Erythema migrans.

EM is the most common manifestation of early Lyme disease, occurring in a high percentage of cases. Because this phase of infection with B. burgdorferi offers an excellent opportunity to treat this potentially systemic infection, front-line physicians must be familiar with its diagnosis. Although much attention has been paid to the classic form--the target lesion or bull's eye--there are variations that are more common. These include uniform coloration, lesions with necrotic or vesicular centers, and lesions with shapes that are not circular or oval. These findings must be interpreted in epidemiologic context. Serologic testing at this phase of the illness should not be done. It is unnecessary and potentially misleading; false-positive and false-negative tests can occur. Diagnosis is clinical. Prompt initiation of appropriate antibiotic therapy for 3 weeks cures most patients at this early stage of the disease. Clinicians should be aware that 15% of patients may be coinfected with a second tick-borne pathogen, which could alter the usual clinical manifestations and the response to treatment.