Table 2.

Differential Diagnosis of Erythema Migrans.

Condition	Characteristics
Single erythema migrans lesion	Erythematous macule or papule at site of tick bite (although the tick is often not seen); enlarges relatively rapidly to 5–30 cm or more in diameter; typically flat and annular; usually uniformly erythematous or with heightened central erythema; may have central clearing; without treatment, persists for average of 3–4 wk*
Nummular eczema	Lesion usually smaller and less erythematous than erythema migrans lesion; does not enlarge rapidly; pruritic; well demarcated; skin may be thickened or weepy
Tinea (ringworm)	Rash with raised margins and scale on the edges; central clearing is typical; pruritic
Granuloma annulare	Small (2–5 cm in diameter), circular rash with erythematous papules and clear center; develops over weeks; often on dorsum of extremities
Cellulitis	Area of inflammation often at site of trauma to skin; warm; enlarges rapidly; rarely circular; may be tender and associated with fever
Insect bite	Often raised papule with central punctum; pruritic; usually smaller than erythema migrans lesion; rarely continues to enlarge
Spider bite	Necrotic lesion with central eschar; often very painful
Hypersensitivity to tick bite	Small lesion, does not expand as erythema migrans does; present at time tick bite is recognized or soon after; uniformly erythematous; often pruritic
Multiple erythema migrans lesions	Multiple ringlike lesions; typically do not enlarge rapidly; a larger, primary lesion may be present; often associated with systemic symptoms
Erythema multiforme	Multiple lesions, often quite small; mucosa, palms, and soles may be involved; cause may be apparent (e.g., drug or infection)
Urticaria	Pruritic, raised lesions; may appear and disappear rapidly

^*A similar lesion is found in southern tick-associated rash illness (STARI), which occurs primarily in southeastern and south central states. STARI does not have extracutaneous manifestations. The cause of STARI is unclear; no diagnostic test is available.⁸