Which bacterial infection is associated with a 'bull's-eye' rash?

The 'bull's-eye' rash, also known as erythema migrans, is a characteristic skin finding associated with Lyme disease, which is caused by the spirochete Borrelia burgdorferi. This rash typically appears as a red circle with a central clearing, resembling a target or 'bull's-eye', and is often the first sign of Lyme disease following a bite from an infected tick, primarily the black-legged tick (Ixodes scapularis).

The presentation of this rash usually occurs within a few weeks to months after exposure to the tick, and it is an important clinical indicator that helps in the diagnosis of Lyme disease, especially in endemic regions. Notably, the rash may vary in size and is not always symmetric, but its distinctive appearance is what associates it most closely with this particular infection. Understanding this identifier helps healthcare providers differentiate Lyme disease from other tick-borne illnesses and guides appropriate treatment plans.