Erythema migrans

Specialty: Skin.

• erythema chronica migrans
• expanding target lesion
• initial Lyme plaque

Why it occurs

• Infection by spirochetes of the *Borrelia burgdorferi* complex transmitted by the bite of a tick of the genus *Ixodes* (Lyme disease in its early localized phase)
• Non-infectious tick bite simulating erythema migrans (persistent local hypersensitivity reaction)
• Spider bite (local reaction with limited central clearance)
• Reactions to drugs with transient expansive annular morphology
• Erythema annulare centrifuge (chronic idiopathic inflammatory dermatosis).

Initial workup

Clinical diagnosis of the expansile lesion (>5 cm in diameter with central clearance and bull's-eye appearance). In non-endemic areas or doubtful cases, confirmation by serology for *Borrelia burgdorferi* (ELISA test followed by confirmatory Western Blot). PCR in skin or synovial fluid biopsy if there is high clinical suspicion with negative serology.

Standard management

• Doxycycline — antibiotic of choice orally, dose of 100 mg every 12 hours for 14 to 21 days; contraindicated in children under 8 years of age and pregnant women
• Amoxicillin — alternative antibiotic of choice in pregnant patients or young children, dose of 500 mg every 8 hours orally for 14 to 21 days
• Cefuroxime axetil — second-line alternative in those allergic to penicillins, dose of 500 mg every 12 hours orally
• Paracetamol or ibuprofen to mitigate myalgia and associated headaches.