Erythema migrans (early Lyme disease)
Erythema chronicum migrans; ECM; EM; Lyme erythema; early localised Lyme borreliosis
Erythema migrans is the pathognomonic cutaneous sign of early localised Lyme borreliosis — caused in the UK predominantly by Borrelia afzelii and B. garinii (in contrast to North-American B. burgdorferi sensu stricto). It presents 1–4 weeks after an Ixodes tick bite as a slowly expanding annular erythematous patch > 5 cm with or without central clearing. UK Lyme is increasingly diagnosed — the cumulative reported incidence has risen substantially since 2010, with endemic foci in the Scottish Highlands, Lake District, New Forest and Thetford / Suffolk. Diagnosis of EM is clinical (serology often negative in early disease). Doxycycline 100 mg twice daily for 21 days is first-line; amoxicillin or azithromycin alternatives.
Clinical features
- Slowly expanding annular erythematous patch / plaque, > 5 cm at maximum diameter (often 5–15 cm).
- Central clearing in classic cases ("bull's-eye"); however, uniform / homogeneous expanding erythema is also typical in European EM.
- Develops 3–30 days after a tick bite (median 7–14 days); painless or mildly pruritic.
- Common sites — at site of tick bite; thigh, groin, axilla, scalp, lower limb most common in UK.
- Constitutional symptoms — fatigue, headache, myalgia, low-grade fever, lymphadenopathy in ~ 30–50%.
- Tick exposure history not always volunteered; ask specifically about countryside / forest / deer-park exposure.
Diagnosis
- EM is a clinical diagnosis — typical expanding annular patch > 5 cm after relevant exposure does not require laboratory confirmation.
- Serology (ELISA + Western blot) — has low sensitivity in early disease (often negative at presentation); takes 4–6 weeks for seroconversion. Do not delay treatment for serology.
- Tissue PCR / culture — research / specialist use; rarely needed.
- If diagnosis is uncertain, alternative differential includes tinea corporis, cellulitis, erythema annulare centrifugum, granuloma annulare, urticarial vasculitis, fixed drug eruption, contact dermatitis, large plaque parapsoriasis.
- Skin biopsy unhelpful — Borrelia rarely seen on histology.
Management
- First-line — doxycycline 100 mg orally twice daily for 21 days (adults and children > 12).
- Children < 12 / pregnancy / lactation — amoxicillin 1 g three times daily for 21 days.
- Penicillin / doxycycline allergy — azithromycin 500 mg daily for 17 days (lower efficacy; reserve).
- Single-dose doxycycline (200 mg) as post-exposure prophylaxis is NOT routinely used in the UK (CDC recommends in select scenarios after high-risk Ixodes scapularis bite).
- Jarisch-Herxheimer reaction — uncommon; fever, malaise, exacerbation of rash 4–12 hours after first dose.
- Resolution — rash typically resolves within 1–2 weeks; reassess at 4 weeks; serological seroconversion confirms exposure but treatment success is clinical.
- Tick removal — fine-tipped tweezers, perpendicular gentle pull at point of attachment; avoid burning, alcohol or chemicals which may increase regurgitation.
- If untreated — risk of disseminated Lyme (CNS, cardiac, joint) and late cutaneous manifestations (acrodermatitis chronica atrophicans, borrelial lymphocytoma cutis) over months to years.
References
- NICE NG95. Lyme disease. London: NICE; 2018 (last updated 17 October 2018).
- UK Health Security Agency. Lyme disease: management and prevention. GOV.UK; accessed 18 May 2026.
- Lantos PM et al. IDSA / AAN / ACR clinical practice guidelines for the prevention, diagnosis, and treatment of Lyme disease. Clin Infect Dis; 2021.
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