Scabies

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Introduction

An intensely pruritic infestation caused by the human mite Sarcoptes scabiei. It is contracted by close contact and rarely via fomites. Occurs commonly in children and inmates of overcrowded institutions such as prisons and boarding houses. Infection of households is common. Sexual intercourse is also another possible method of spread amongst adults. Sharing a bed or using the same underwear will also suffice to contract the disease. 

Clinical Features

  • Severe pruritis worse at night is characteristic
  • The typical lesion is the burrow - it is hardly seen because of the marked excoriation and secondary infection on the skin
  • Papulo-pustular eruptions with excoriation and impetignization
  • Characteristic sites of predilection:
    • Interdigital spaces of the fingers
    • Flexural surfaces of the wrist
    • Extensor surfaces of the elbows and knees
    • Anterior axillary area
    • Nipples
    • The phallus (especially in adults) 
  • General immune status and experience with S. scabiei play a role 
    • In a normal host, the initial infection is asymptomatic for about 3-6 weeks during which time the individual is capable of transmitting the disease
  • All family or living unit members must therefore be treated, not just those who are itching
  • After re-infestation, symptoms appear within 24 hours

Crusted scabies (Norwegian scabies)

  • An uncommon variant of scabies
  • Patient fails to mount a resistance and the mites proliferate dramatically
  • May be found among HIV/AIDS patients, institutionalized inmates like prisoners, refugees and psychiatric patients 

Differential Diagnoses

  • Infantile acropustulosis
  • Atopic dermatitis 
  • Papular acral dermatitis of childhood
  • Dermatitis herpetiformis

Complications

Secondary bacterial infection leading to acute glomerulonephritis

Investigations

  • Burrow scraping on to a glass slide for microscopy
  • Video dermatoscopy

Treatment goals

  • Treat the infestation
  • Treat secondary bacterial infection
  • Relieve pruritis

Drug Treatment

Scabicides:

  • Tetmosol soap to bathe for 2-4 weeks
  • Permethrin 5% cream
    • Adult: apply over the whole body and wash off after 8-12 hours
    • Child: supervision required with application and rinsing

Or

Benzyl benzoate 25% in emulsion

  • Adult: apply over the whole body; repeat without bathing next day and wash off 24 hours later. If necessary, apply a third time. 
  • Child: Benzyl benzoate is an irritant and should be avoided in children

Or

Precipitated sulphur 5-10% in petroleum jelly 

  • Adult and child: apply over all the body daily for 7-10 days

Anti-helminthic:

Ivermectin

  • Adult: Single 200 microgram/kg oral dose for crusted scabies
  • Child: over 5 years: 200microgram/kg daily for 2 days

Antihistamine:

Chlorpheniramine

  • Adult 4mg orally every 4-6 hours; maximum 24mg a day 
  • Child
    • 1 month - 2 years 1mg orally every 12 hours; 
    • 2-5 years: 1mg every 4-6 hours
    • 6-12 years: 2mg every 4-6 hours

Topical antipruritic: 

Crotamiton cream (for residual itching) 

  • Adult: apply every 8-12 hours
  • Child: less than 3 years: apply one daily only