erythema migrans a bullseye rash at the portal of entry is associated with
Erythema chronicum migrans Erythema chronicum migrans (New Latin, literally, "chronic migrating redness") refers to the rash often (though not always) seen in the early stage of Lyme disease. It can appear anywhere from one day to one month after a tick bite. This rash does not represent an allergic reaction to the bite, but rather an actual skin infection of one of the Lyme bacteria species from the genus Borrelia. "Erythema migrans is the only manifestation of Lyme disease in the United States that is sufficiently distinctive to allow clinical diagnosis in the absence of laboratory confirmation."[1] It is a pathognomonic sign:[2] a physician-identified rash warrants an instant diagnosis of Lyme disease and immediate treatment without further testing, even by the strict criteria of the Centers for Disease Control and Prevention. Such target lesions (bull's-eye rashes) are characteristic of Borrelia infections, and no other pathogens are known that cause this form of rash. It is also true, though, that the rash in Lyme disease may also be solid rather than bullseye-shaped,[3] so self-diagnosis cannot rule out Lyme disease, and a doctor visit is advisable.[3]