Derm Cases · Dermatology / Infectious Disease / Emergency Medicine
OSCE — expanding annular erythema after a tick bite: erythema migrans
An 8-minute OSCE station on clinical diagnosis of erythema migrans without early serology, stage-based Lyme complications (carditis, neuroborreliosis), first-line doxycycline/amoxicillin regimens, IV ceftriaxone indications, and post-exposure prophylaxis counselling.
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Study tools
Target exams
NEET-PGINICETUSMLEPLABMRCP
Prompt
An 8-minute OSCE station on clinical diagnosis of erythema migrans without early serology, stage-based Lyme complications (carditis, neuroborreliosis), first-line doxycycline/amoxicillin regimens, IV ceftriaxone indications, and post-exposure prophylaxis counselling.
Brief (to candidate)
A 28-year-old trekking guide returns from a wooded endemic area with a slowly expanding, non-pruritic annular erythematous plaque (12 cm) on the thigh, mild fever and myalgia, 10 days after an engorged tick bite. ECG is normal; neurology is normal. You have 8 minutes to diagnose, decide on serology, prescribe first-line antibiotics, and counsel on red-flag dissemination and prevention.
[4]Candidate instructions
- Recognise erythema migrans as a clinical diagnosis.
- Explain why early serology is not required (and may mislead).
- Prescribe stage-appropriate oral antibiotics with dose and duration.
- Screen verbally for carditis and neuroborreliosis and state IV therapy triggers.
- Counsel on tick removal, prophylaxis decision-making, and prognosis.
Examiner checklist (mark each domain / 10)
| Domain | Key actions expected |
|---|---|
| Recognition | Expanding annular/targetoid erythema days–weeks after Ixodes exposure = erythema migrans (early-localised Lyme)[1][3] |
| Diagnosis principle | Treat clinically in endemic exposure; early serology sensitivity only ~30–40%; do not delay antibiotics for titres[2][4] |
| First-line oral Rx | Doxycycline 100 mg PO BD for 14–21 days first-line; alternatives amoxicillin 500 mg TDS or cefuroxime 500 mg BD (pregnancy/young children/intolerance)[2][4] |
| Dissemination red flags | Multiple EM lesions; AV block/syncope → admit, telemetry, IV ceftriaxone 2 g daily; meningitis, radiculopathy, bilateral facial palsy → neuroborreliosis work-up and parenteral therapy[4] |
| Co-infection | Unwell with cytopenias/high fever → consider anaplasmosis/babesiosis from same tick; doxycycline covers Anaplasma[2] |
| Prophylaxis | After high-risk engorged Ixodes bite in highly endemic area: single-dose doxycycline 200 mg within 72 h offered by shared decision-making, not mandatory for every bite[4] |
| Communication | Reassure excellent cure rates of early EM; reject prolonged antibiotics for post-treatment symptom syndromes without objective infection |
Model key actions
- Diagnose erythema migrans clinically and start doxycycline 100 mg BD × 14–21 days without waiting for serology.[3][4]
- Safety-net cardiac and neurological dissemination needing IV ceftriaxone.[4]
- Counsel prevention and selective post-exposure prophylaxis.[2][4]
Common errors
- Ordering and waiting for serology before treating classic EM.
- Using azithromycin first-line when doxycycline/amoxicillin available.
- Missing AV block / facial palsy red flags.
- Routine prolonged IV antibiotics for uncomplicated EM.
- Prescribing months of antibiotics for fatigue after adequate treatment (PTLDS).
References4ShowHide
- [1]Steere AC, Strle F, Wormser GP, et al. Lyme borreliosis. Nature Reviews Disease Primers, 2016.PMID 27976670
- [2]Sanchez E, Vannier E, Wormser GP, et al. Diagnosis, Treatment, and Prevention of Lyme Disease, Human Granulocytic Anaplasmosis, and Babesiosis: A Review. JAMA, 2016.PMID 27115378
- [3]Kullberg BJ, Vrijmoeth HD, van de Schoor F, et al. Lyme borreliosis: diagnosis and management. BMJ, 2020.PMID 32457042
- [4]Lantos PM, Rumbaugh J, Bockenstedt LK, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America (IDSA), American Academy of Neurology (AAN), and American College of Rheumatology (ACR): 2020 Guidelines for the Prevention, Diagnosis and Treatment of Lyme Disease. Clinical Infectious Diseases, 2021.PMID 33417672