home or prison.

People in contact with someone with crusted scabies may become very itchy, with tiny red spots and blisters on exposed areas; these people are not always infested themselves but must be treated with insecticides just in case they are carrying the mite.

Crusted scabies - Images copyright DermNet (NZ)

Crusted plaque Mite and eggs under microscope

Diagnosis

Scabies can affect children, young adults and the elderly in every community. Think of it if you or your child has developed a widespread itchy rash, especially if there's been close contact with another itchy person. However, not everyone who itches has scabies; dermatitis is not contagious and is much more common in New Zealand.

If you or your doctor consider scabies a possible explanation for an itchy rash, the diagnosis can be confirmed by microscopic examination of the contents of a burrow. However, even experienced dermatologists only recover a mite or an egg in about 50% of cases of scabies. Occasionally, a skin biopsy is necessary and typical histological features may be seen on pathology.

A new technique using dermoscopy makes it easier to identify a mite at the end of a burrow by its characteristic “jet-plane” appearance.

Treatment

Scabicides are chemical insecticides used to treat scabies. Those available in New Zealand include:

  • 25% benzyl benzoate lotion, applied daily for 3 days
  • 5% permethrin cream, left on for 8-10 hours
  • 0.5% aqueous malathion lotion, left on for 24 hours.

Gamma benzene hexachloride cream is no longer recommended because of resistance and potential toxicity. Sulphur and crotamiton were popular in the past but are relatively weak scabicides.

The scabicide has to be applied before bed to the whole body from the chin to soles. The scalp and face also need to be treated in children under 2 years, those confined to bed, and some others with reduced resistance.

A repeat treatment a week later is often recommended. It should not be repeated for several weeks after that without medical advice. Overuse of insecticides will irritate the skin.

Linen and household cleaning after treatment

Each treatment with scabicide should be followed the next morning by hot-wash laundering or dry cleaning of sheets and pillow cases and any clothes worn against the skin over the last week. Non washable items should be sealed in a plastic bag and stored above 20°C for one week. Alternatively they can be frozen below -20°C for 12 hours.

Rooms should be thoroughly cleaned with normal household products. Fumigation or specialised cleaning is not required. Carpeted floors and upholstered furniture should be vacuumed and all areas cleaned with normal household products. The vacuum bag should then be discarded and furniture covered by plastic or a sheet during treatment and for 7 days after.

Itch improvement

Most people's itch improves within a few days of treatment but it may take 4-6 weeks for the itch and rash of scabies to clear completely because of dead mites at the skin surface. These will be slowly cast off.

To reduce the risk of the treatment failing:

  • ensure the scabicide is applied to the whole body from the chin down
  • leave it on for the recommended time and reapply it after washing
  • apply the scabicide under fingernails using a soft brush
  • obtain antibiotics from your doctor if there is crusting and secondary infection
  • ensure all close contacts are treated, whether or not they are itchy.

Persistent rash

Occasionally a rash persists even though every mite has been killed. Reasons for this include:

  • Scabies nodules may take several months to settle down. They are not infectious. A topical steroid may help; apply it accurately to each bump.
  • The scabies can result in dermatitis. Dermatitis can be due to the mite, the scratching, the treatment or other factors. Persistently itchy patches should be treated with frequent applications of emollients and mild topical steroids.
  • The diagnosis may be incorrect. Scabies can be confused with a number of other skin conditions, particularly dermatitis and papular urticaria. If you have an itchy rash, your doctor may treat you for scabies 'just in case', even when it is more likely you have another skin disorder. This is because it is important to treat scabies vigorously to prevent other people catching it.
  • Resistance to treatment. Scabies occasionally appears to be resistant to the prescribed scabicide. Obtain advice from your doctor; a different scabicide or other treatment may be prescribed. You may be referred to a dermatologist.

Oral ivermectin has proved very effective and is now considered treatment of choice for crusted scabies and other resistant cases.

Original material provided by the New Zealand Dermatological Society Inc. (DermNet NZ), June 2012 and reviewed by everybody, September 2012.

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