Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei)

Overview

Plain-Language Overview

Scabies is a contagious skin condition caused by tiny mites called Sarcoptes scabiei that burrow into the skin. It mainly affects the skin and causes intense itching, especially at night. The mites create small tunnels or burrows under the skin, which can lead to a rash and discomfort. This condition spreads easily through close physical contact with an infected person. The itching and rash can interfere with sleep and daily activities. It often appears between the fingers, wrists, and other warm areas of the body. Scabies requires medical diagnosis and treatment to eliminate the mites and relieve symptoms.

Clinical Definition

Scabies is a parasitic infestation of the skin caused by the mite Sarcoptes scabiei var. hominis. The female mite burrows into the stratum corneum of the epidermis, laying eggs and triggering a type IV hypersensitivity reaction. This leads to intense pruritus, especially nocturnal, and characteristic skin burrows. The infestation primarily affects areas with thin skin such as interdigital spaces, wrists, and genitalia. Secondary bacterial infections may complicate the clinical picture. Diagnosis is clinically significant due to the high transmissibility and potential for outbreaks in crowded settings. The hallmark features include intense itching, visible burrows, and a papular rash.

Inciting Event

Initial infestation occurs after prolonged skin-to-skin contact with an infested person.
Exposure to contaminated bedding or clothing can occasionally transmit mites.
Outbreaks often begin with a single index case in a crowded environment.
Reinfestation can occur after incomplete or delayed treatment.

Latency Period

Symptoms typically develop 2 to 6 weeks after initial mite exposure in a naïve host.
In previously sensitized individuals, symptoms may appear within 1 to 4 days.
Latency reflects the time needed for immune sensitization and hypersensitivity to develop.

Diagnostic Delay

Early symptoms are often mistaken for eczema or allergic dermatitis.
Lack of awareness of burrow lesions leads to missed clinical clues.
Pruritus may be attributed to other dermatologic conditions or dry skin.
Patients may delay seeking care due to mild initial symptoms or stigma.

Clinical Presentation

Signs & Symptoms

Intense pruritus, often worse at night, is the hallmark symptom of scabies.
Burrows appear as thin, serpiginous, grayish lines on the skin surface.
Papules and vesicles develop due to hypersensitivity to mite antigens.
Secondary excoriations and crusting result from persistent scratching.
Widespread rash may occur in immunocompromised patients or in crusted scabies.

History of Present Illness

Patients report intense nocturnal pruritus worsening over weeks.
Pruritic papules and linear burrows appear on interdigital spaces, wrists, and genitalia.
Symptoms often spread to family members or close contacts.
Secondary excoriations and crusting develop from persistent scratching.
In severe cases, widespread crusted plaques may be present.

Past Medical History

Previous episodes of scabies infestation increase risk of reinfestation.
History of immunosuppressive conditions or medications may worsen presentation.
Chronic skin diseases like eczema can complicate diagnosis.
Prior treatment with topical steroids may mask symptoms.

Family History

Close household contacts often have concurrent scabies infestation.
No known genetic predisposition or familial syndromes are associated with scabies.
Family outbreaks are common due to direct transmission among members.

Physical Exam Findings

Presence of linear burrows especially in the interdigital spaces, wrists, and axillae is characteristic of scabies.
Erythematous papules and vesicles commonly appear on the trunk, hands, and feet.
Excoriations due to intense scratching are frequently observed.
Nodules may be present in the genital area or axillae in chronic cases.
Secondary bacterial infection signs such as impetigo may be seen due to skin breakdown.

Diagnostic Workup

Diagnostic Criteria

Diagnosis is established by identifying the mite, eggs, or fecal pellets via skin scraping and microscopic examination. The presence of characteristic burrows on physical exam supports the diagnosis. Intense pruritus, especially worsening at night, and a typical distribution of lesions are key clinical clues. Dermoscopy can aid in visualizing the mite as a 'delta wing' sign. Definitive diagnosis requires direct visualization of the parasite or its products.

Pathophysiology

Key Mechanisms

Infestation by the mite Sarcoptes scabiei leads to intense cutaneous hypersensitivity reaction.
Mite burrowing into the stratum corneum causes direct skin damage and triggers immune-mediated pruritus.
Type IV hypersensitivity reaction to mite antigens and feces results in inflammatory skin lesions.
Scratching causes excoriations and may lead to secondary bacterial infection.
Transmission of mites via close skin-to-skin contact facilitates spread and reinfestation.

Involvement	Details
Organs	Skin is the main organ affected by Sarcoptes scabiei infestation, manifesting with burrows, papules, and intense pruritus.
Tissues	Epidermis is the primary site of mite burrowing and skin lesion formation in scabies.
Tissues	Dermis contains inflammatory infiltrates including eosinophils and lymphocytes that contribute to pruritic papules and nodules.
Cells	Langerhans cells act as antigen-presenting cells in the skin, initiating the immune response to Sarcoptes scabiei infestation.
	Eosinophils contribute to the inflammatory response and tissue damage in scabies through release of cytotoxic granules.
	T lymphocytes mediate the delayed-type hypersensitivity reaction responsible for intense pruritus and skin inflammation.
Chemical Mediators	Histamine released from mast cells causes intense pruritus and contributes to the allergic reaction in scabies.
	Interleukin-4 (IL-4) promotes a Th2 immune response that drives eosinophil recruitment and IgE production in scabies.
	Tumor necrosis factor-alpha (TNF-α) amplifies local inflammation and tissue damage in the skin.

Treatments

Pharmacological Treatments

Permethrin 5% cream
- Mechanism:
  - Disrupts sodium channel function in the nerve cells of Sarcoptes scabiei, causing paralysis and death of the mite.
- Side effects:
  - Local skin irritation
  - Pruritus exacerbation
  - Erythema
- Clinical role:
  - First-line
Ivermectin
- Mechanism:
  - Binds to glutamate-gated chloride channels in the nerve and muscle cells of Sarcoptes scabiei, causing paralysis and death.
- Side effects:
  - Mild dizziness
  - Gastrointestinal upset
  - Transient rash
- Clinical role:
  - Second-line
Benzyl benzoate
- Mechanism:
  - Acts as a neurotoxin to Sarcoptes scabiei mites, leading to their death.
- Side effects:
  - Skin irritation
  - Burning sensation
  - Contact dermatitis
- Clinical role:
  - Alternative first-line

Non-pharmacological Treatments

Wash all clothing, bedding, and towels in hot water and dry on high heat to kill mites and eggs.
Avoid close skin-to-skin contact with infected individuals until treatment is complete.
Vacuum furniture and carpets thoroughly to remove mites from the environment.

Prevention

Pharmacological Prevention

Permethrin 5% cream applied to all household contacts prevents spread.
Oral ivermectin is used for mass prophylaxis in institutional outbreaks.
Topical lindane may be used but is less preferred due to neurotoxicity risk.

Non-pharmacological Prevention

Washing clothing and bedding in hot water to kill mites prevents reinfestation.
Avoiding close skin-to-skin contact with infected individuals reduces transmission.
Environmental decontamination including vacuuming furniture and carpets helps control spread.
Screening and treating close contacts promptly prevents outbreaks.

Outcome & Complications

Complications

Post-scabetic eczema due to persistent hypersensitivity reaction after mite eradication.
Secondary bacterial infections leading to cellulitis, abscess, or systemic infection.
Crusted scabies can cause widespread skin involvement and increased transmission risk.
Glomerulonephritis may rarely develop secondary to streptococcal skin infection.

Short-term Sequelae	Long-term Sequelae
Persistent pruritus for weeks after treatment due to ongoing hypersensitivity. Secondary bacterial superinfection causing local inflammation and pain. Sleep disturbance from nocturnal itching.	Chronic dermatitis or lichenification from repeated scratching and inflammation. Post-inflammatory hyperpigmentation or scarring in affected areas. Rarely, renal complications from streptococcal glomerulonephritis.

Differential Diagnoses

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei) versus Atopic Dermatitis

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei)	Atopic Dermatitis
Intense pruritus with burrows and papules primarily in interdigital webs and wrists	Chronic relapsing course with dry, scaly, erythematous plaques often on flexural surfaces
Can affect all ages but often presents in older children and adults	Typically begins in infancy or early childhood
Requires scabicidal agents like permethrin or ivermectin	Improves with emollients and topical corticosteroids

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei) versus Contact Dermatitis

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei)	Contact Dermatitis
Exposure to infested individuals or fomites with widespread pruritic burrows	Recent exposure to irritants or allergens localized to contact area
Presence of burrows containing mites in stratum corneum	Eczematous inflammation with spongiosis without burrows
Requires eradication of mites with topical or oral scabicides	Improves with avoidance of allergen and topical steroids

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei) versus Pediculosis (Lice Infestation)

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei)	Pediculosis (Lice Infestation)
Infestation by Sarcoptes scabiei mites burrowing in skin	Infestation by lice species visible on hair shafts or scalp
Pruritus with burrows on interdigital webs, wrists, and genital areas	Pruritus localized to scalp, neck, or pubic area with visible nits
Microscopic visualization of mites or eggs from skin scrapings	Identification of lice or nits on hair shafts

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei) versus Bullous Pemphigoid

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei)	Bullous Pemphigoid
Can affect all ages but often younger than typical bullous pemphigoid patients	Typically affects elderly patients
Intense pruritus with burrows and papules, no bullae	Tense bullae on erythematous or normal skin with less pruritus
Identification of mites or eggs on skin scraping microscopy	Linear IgG and C3 deposition along basement membrane on immunofluorescence

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei) versus Tinea Corporis

Scabies (Intense Pruritus, Skin Burrows - Sarcoptes scabiei)	Tinea Corporis
Burrows and papules caused by mite infestation	Annular, scaly plaques with central clearing caused by dermatophyte fungi
Microscopic identification of mites or eggs in skin scrapings	Positive KOH prep showing septate hyphae
Requires topical or oral scabicides	Responds to topical antifungals