12 Infectious Diseases Identified by Their Characteristic Rashes

8. Erythema Migrans - The Expanding Target

Erythema migrans, the pathognomonic skin lesion of early Lyme disease caused by Borrelia burgdorferi, presents as an expanding erythematous patch with characteristic features that make it one of the most diagnostically significant rashes in infectious disease medicine. The lesion typically appears 3 to 30 days after a tick bite, beginning as a small red area that gradually expands centrifugally at a rate of several centimeters per day. The classic presentation includes central clearing, creating the characteristic "bull's-eye" or target-like appearance, though this central clearing occurs in only about 20% of cases. More commonly, the lesion presents as a uniformly erythematous, expanding patch with well-defined borders and minimal central clearing. The rash is typically asymptomatic or only mildly pruritic, and the affected skin may feel warm to the touch but is rarely painful. The expanding nature of the lesion is pathognomonic for erythema migrans and helps distinguish it from other tick-borne illnesses or local reactions to tick bites. Multiple lesions may occur in cases of disseminated early Lyme disease, representing hematogenous spread of the spirochete. The size of erythema migrans lesions can vary significantly, ranging from a few centimeters to over 30 centimeters in diameter, and the lesions may have irregular shapes as they expand around anatomical structures. Without treatment, erythema migrans typically resolves spontaneously over several weeks to months, but early antibiotic therapy can prevent progression to disseminated Lyme disease and its associated complications.