Overview
Definition:
Scabies is a contagious infestation of the skin by the human itch mite, Sarcoptes scabiei var
hominis
it causes intense pruritus and a characteristic rash, often worse at night.
Epidemiology:
Scabies is a global health problem affecting all socioeconomic groups, with an estimated 300 million cases worldwide annually
children and elderly are particularly vulnerable
Transmission occurs through prolonged direct skin-to-skin contact.
Clinical Significance:
Scabies is a common pediatric dermatological condition that causes significant morbidity due to intense itching, secondary bacterial infections, and psychological distress
prompt and effective treatment is crucial for the child and household contacts.
Clinical Presentation
Symptoms:
Intense pruritus, often nocturnal
Burrows, papules, vesicles, and excoriations, commonly found in characteristic locations
Secondary impetiginization from scratching.
Signs:
Linear or serpiginous burrows, particularly in finger webs, wrists, elbows, axillae, areolae, belt line, and genitalia
Papules, vesicles, and urticarial plaques
Crusted scabies (a severe form) presents with thick, hyperkeratotic plaques.
Diagnostic Criteria:
Diagnosis is primarily clinical, based on characteristic lesions and distribution, and intense pruritus
Definitive diagnosis requires visualization of mites, eggs, or scybala (feces) under microscopy from skin scrapings
The Menzies Scoring System can aid diagnosis.
Diagnostic Approach
History Taking:
Detailed history of pruritus onset and progression, especially nocturnal worsening
History of similar symptoms in household contacts or sexual partners
Previous treatments and their efficacy
Presence of comorbidities like immunocompromise or eczema.
Physical Examination:
Systematic examination of the entire skin surface, paying close attention to characteristic sites: finger webs, interdigital spaces, wrists, elbows, axillae, areolae, waistline, and genitalia
Look for burrows, papules, vesicles, and signs of excoriation and secondary infection.
Investigations:
Microscopic examination of skin scrapings: potassium hydroxide (KOH) mount to visualize mites, eggs, or scybala
Dermoscopy can also aid in identifying the mite or burrows
Biopsy is rarely indicated but may show characteristic inflammatory infiltrates and mite parts.
Differential Diagnosis:
Atopic dermatitis
Allergic contact dermatitis
Urticaria
Folliculitis
Insect bites
Psoriasis
Lichen planus
Differential diagnosis is crucial as symptoms can overlap, and misdiagnosis can lead to delayed or ineffective treatment.
Management
Initial Management:
Immediate treatment of the patient and all close contacts (household members, sexual partners) simultaneously is crucial to prevent reinfestation
Environmental decontamination: washing all clothing, bedding, and towels in hot water and drying on a hot cycle.
Medical Management:
Topical permethrin 5% cream: applied from neck down, left on for 8-14 hours, then washed off
Repeat in 7 days
Oral ivermectin: typically 200 mcg/kg single dose, repeated in 7-14 days
For crusted scabies, oral ivermectin may be given more frequently (e.g., every 3-7 days) along with topical treatment.
Pediatric Dosing Considerations:
Permethrin 5% cream is generally safe and effective in infants and children over 2 months
For infants under 2 months, malathion 0.5% lotion or benzyl benzoate 10% lotion may be considered, though less preferred
Oral ivermectin is typically used for children >15 kg
For younger children, ivermectin is generally contraindicated or used off-label with extreme caution and expert consultation.
Supportive Care:
Management of pruritus: oral antihistamines and topical corticosteroids may be used to alleviate itching, especially if inflammation is present
Treatment of secondary bacterial infections: antibiotics for impetiginization
Patient education on transmission and treatment adherence is vital.
Comparative Efficacy
Permethrin:
Highly effective with excellent safety profile
Considered first-line treatment by many guidelines
Requires careful application and adherence to repeat dosing
May have lower efficacy against crusted scabies.
Ivermectin:
Effective, particularly for widespread infestations, institutional outbreaks, and crusted scabies
Convenient oral administration
Potential for drug interactions and side effects (e.g., dizziness, nausea)
Generally not recommended for infants under 15 kg or pregnant/lactating women unless benefits outweigh risks.
Resistance:
Scabies mite resistance to pyrethroids (like permethrin) has been reported, though it remains rare
Resistance to ivermectin is also a concern, necessitating careful monitoring of treatment response.
Complications
Early Complications:
Secondary bacterial infections (impetigo, cellulitis, erysipelas) due to excoriations
Severe pruritus leading to insomnia and psychological distress.
Late Complications:
Post-scabetic nodules (persistent, pruritic nodules, especially in children)
Chronic scratching can lead to lichenification
In severe, untreated cases or in immunocompromised individuals, generalized exfoliative dermatitis and severe secondary infections leading to sepsis can occur.
Prevention Strategies:
Prompt treatment of index case and all contacts
Thorough decontamination of personal items and living environment
Isolation of affected individuals in institutional settings until treatment is completed.
Key Points
Exam Focus:
DNB/NEET SS exams will focus on typical presentation, characteristic locations of lesions, the importance of treating contacts simultaneously, and differentiating between permethrin and ivermectin indications and contraindications, especially in pediatric populations
Understanding dosing for specific age/weight groups is key.
Clinical Pearls:
Always ask about nocturnal pruritus and symptoms in contacts
Treat the entire household simultaneously
Educate patients and caregivers on proper application of topical agents and environmental cleaning
Pruritus may persist for several weeks post-treatment due to hypersensitivity reaction, even if mites are eradicated.
Common Mistakes:
Failure to treat all contacts, leading to reinfestation
Inadequate application of topical agents (e.g., not covering the entire body)
Not repeating treatment when indicated
Misdiagnosing scabies as another dermatosis
Prescribing ivermectin inappropriately in young children or pregnant women without careful consideration.