Overview
Clinical Orientation
An expanding erythematous lesion after outdoor exposure is the Lyme clue that should change the plan before a laboratory result returns.
An expanding erythematous lesion after outdoor exposure is the Lyme clue that should change the plan before a laboratory result returns. In Canada, risk depends on geography: infected *Ixodes scapularis* transmits Lyme disease in eastern and central Canada, while *Ixodes pacificus* is the relevant vector in British Columbia. The clinical task has two tracks. Recognize early localized disease and treat a typical erythema migrans lesion clinically; do not let an early negative serologic result delay therapy. At the same time, look for dissemination. Facial palsy, meningitis or radicular pain suggests neuroborreliosis; syncope, palpitations, bradycardia, or conduction abnormalities raises concern for Lyme carditis; a swollen large joint points toward Lyme arthritis. Carditis is the time-critical presentation because atrioventricular block can worsen abruptly. The same tick may transmit other pathogens. High fever, cytopenias, haemolysis, or an unexpectedly severe illness should prompt consideration of Anaplasma or Babesia rather than attributing every symptom to Lyme disease alone.