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.
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
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.
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).
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.
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 mite.
Blisters and pustules on the palms and soles are characteristic of scabies in infants.
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 bedbound elderly patients.
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