About This Page
This is a clinician-written, evidence-based guide aligned to the MCC Examination Objectives. It is structured by clinical presentation — the way the MCCQE tests and the way patients actually present. Management reflects current Canadian guidelines (CMA, CFPC, CPS). Always cross-reference with institutional protocols and clinical judgment.
Key points
- In Canada, Lyme disease risk follows the expanding range of Ixodes ticks; local epidemiology matters
- Erythema migrans after compatible tick exposure is a clinical diagnosis — do not wait for serology
- Serology is often negative early and is most useful in disseminated or later manifestations
- Post-exposure prophylaxis is only for specific high-risk Ixodes tick bites within the right time window
- Avoid testing asymptomatic low-risk bites because false positives can cause harm
Approach to the Presentation
Tick-borne illness on the MCCQE1 is usually a Lyme disease reasoning problem. The Canadian approach begins with geography, tick identification, attachment duration, timing, symptoms and pregnancy/age factors. Erythema migrans is an expanding rash, typically greater than 5 cm, developing days to weeks after a bite and is treated clinically. Testing depends on disease stage: early localized disease may be seronegative, while two-tier serology is appropriate for compatible disseminated disease. Consider other tick-borne infections only when geography, travel, cytopenias, high fever or severe systemic features suggest an alternative or coinfection.
Differential Diagnosis
| diagnosis | likelihood | key features | distinguishing test |
|---|---|---|---|
| Early disseminated Lyme disease | must-not-miss | Multiple EM lesions, facial nerve palsy, meningitis/radiculitis, migratory arthralgia or carditis symptoms | Two-tier Lyme serology; ECG for carditis symptoms; CSF if meningitis |
| Lyme carditis | must-not-miss | Young patient with tick exposure and fluctuating AV block, palpitations, syncope, dyspnea or chest pain | ECG showing AV block; Lyme serology; telemetry if significant conduction disease |
| Septic arthritis | must-not-miss | Hot painful joint, fever, inability to bear weight or severe pain with passive movement | Urgent joint aspiration for cell count, Gram stain, culture and crystals |
| Early localized Lyme disease | common | Expanding erythematous rash usually >5 cm days to weeks after Ixodes exposure, with possible fever, fatigue, headache or myalgia; not always bullseye | Clinical diagnosis in compatible exposure; serology may be negative early |
| Cellulitis or hypersensitivity reaction | common | Small itchy local reaction within 24-48h, usually <5 cm and not progressively expanding; cellulitis is warm/tender | Clinical timing and appearance |
| Bell palsy unrelated to Lyme | common | Isolated acute facial weakness without tick exposure/endemic risk or systemic features | Clinical; Lyme serology if exposure/geography compatible |
| Viral syndrome | common | Fever/myalgia without EM or objective Lyme features; often sick contacts or respiratory/GI symptoms | Clinical; avoid Lyme testing if pre-test probability low |
| Lyme arthritis | less common | Intermittent or persistent mono/oligoarthritis, classically knee, weeks to months after exposure | Two-tier serology; joint aspiration if septic arthritis is in differential |
| Anaplasmosis / babesiosis | rare | High fever, chills, cytopenias or transaminitis after tick exposure; babesiosis may cause haemolysis, severe in asplenia | CBC/LFTs, blood smear/PCR/serology depending on pathogen |
Red Flags & Key History
Symptoms
Syncope, palpitations, dyspnea or chest pain after possible Lyme exposure — carditis/AV block concern
Severe headache, meningism, radicular pain, facial palsy or neurological deficit
Hot painful joint with inability to bear weight — septic arthritis must be excluded
Asplenia, immunocompromise, severe haemolysis or high fever after tick exposure
Expanding rash >5 cm developing over days after tick exposure
Tick species, attachment duration and local endemicity determine prophylaxis and testing decisions
Signs
Erythema migrans: expanding annular or homogeneous erythematous lesion; bullseye not required
Facial nerve palsy, meningism or radiculopathy
Bradycardia, irregular pulse or signs of AV block
Large knee effusion with limited pain may suggest Lyme arthritis but septic arthritis must be considered
Approach to Investigation
First-line
Clinical diagnosis of erythema migransDo not delay treatment for serology when classic EM and exposure risk are present
Two-tier Lyme serologyAppropriate for compatible disseminated/late manifestations; early localized disease may test negative
ECGFor palpitations, syncope, chest symptoms, dyspnea or suspected Lyme carditis
CBC and liver enzymesIf febrile systemic illness or concern for anaplasmosis/babesiosis coinfection
Second-line
Joint aspirationIf acute monoarthritis and septic arthritis is in the differential
CSF studiesIf meningitis/radiculitis suspected; coordinate with serology and specialist input
Blood smear/PCR/serology for coinfectionsWhen severe fever, cytopenias, haemolysis, travel/geography or asplenia suggests babesiosis/anaplasmosis
Specialist
Infectious diseases consultationFor carditis, neuroborreliosis, pregnancy uncertainty, coinfection, treatment failure or complex serology
Cardiology/telemetryFor suspected Lyme carditis with PR prolongation, AV block, syncope or significant symptoms
Management Principles
PHAC Lyme disease guidance for health professionals1
Tick bite without symptoms
- Remove tick with fine-tipped tweezers, clean area and document species/attachment estimate if possible
- Assess prophylaxis criteria: likely Ixodes tick, endemic risk, adequate attachment duration, presentation within appropriate time window and no contraindication
- Do not order Lyme serology for an asymptomatic tick bite
2
Erythema migrans
- Treat clinically when exposure and rash are compatible
- Do not wait for serology because early tests may be negative
- Use age/pregnancy-appropriate antibiotic choice according to Canadian/local guidance
3
Disseminated disease
- Order two-tier serology and syndrome-specific tests
- ECG and telemetry for carditis symptoms or conduction abnormalities
- Evaluate neurological and joint manifestations carefully; exclude septic arthritis when acute hot joint
4
Prevention counselling
- Tick checks, prompt removal, repellents, protective clothing and awareness of local risk areas
- Explain that nonspecific chronic symptoms alone are not diagnostic of active Lyme disease without objective evidence
Complications & Pitfalls
- Testing too early: Early EM can be seronegative, so classic rash is treated clinically.
- Testing low-risk bites: Low pre-test probability increases false positives.
- Missing carditis: Syncope or palpitations after Lyme exposure needs ECG.
- Bullseye myth: EM does not need central clearing.
- Septic arthritis confusion: A hot painful joint still needs aspiration when septic arthritis is plausible.
MCCQE1 Exam Tips
- 1Expanding >5 cm rash after tick exposure = erythema migrans; treat clinically without waiting for serology
- 2Serology is most useful for disseminated/late Lyme, not very early localized disease
- 3Tick prophylaxis is conditional, not automatic after every tick bite
- 4Lyme carditis presents with AV block, palpitations or syncope; get ECG
- 5Facial palsy in an endemic area with tick exposure should prompt Lyme consideration
- 6Local bite reaction appears quickly and is usually small/itchy; EM expands over days
- 7Canadian epidemiology matters: ask where the exposure occurred
Put your reading into practice
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