Scabies is the hypersensitivity reaction to parasitic infestation with the mite Sarcoptes scabiei. Human scabies is Sarcoptes scabiei var. hominis.
The mite itself is less than 0.5mm long and so effectively invisible. There are various subspecies of Sarcoptes scabiei more effective in colonising other mammals, and at the most extreme capable of causing sarcoptic mange. The typical itching along the mite's burrow only begins 3 to 4 weeks after initial infestation, usually by human to human contact, occasionally by animal to human contact and almost as rarely by fomites. It is actually quite hard to transmit via bedding and clothing, with a 1% infection rate if you climb in nude to a bed just vacated by someone with less than a 20 mite infestation. As symptoms are related to hypersensitivity, they may not develop for several weeks after initial infection, and are not proportional to the number of mites - potentially only 1 to 5 mites are involved, although in the immunosuppressed presenting with Crusted scabies there will be thousands.
Should be by clinical history and examination. Skin scrapings will rarely be diagnostic except in Crusted scabies.
Often chronic signs and symptoms, with a history of failed attempts at symptom control with topical steroids etc.
- Itching worse at night
- The burrows take a few days to become visible
- Papules, evolving to vesicles and bullae
Can mimic other skin conditions and secondary infection possible. Dull red/brown nodules can develop and last for months (elbows, anterior axillary folds, genitals). Protective immunity can occur. Norwegian or Crusted scabies is seen particularly but not exclusively in the immunosuppressed, is a psoriaform condition with a heavy burden of mites, hence very infectious, but may not be particularly itchy.
- Pyoderma, staphylococcal or Streptococcal secondary infection
- Acute post-streptococcal glomerulonephritis
- Generalised lymphadenopathy esp in Crusted scabies
- Appropriate scabocide applied to whole body (scalp, neck, face, ears, genitals, periungal area) esp young children. Ideally repeat after 7 days.
- Second line
- In crusted scabies
- wash bedding/clothes at 60°C
- seal in bag for several days
- Treat contacts, ideally all household members at the same time!
Symptoms may take several weeks to settle after successful treatment.
- Clinical Knowledge Summaries Scabies guidance from National Library for Health (formerly Prodigy)
- The treatment of scabies from Australian prescriber]
- US guidelines from CDC