TOACS FCPS Station · Erythema Migrans in Lyme Disease

📷 Clinical Photograph – Erythema Migrans

Clinical photograph showing an expanding erythematous rash with central clearing (bull's-eye) on the thigh, characteristic of erythema migrans in Lyme disease

Figure 1 · Erythema Migrans · Early localized Lyme disease

🔍 Key clinical features:

✓ Expanding erythematous rash – enlarges over days to weeks
✓ Central clearing – classic 'bull's-eye' appearance (may be uniformly erythematous)
✓ Painless, not pruritic – often asymptomatic
✓ Appears 3–30 days after tick bite – at the site of the bite
✓ Pathognomonic – clinical diagnosis, no serology needed to treat

📋 Clinical scenario (examiner prompt)

A 10‑year‑old child is brought in during July with a large, expanding rash on the thigh. The rash started as a small red spot 7 days ago and has grown to about 10 cm in diameter with a central clearing (a 'bull's-eye' appearance). The child recalls a tick bite 2 weeks ago while camping. He is afebrile and otherwise well, but his mother is concerned about the rash.

Expanding rash Central clearing (bull's-eye) Tick bite history Endemic area

🧑‍⚕️ Examiner tasks · TOACS

1. Identify the diagnosis from the clinical image and context.

2. Describe the clinical features (erythema migrans – expanding rash, central clearing, painless).

3. Explain the etiology and transmission (Borrelia burgdorferi, Ixodes ticks).

4. Discuss diagnosis and management (clinical diagnosis, oral antibiotics, prevention).

⚠️ Key concept: Erythema migrans is the hallmark of early localized Lyme disease. It is a clinical diagnosis – serology is not needed for EM. Treatment is with oral antibiotics: doxycycline (for children ≥8 years) or amoxicillin (for younger children) for 10–14 days. Prognosis is excellent with appropriate treatment.

🎯 Expected answers (for examiners)

• Diagnosis: Erythema migrans (early localized Lyme disease)
• Clinical features: Expanding erythematous rash, central clearing (bull's-eye), painless, at site of tick bite
• Etiology: Borrelia burgdorferi transmitted by Ixodes scapularis (deer tick) in the eastern US
• Diagnosis: Clinical – serology not needed for EM; can be negative early
• Treatment: Doxycycline (≥8y) 4.4 mg/kg/day BID ×10-14 days; Amoxicillin (<8y or pregnant) 50 mg/kg/day TID ×14 days
• Prevention: Tick checks, DEET, permethrin, single-dose doxycycline for high-risk bites

📌 Erythema migrans – diagnostic pearls:
• Pathognomonic for Lyme disease – treat without serology
• Classic appearance: Expanding rash with central clearing ('bull's-eye')
• May be uniformly erythematous – not all have central clearing
• Appears 3–30 days after tick bite
• Treatment with oral antibiotics is highly effective (>95% cure)

⚡ Quick FCPS‑style MCQ

A child with an expanding erythematous rash with central clearing on the thigh after a tick bite. The most appropriate next step is:

A. Order Lyme serology (ELISA and Western blot) B. Treat with oral doxycycline or amoxicillin based on clinical diagnosis C. Perform skin biopsy D. Observe without treatment

📌 Topic summary · Erythema Migrans in Lyme Disease

Etiology
Borrelia burgdorferi

Vector
Ixodes scapularis (deer tick)

Classic rash
Expanding 'bull's-eye' (erythema migrans)

Diagnosis
Clinical (no serology needed for EM)

Treatment
Oral doxycycline or amoxicillin ×10-14 days

Prognosis
Excellent (>95% cure)

Feature	Erythema Migrans (Lyme Disease)
Stage	Early localized
Onset	3–30 days after tick bite
Rash characteristics	Expanding erythematous rash, central clearing (bull's-eye), painless
Serology at this stage	Often negative (sensitivity ~30-50%)
Treatment	Doxycycline (≥8y) or Amoxicillin (<8y) ×10-14 days
Prevention	Tick checks, DEET, permethrin, single-dose doxycycline for high-risk bites

🕷️ TOACS FCPS Station · Erythema Migrans in Lyme Disease

📷 Clinical Photograph – Erythema Migrans

🧑‍⚕️ Examiner tasks · TOACS

📌 Topic summary · Erythema Migrans in Lyme Disease