Erythema chronicum migrans is a circular rash that often appears in the early stages of Lyme disease. Approximately of people with Lyme disease will have this rash. While you might know it as a bull’s-eye rash, it can also appear as a solid circle. A Lyme disease diagnosis can be made if you have an erythema migrans rash and have recently been bitten by a tick, or if you were in an area where a bite was likely, such as the woods.

Erythema migrans is a large rash, usually measuring about 2 to 2.5 inches and expanding gradually. It can reach up to 12 inches or more. The rash appears where you were bitten by a tick somewhere between after the bite. For most people, it will appear within 7 to 10 days.

Many people notice a bump or redness immediately after a tick bite, but this usually goes away in a few days and is not a sign of Lyme disease.

An erythema migrans rash may feel warm when you touch it, but it’s rarely painful or itchy. Because the rash is an early sign of Lyme disease, you might also experience:

  • fever
  • headache
  • chills
  • joint aches
  • swollen lymph nodes

Once the rash appears, it will begin to expand and can reach over six inches wide. In some people, the rash can start to clear as it gets bigger, causing the bull’s-eye rash many people associate with Lyme disease. However, solid, circular rashes are the most common type in the United States. Some people with darker complexions might have a rash that looks like a bruise.

Erythema migrans is a hallmark symptom of Lyme disease. If you think you might have been bitten by a tick and you’ve developed a circular rash, call your doctor as soon as possible. Medical attention is particularly important if you have other symptoms of early-stage Lyme disease, which are usually flu-like.

If left untreated, Lyme disease can cause:

  • joint inflammation and pain
  • irregular heartbeat
  • shortness of breath
  • hand and foot numbness
  • facial palsy (droop or loss of muscle tone on one or both sides of the face)
  • dizziness
  • short-term memory loss

If treated early, Lyme disease can almost always be cured. Call your doctor as soon as you see erythema migrans.

Erythema migrans is not a reaction to a tick bite, but rather an outward sign of an actual Lyme disease infection in your skin. When the rash first appears, Lyme disease has likely just started to spread through your body, but is still more localized.

Erythema migrans is only caused by Lyme disease. A similar-looking circular rash is caused by a bite from a lone star tick, which is different from the tick that causes Lyme disease. But this rash will never be in the shape of a bull’s-eye.

A doctor can diagnose you with Lyme disease if you have erythema migrans and think you might have been bitten by a tick recently or were in a place where a tick bite could have occurred. It’s the only symptom that doctors can use to diagnose Lyme disease without blood tests.

While there are other rashes that look like erythema migrans, this rash is the only one of similar-looking rashes that will expand quickly and for many days after it appears. It’s also the only one that could appear as a bull’s-eye rash.

Even if a doctor diagnoses you with Lyme disease based on erythema migrans, they will do a blood test to confirm the diagnosis. An enzyme-linked immunosorbent assay (ELISA) detects antibodies to the bacteria that cause Lyme disease. It’s the most commonly used test.

Lyme disease is treated with oral antibiotics, such as doxycycline (Acticlate, Doryx, Vibra-Tabs) or amoxicillin. A 14- to 21-day course of treatment will effectively treat the disease in most people. If your Lyme disease is in a later stage with neurological symptoms, you might need intravenous (IV) antibiotics.

Because an erythema migrans rash is a Lyme disease infection in the skin, any treatment for Lyme disease will also treat erythema migrans. If the rash is itchy or uncomfortable, you can try using an antihistamine to relieve itching or a cold compress to cool the rash area. However, you should ask your doctor before taking any medication for the rash specifically.

In most people, erythema migrans fades within three to four weeks, although for some people it can last for months. If you develop Lyme disease, you might also develop smaller rashes on other parts of your body as the disease spreads. Erythema migrans should totally clear after a successful course of Lyme disease treatment.

If you’re in wooded or high grass areas during tick season (May through mid-July), it’s important that you take precautions to avoid tick bites. This is the best way to prevent Lyme disease. To decrease your risk of bites, follow these tips:

  • Wear long pants and long-sleeve shirts outdoors. It might be hot, but long clothing will decrease the area that ticks can bite. Take extra care to make sure you don’t get dehydrated or overheated.
  • Use insect repellent (bug spray) with DEET. Insect repellent with 10 percent DEET will protect you for about two hours. Don’t use more DEET than what is required for the time you’ll be outside, and don’t use DEET on the hands of young children or the faces of children younger than 2 months. You can also use tick repellent spray containing permethrin on your clothes.
  • Check yourself, your children, and your pets for ticks as soon as you come indoors. If you do find a tick, use tweezers to remove it by pulling gently near its head or mouth. Check to make sure all tick parts have been removed and then disinfect the site. Learn more about identifying and removing ticks.
  • Put your clothing in the dryer for ten minutes on high heat to kill any ticks that might have attached themselves.