Scabies

From Ganfyd

Scabies is the hypersensitivity reaction to parasitic infestation with the mite Sarcoptes scabiei. Human scabies is Sarcoptes scabiei var. hominis.

Scabies was the first disease to be demonstrated to be caused by a microscopic organism. This relationship between the skin lesions and the mite were described by Giovan Cosimo Bonomo and Diacinto Cestoni in 1687.[1]

Aetiology

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.

Diagnosis

Should be by clinical history and examination. Skin scrapings will rarely be diagnostic except in Crusted scabies.

Clinical

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
  • Most easily seen fingers, wrists, penis
• 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.

Associations

• Pyoderma, staphylococcal or Streptococcal secondary infection
  • Acute post-streptococcal glomerulonephritis
• Generalised lymphadenopathy esp in Crusted scabies

Treatment

1. Appropriate scabocide applied to whole body (scalp, neck, face, ears, genitals, periungal area) esp young children. Ideally repeat after 7 days.
   • Permethrin 5% cream [Lyclear] - not 1% as used for head lice. Safe in infants although rarely CNS side effects.
   • Benzyl benzoate 10% cream or 25% lotion where cost an issue
2. Second line
   • Lindane 1%
   • Malathion 0.05% - safe in pregnancy.
   • Certain essential oils seem very effective - Tea tree (Melaleuca alternifolia) oil, bush tea (Lippia multiflora) oil, and paste extracts of neem (Azadirachta indica) or turmeric(Curcuma longa)
   • Ivermectin
3. In crusted scabies
   • wash bedding/clothes at 60°C
   • seal in bag for several days
4. Treat contacts, ideally all household members at the same time!

Symptoms may take several weeks to settle after successful treatment.

External links

• Clinical Knowledge Summaries Scabies guidance from National Library for Health (formerly Prodigy)
• The treatment of scabies from Australian prescriber]
• US guidelines from CDC

References