Erythema Erythema migrans 2017-06-18T20:57:08Z An expanding erythematous (red) skin lesion, usually round or oval, by definition at least 5 cm in size (in largest diameter). Erythema chronicum migrans Erythema migrans develops at the site of the deposition of Borrelia burgdorferi 7 to 14 days (range, 1-36 days) following the detachment of certain Ixodes ticks. The most important differential diagnosis is a localized and transient inflammatory reaction to the bite of an arthropod that is not associated with infection and resolves spontaneously within a day or two, and is usually less than 5 cm in size.