scabies
SCABIES
Scabies is a contagious disease caused by the mite Sarcoptes scabiei var hominis.
The affected individual usually complains of having a highly pruritic rash that occurs at night.
It occurs more often in children <15 years of age, sexually active young adults, the immunocompromised and in persons living in crowded living conditions (eg nursing homes, military barracks).
Transmission is typically by direct skin contact with an infected person and in adults, sexual transmission is common.

Classification

Classical Scabies

  • Occurs in immunocompetent patients
  • Mite burden is on average 5-15 mites/host during an initial episode

Primary Lesions

  • Burrows
    • Patient may have linear, curved or S-shaped silvery lined burrows with a mean length of 0.5 cm
    • Burrows and other primary lesions may not be seen because they may have been destroyed by scratching or secondary bacterial infection
    • Usually found on the finger webs, sides of the hands and feet, wrists, buttocks, penis or scrotum
    • In the infant: Usually found on the palms and soles
    • May enhance the sighting of burrows with blue or black fountain ink because the burrow absorbs the ink and is highlighted as a dark line
  • Vesicles and papules
    • Vesicles are usually seen at the start of the burrow 
      • Isolated pinpoint vesicles which are filled with serous fluid may be present
    • Papules are small, often excoriated with hemorrhagic crusts on top
      • May be from a hypersensitivity reaction and rarely contain mites
    • Most commonly found on the finger webs
    • In the infant: Vesicles or pustules may be found on the palms and soles
  • Nodules
    • Persistent dome-shaped papules measuring 5-6 mm in diameter which are firm, erythematous and extremely pruritic
    • Most often lesions are found on covered areas (eg axillary, groin and male genitalia)
    • Mites are typically absent from these lesions
    • Probably represent an exaggerated inflammatory response to the mite or its products
    • May be a hypersensitivity reaction to prior or currently active scabies infestation
    • May persist long after successful treatment and may require intralesional corticosteroid injection

Secondary Lesions

  • Often predominate the clinical picture as they are a result of infection or scratching
  • Pinpoint erosions are the most common secondary lesions
  • Pustules are a sign of secondary infection
  • Scaling, erythema and eczematous inflammation can occur due to excoriation or irritation

Distribution of Lesions

  • Adults and older children
    • Typically occur from the neck down: The interdigital web spaces of the hands, the flexor surface of the forearms (wrist and elbow), axillary folds, umbilicus and belt line, buttocks, ankles, groin, penis, scrotum, areola and nipples
    • Scalp and face are usually not infected in the adult
  • Infants
    • Lesions are usually distributed on the palms and soles, and occasionally on the face and scalp

Crusted (Norwegian Scabies)

  • Also known as Boeck scabies, scabies crustosa or keratotic scabies
  • Highly contagious form of scabies with the affected person being infested with hundreds to millions of mites

Clinical Presentation

  • Typically occurs in immunocompromised patients and patients with severe systemic illness, neurologic/mental disorders or in residents or in residents of long-term care facilities
  • Itching may be absent or mild
  • Lesions on the skin consist of generalized, poorly defined, erythematous, fissured plaques covered by scales and crusts
    • Plaques have yellow-to-brown, thick, verrucous appearance on bony prominences (eg iliac crest, elbows, finger joints)
  • Nails may be dystrophic, discolored and often thickened
  • Patients may present with hyperkeratotic crusted plaques which may be generalized or localized to the hands or feet
  • Occasionally patients may have psoriasiform or eczematous lesions with fine, powder-like, red, scaling eruptions on the face, neck, scalp and trunk
  • In some cases, generalized lymphadenopathy and eosinophilia are present

Laboratory Tests

Diagnostic tests

Microscopic Identification

  • Diagnosis is confirmed by microscopic identification of the mites, eggs or mite feces
  • Specimen should be taken by scrapping the burrows or the newest lesions without excoriations and eczematization
    • A drop of mineral oil should be placed on the lesion and then scraped with a sterile #15 surgical blade and may be viewed directly under a light microscope
    • 10% potassium hydroxide (KOH) solution may be used to decrease the amount of keratinic debris which is helpful when diagnosing crusted scabies
    • Dermoscopy may be used as a guide for procurement of skin samples
  • When skin specimens are negative but a strong clinical suspicion remains, a successful trial of scabicide therapy may confirm the diagnosis 

Dermoscopy

  • May be used to identify skin burrows, mites, eggs
  • Characteristic finding is a dark, triangular shape which represents the head of the adult female mite within a burrow ("delta wing" sign)
Editor's Recommendations
Most Read Articles
20 Sep 2019
Both adalimumab and phototherapy increase overall and skin-related quality of life (QoL) in psoriasis patients, although phototherapy appears to be better, a recent study has shown.
Roshini Claire Anthony, 24 Sep 2019

Once-daily oral omadacycline demonstrated noninferiority to twice-daily linezolid in treating acute bacterial skin and skin structure infections (ABSSSI), the phase III OASIS-2* study showed.