Snapshot A 30-year-old male presents to the urgent care clinic for a round rash on his thigh. He reports that he noticed this rash 3 days ago while showering and has since expanded in size. He denies any pain, itching, or blistering from this rash. Two weeks ago, he had gone hiking in Connecticut while visiting relatives but denies pulling off any ticks, though he admits he did not thoroughly check after his hike. He denies any fevers or chills. On physical exam, there is an annular erythematous rash with central clearing on the left thigh with a diameter of 7 cm. He is given a course of antibiotics for presumed Lyme disease. Introduction Classification Borrelia burgdorferi a spirochete (spiral-shaped bacteria) does not gram-stain well due to thin cell walls transmission Ixodes tick clinical syndrome Lyme disease, a tick-borne illness characterized by 3 stages (see below) Epidemiology incidence northeast and upper midwest US risk factors tick bite hiking and other outdoor activities Associated conditions ehrlichiosis same tick vector babesiosis same tick vector Prognosis erythema migrans may resolve without treatment excellent prognosis with treatment Presentation Symptoms early localized stage erythema migrans (within 1 month of tick bite) expanding area of round erythema with central clearing, resulting in the classic targetoid lesion classic "bulls-eye" rash flat or raised not tender or pruritic systemic symptoms fever myalgia arthralgia headache fatigue early disseminated stage multiple erythema migrans neurologic symptoms facial nerve palsy, typically bilateral meningitis mononeuritis multiplex Lyme carditis third-degree (complete) atrioventricular block myopericarditis late stage arthritis involving knees, classically encephalopathy polyneuropathy Studies Labs serologic detection of immunoglobulins using enzyme-linked immunosorbent assay (ELISA) Making the diagnosis based on clinical presentation Differential Erythema multiforme distinguishing factors also presents with an annular rash though often presents with numerous annular lesions that are smaller and may show blistering not associated with a tick bite often associated with viral infections or medications Treatment Medical doxycycline or amoxicillin indications erythema migrans Lyme arthritis ceftriaxone indications neurologic Lyme disease Lyme carditis other disseminated Lyme disease macrolides indication second-line in those who are allergic to first-line medications Complications Chronic arthritis Post-Lyme syndrome although there is no biologic evidence or accepted definition for symptomatic chronic Lyme disease after completion of initial antibiotics, many patients report fatigue