Bull's-eye rash (erythema migrans)

A bull's-eye rash — medically called erythema migrans — is a red patch that starts at the site of a tick bite and slowly enlarges over days, often reaching several centimetres or more across. In its classic form the centre clears as the edge expands, producing concentric rings that resemble a target or bull's-eye, though many cases are simply a uniform, expanding red or oval patch without the clear centre. It is usually not itchy or painful, which is part of why it can be missed. This rash is the hallmark early sign of Lyme disease, an infection carried by ticks, and it typically appears from a few days up to a month after the bite — often after time spent in grassy or wooded areas. Its importance is that it is diagnostic enough on its own: because antibodies may not have developed this early, a classic erythema migrans in the right setting is treated as Lyme disease without waiting for a blood test. Not every rash after a tick bite is erythema migrans — a small, quickly appearing reaction right at the bite is usually just a local response — but a rash that expands over days is the one to act on. Recognising it early allows prompt, highly effective antibiotic treatment before the infection can spread.

• Lyme disease (erythema migrans) — the classic cause; an expanding rash days to weeks after a tick bite.
• A local tick-bite reaction — small and appearing within hours to a day, not expanding; usually harmless.
• Ringworm (a fungal infection) — a ring-shaped rash, but with a different history and often itchy and scaly.
• Other insect-bite reactions or cellulitis — usually with different features (pain, warmth, rapid spread).

A classic expanding bull's-eye rash after possible tick exposure is diagnosed clinically as early Lyme disease and treated without waiting for a blood test, because Lyme antibodies may not yet have developed at this early stage — a negative test does not rule it out. Blood tests (two-tier Lyme serology) are more useful later, for suspected joint, nerve or heart involvement, and inflammatory markers (ESR, CRP) may be checked. If the diagnosis is uncertain — for example distinguishing it from ringworm or a simple bite reaction — the appearance, how it evolves and the exposure history guide the doctor. Mediora.AI can show Lyme antibody and inflammatory results in context, but a spreading bull's-eye rash is primarily a clinical diagnosis that should prompt prompt medical review.