What are the differential diagnoses and management for a linear scratch‑like rash?

Linear Scratch-Like Rash: Differential Diagnosis and Management

The most critical first step is to determine whether this represents a parasitic migratory larval infection—particularly cutaneous larva migrans, larva currens from Strongyloides, or onchocerciasis—as these require specific antiparasitic therapy and have distinct migration patterns that guide diagnosis.

Key Diagnostic Features by Migration Speed

The speed and pattern of linear progression immediately narrows your differential:

Rapid Migration (Hours)

• Larva currens (Strongyloides stercoralis): Moves 5-10 cm per hour, typically around trunk, upper legs, and buttocks as an itchy, linear urticarial rash 1
• This is the fastest-moving parasitic rash and indicates active strongyloidiasis requiring urgent treatment 1

Moderate Migration (Days)

• Cutaneous larva migrans: Advances 1-2 cm per day as a characteristic serpiginous, itchy rash caused by dog/cat hookworm larvae 1
• Clinical diagnosis is sufficient without laboratory confirmation 1

Slow/Static Linear Lesions

• Neurotic excoriations: Self-inflicted linear erosions, scabs, and scars that are similar in size/shape, grouped on accessible body sites (extensor extremities, face, upper back) 2
• These represent a physical manifestation of psychological distress rather than infection 2

Travel and Exposure History: Critical Decision Points

Recent Travel to Endemic Areas

If the patient has traveled to tropical/subtropical regions within the past 20 months:

• Onchocerciasis presents with diffuse pruritic dermatitis over legs and buttocks, may have subcutaneous nodules, and has an 8-20 month incubation period 1
• Requires nocturnal blood microscopy (10 pm to 2 am) and serology 1
• Treatment: Doxycycline 200 mg daily for 6 weeks plus ivermectin 200 μg/kg monthly for 3 months 1

Soil/Beach Exposure

• Cutaneous larva migrans from walking barefoot on contaminated sand or soil 1
• Treatment: Ivermectin 200 μg/kg single dose OR albendazole 400 mg once daily for 3 days 1

No Travel History

Consider non-parasitic causes:

• Neurotic excoriations (psychological) 2
• Drug eruptions 1
• Contact dermatitis in linear distribution
• Phytophotodermatitis (plant exposure + sun)

Immediate Management Algorithm

Step 1: Assess Migration

• If actively migrating: Measure and mark the leading edge; reassess in 2-4 hours
  • Rapid movement (cm/hour) = larva currens → treat for strongyloidiasis 1
  • Slow movement (cm/day) = cutaneous larva migrans → treat with ivermectin or albendazole 1

Step 2: Evaluate for Systemic Features

• Check for eosinophilia: Common in parasitic infections 1
• Assess for respiratory symptoms: Tropical pulmonary eosinophilia or disseminated strongyloidiasis 1
• Look for other skin findings: Nodules (onchocerciasis), urticaria (multiple helminth infections) 1

Step 3: Treatment Based on Most Likely Diagnosis

For cutaneous larva migrans (most common in travelers):

• Ivermectin 200 μg/kg as single oral dose 1
• Alternative: Albendazole 400 mg once daily for 3 days 1

For suspected larva currens/strongyloidiasis:

• Requires stool examination and serology 1
• Treatment is more complex and requires specialist input due to risk of hyperinfection syndrome 1

For non-migratory linear lesions without travel history:

• Consider biopsy if diagnosis unclear 1
• Evaluate for psychological factors if pattern suggests neurotic excoriations 2
• Rule out drug reactions, especially if recent medication changes 1

Critical Pitfalls to Avoid

• Missing strongyloidiasis: Larva currens moves much faster than cutaneous larva migrans; failure to recognize this can lead to life-threatening hyperinfection syndrome, especially if the patient becomes immunosuppressed 1
• Treating empirically without travel history: Parasitic causes are exceedingly rare without appropriate geographic exposure 1
• Overlooking psychological causes: Linear lesions that are uniform, grouped on accessible sites, and non-migratory strongly suggest neurotic excoriations requiring psychotropic medication and counseling rather than antiparasitics 2
• Assuming all linear rashes are parasitic: The differential includes drug eruptions, contact dermatitis, and phytophotodermatitis, which require entirely different management 1

When to Obtain Tissue Diagnosis

Biopsy or aspiration should be performed early when 1:

• Diagnosis remains unclear after history and examination
• Patient is immunocompromised (broader differential including fungal, viral, bacterial causes)
• No response to empiric antiparasitic therapy within expected timeframe
• Concern for malignancy, vasculitis, or other systemic disease