Scabies Initial Assessment

Classical Scabies

The primary symptom of scabies is generalized pruritus, which is usually worse at night. Pruritus is caused by a delayed (type IV) hypersensitivity reaction to the mite and its products (saliva, eggs, and feces) once the host becomes sensitized. Hypersensitivity occurs 3-6 weeks after infestation but occurs within 1-3 days with re-infestation due to prior sensitization. Primary lesions may or may not be seen because they may be destroyed by scratching or secondary bacterial infection.

Primary Lesions

Burrows are linear, curved, or S-shaped silvery lined burrows with a mean length of 0.5 cm. The infestation is usually found on the finger webs, sides of the hands and feet, wrists, buttocks, axilla, nipples, penis, or scrotum. In young children and infants, this is usually found on the palms and soles. The application of blue or black fountain ink may enhance the sighting of burrows, as the ink is absorbed by the burrow 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. These may be from a hypersensitivity reaction and rarely contain mites. These are most commonly found on the finger webs. In young children and infants, the vesicles or pustules may be found on the palms and soles.

Nodules

Nodules are 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 axillae, groin, buttocks, and male genitalia). Mites are typically absent from these lesions. Nodules probably represents an exaggerated inflammatory response to the mite or its products. These may be a hypersensitivity reaction to a prior or currently active scabies infestation. Nodules may persist long after successful treatment and require intralesional corticosteroid injection.

Secondary Lesions

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

The lesions typically occur from the neck down such as the interdigital web spaces of the hands, the flexor surface of the forearms (wrist and elbow), axillary folds, the umbilicus and belt line, buttocks, ankles, groin, penis, scrotum, areola, and nipples. The scalp and face are usually not infected in adults, and in very young children, the head is usually spared. The back area is rarely affected in either adults or children.

Infants

Lesions are usually distributed on the palms and soles, occasionally on the face and scalp.

Crusted Scabies

Crusted scabies typically occur in immunocompromised patients, patients on long-term topical corticosteroids, patients with severe systemic illness or neurologic/mental disorders, and 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 a yellow-to-brown, thick, verrucous appearance on bony prominences (eg iliac crest, elbows, and 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. 

The diagnosis of scabies is based on the clinical presentation and confirmed by microscopic identification of mites, eggs, or mite feces.

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

Level A: Confirmed scabies This diagnosis is made based on the identification components of mites, mite egg, or scibala, which can be observed through definitive visualization of the following components, with at least one of the following factors:
A1	Mites, eggs or feces on light microscopy* of skin samples/scrapes
A2	Mites, eggs or feces visualised on individual using high-powered imaging device
A3	Mite visualised on individual using dermoscopy
Level B: Clinical scabies This diagnosis is made based on features of patient’s history and skin examination, with at least one of the following factors:
B1	Scabies burrows (cuniculi)
B2	Typical lesions affecting male genitalia
B3	Typical lesions in a typical distribution and 2 history features
Level C: Suspected scabies This diagnosis is made based on features of patient’s history and skin examination, with at least one of the following factors:
C1	Typical lesions in a typical distribution and 1 history feature
C2	Typical lesions in a typical distribution and 2 history feature
History features:
H1	Itch/pruritus
H2	Close contact with an individual who has itch or typical lesions in a typical distribution

*If the result of microscopic examination is negative, the clinical diagnosis (level B) can still be established and 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.