Erythema migrans

Erythema migrans is an expanding skin lesion associated with early Lyme disease, commonly developing near the site of an infected tick bite. The name describes the outward enlargement of the affected area, not movement of the entire patch across the body. Its appearance varies: central clearing may create rings, but a uniform expanding patch can also occur.^[ref]

The time course is part of the observation. Recording the border and size on successive days provides information that a single photograph cannot supply. An immediate small local reaction to a bite and a lesion that enlarges over subsequent days are different patterns. Erythema migrans is not present in every Lyme disease case, so absence of the familiar bullseye image cannot be equated with absence of infection.^[ref]