A bullseye rash after a bite

An expanding red patch appearing three to thirty days after a tick bite is the classic sign of early Lyme disease and needs medical assessment. Importantly, most such rashes are uniformly red — the ring-within-a-ring "bullseye" appears in a minority of cases, so its absence rules nothing out.

What it looks like

The rash of early Lyme disease (erythema migrans) typically:

• appears 3 to 30 days after the bite, most often around week one to two;
• expands over days, often reaching several centimetres or more;
• is usually not itchy and not painful, which is unusual for a rash;
• may be uniformly red, or develop central clearing, or show a dusky centre;
• can appear away from the bite site if the infection has spread.

On deeper skin tones it often presents as a darker or bruise-like patch, and it is more frequently missed for that reason.

What it is not

• A reaction ring appearing within hours. Ordinary bite reactions and large local reactions to stings develop fast and start settling within two days. The Lyme rash appears late and keeps growing.
• Ringworm. Fungal rings are scaly at the edge and itchy; erythema migrans is flat and usually neither.
• STARI, a similar rash following a lone star tick bite in the Southeast, which is not Lyme but looks alike — another reason species and geography matter.

What to do

Photograph it, with something for scale, and mark the edge with a pen so you and a clinician can see whether it is expanding. Then get it assessed — early Lyme disease is treated with a straightforward course of oral antibiotics, and treatment is far simpler at this stage than later.

Note the date of the bite and the date the rash appeared. That interval is one of the most useful things you can bring to the consultation.

Other symptoms in the same window

Fever, chills, headache, muscle and joint aches, swollen glands and marked fatigue in the weeks after a tick bite matter even without any rash at all — a substantial share of people never develop one.