Scabies Diagnosis

International Alliance for the Control of Scabies (IACS) Criteria for the Diagnosis of Scabies

*If the result of microscopic examination is negative, the clinical diagnosis (level B) can still be established & the patients should be treated as scabies
Reference: Engelman D, Yoshizumi J, Hay RJ, et al. The 2020 International Alliance for the Control of Scabies consensus criteria for the diagnosis of scabies. Br J Dermatol. 2020 Nov;183(5):808-820. doi: 10.1111/bjd.18943. PMID: 32034956.

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, armpit, nipples, 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

Clinical Grading Scale for Crusted Scabies
Distribution and Extent of Crusting

• 1 - Wrist, web spaces, feet only (<10% total body surface area [TBSA])
• 2 - Above plus forearms, lower legs, buttocks, trunk or 10-30% TBSA
• 3 - Above plus scalp or >30% TBSA

Crusting/Shedding

• 1 - Mild crusting (<5 mm depth of crust), minimal skin shedding
• 2 - Moderate (5-10 mm crusting), moderate skin shedding
• 3 - Severe (>10 mm crusting), profuse skin shedding

Past Episodes

• 1 - No previous history
• 2 - 1-3 prior hospitalizations for crusted scabies or depigmentation of elbows/knees
• 3 - ≥4 prior hospitalizations for crusted scabies or depigmentation as above plus legs/back or residual skin thickening/ichthyosis

Skin Condition

• 1 - No cracking or pyoderma
• 2 - Multiple pustules and/or weeping sore and/or superficial cracking
• 3 - Deep skin cracking with bleeding, widespread purulent exudates

Grading

• Grade 1 (mild): Total score of 4-6
• Grade 2 (moderate): Total score of 7-9
• Grade 3 (severe): Total score of 10-12

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 or “mini triangle sign”)
• Videodermatoscopy and reflectance confocal microscopy provide more detailed inspection of mites

Polymerase Chain Reaction (PCR)

• Targets the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene of S scabiei and used to diagnose scabies infestation