Scabies

Scabies infestation is caused by the mite Sarcoptes scabiei. Transmission is by skin-to-skin contact. Mites burrow into the skin where they lay eggs. The offspring crawl out onto the skin, and make new burrows.

The incubation period is 2-6 weeks before the onset of itching in people without previous exposure, 1-4 days after re- exposure. The period of communicability is until the mites and eggs are destroyed by treatment. Contact infectivity is high.

Any part of the body can be affected.

Symptoms

• The main symptom, which may take 4 to 6 weeks to develop, is a generalised itch, usually worse at night or when the body is warm (e.g. after a shower).
• The itching is due to a hypersensitivity reaction to the absorption of mite excrement into skin.

Signs

• An itching rash on the body and limbs. Classic sites of infection are flexures, which are warmer – interdigital folds, the wrists, elbows, knees, buttocks, genital region, and under the breasts.
• Characteristic silvery lines may be seen where the mite has burrowed, with the mite sometimes visible at the end of the burrow. However, scratching often obliterates the burrow.
• In the genital region, particularly on the glans penis, the rash becomes papular or nodular.
• In patients with HIV infection or others with suppressed immune function, or in the elderly, the rash is severe and crusted. These lesions team with mites, and are a significant risk to others.

STI Atlas (external site)

• Scrapings, taken from the burrows with a fine needle to reveal the mite, may be examined under light microscopy.
• Usually the rash is characteristic but can be confused with dermatitis or eczema.

• Patients should be advised to avoid contact with their partners or other skin-to-skin contact until they have completed treatment, and their partner and any affected household contacts have completed treatment.
• Patients should be given topical antipruritic creams or tablets. They should be advised that, despite successful treatment, they will continue to itch for a further four weeks due to the debris from the scabies mite in the skin. This advice prevents patients over-treating themselves and, as a result, causing eczema.
• At night, adults should:
  • wash the entire body with soap and water, and then dry
  • apply one of the treatments below, from the neck down.
• The cream should be rubbed in well and left on for 24 hours, then washed off. The patient may require a second dose of treatment a week later.
• Usually, advice is also given to wash all currently used underwear, nightclothes, bed linen and bath towels in hot water, and dry them well.

Standard

• Permethrin 5 per cent cream. Leave on for 24 hours. Repeat in 7 days if necessary.

        or

• Benzyl benzoate 25 per cent lotion. Leave on for 24 hours. Repeat in 7 days if necessary.

Most patients will continue to itch for several weeks, so symptomatic treatment for the itch can be given in the meantime:

• crotamiton 1 per cent lotion or cream (Eurax)

        or

• 1 per cent hydrocortisone in calamine cream twice daily.

Crusted or resistant scabies can be treated with ivermectin. For a person >15kg, oral 200 microgram/kg/dose for 2 doses 7 days apart or 3 doses on days 1,2, and 8. More severe cases will require extra doses on days 9 and 15 (and on days 22 and 29 if very severe). 

Treatment of scabies in HIV-positive patients should be referred to a specialist.

Pregnancy

Permethrin (category B2) is safe during pregnancy.

Australian categorisation system for prescribing medicines in pregnancy (external site).

Related links

• Scabies (Healthy WA)

No follow-up is usually required. If new burrows appear after treatment, then the treatment should be repeated.

Always test for other STIs when sexual transmission is suspected.