Scabies is a contagious skin disease caused by the itch mite Sarcoptes scabiei.
Fertilised female mites burrow into the horny layer of the skin, advance some 2 mm each day, deposit eggs (2-3 per day for 10 days) and then die.
The transfer of the female mite requires close personal contact rather than transient personal contact.
Transmission can occur while:
- sleeping with, or
- having sexual intercourse with an infected person;
- between members of families and households
- and in body contact.
Primary infection: symptoms will appear around 4 weeks after infestation.
In people who have previously had scabies, the symptoms will appear more rapidly, often within hours of infestation. (Due to increased sensitivity.)
The primary lesion, the burrow, is often difficult to see. It is a thread-like furrow or ridge 5-15 mm long. Burrows mostly occur on the medial and flexor (inside) surfaces of the wrists, the sides and webs of the fingers and sometimes around the nipples, penis, scrotum and buttocks. A vesicle (small blister) may appear at the end of a burrow.
In sexually acquired scabies the lesions are often confined to the lower trunk, thighs and genitals.
After about a month the lesions become papular (raised) and irritable, presumably through host sensitisation. Intense itching occurs, especially when the person becomes warm in bed, after exercise, a hot shower or bath.
Usually by the time people seek attention, the eruption has been scratched, burrows disrupted and secondary infection has occurred.
In males, penile or scrotal lumps can be the main or only complaint. The lumps, which may or may not itch, are seen as inflamed, reddish-brown, indurated nodules up to 12mm across. They are commonly seen on the penis, especially the glans, and on the scrotum. Careful searching may reveal scabetic lesions in some of the usual sites.
In people who shower frequently, the clinical signs of scabies may be minimal and not typical, and burrows especially difficult to find.
The prior use of topical applications containing steroids frequently masks the clinical features and hinders accurate diagnosis.
The distribution of the rash, the presence of burrows, the multiplicity of lesions, the intense irritation (especially when in bed or while warm) and the possible infestation of those in close personal contact make for a clinical diagnosis of scabies.
The diagnosis is confirmed by finding a whole acarus, an identifiable part or some eggs. Preferably using good natural light and a binocular loupe, a search should be made among the lesions for burrows. If the overlying epidermis is thin, the mite can sometimes be seen at the anterior end of a burrow. The burrow should be opened with a triangular, cutting-edge needle on the point of which the mite can be transferred to a glass slide for low power microscopic examination.
If no burrows can be found and a mite cannot be isolated, microscopy of skin scrapings taken over old burrows or papules, under a drop of potassium hydroxide 10% solution, will sometimes show parts of an acarus or some eggs.