Scabies Diagnosis
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
- Vesicles are usually seen at the start of the burrow
- 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)