What Is Erythema Migrans?

Erythema migrans is a circular skin rash that appears as one of the first symptoms of Lyme disease, an infection spread by ticks. It’s also known as a bull’s-eye rash, because the center of the red area is oftentimes clear, giving it the appearance of a bull’s-eye. Although erythema migrans is most commonly associated with early-stage Lyme disease, it can also be caused by other tick-borne illnesses.

Continue reading to learn more about erythema migrans, including the symptoms of erythema migrans, how long it lasts, and whether it is always caused by Lyme disease. 

Erythema Migrans Symptoms

The primary symptom of erythema migrans is a red rash. The rash often has these characteristics:

• Is circular or oval, though it can also be other shapes
• Is red and slightly raised
• Expands over time
• Measures 4–20 inches across and, on average, covers six inches of skin
• Often has a noticeable bite mark or scratch at the center
• Has a clearly defined border
• Leaves a bluish tinge in areas where it has cleared
• Can last for weeks or months if left untreated 

When erythema migrans is caused by Lyme disease, it most often appears seven to 10 days after a tick bite. However, it can appear as long as 33 days after a bite. You might experience other Lyme disease symptoms, including fever, headache, and chills. 

Erythema migrans almost always appears at the site of a tick bite.

Causes

The most common cause of erythema migrans is Lyme disease. Lyme disease is caused by a bacterium called Borrelia burgdorferi that's acquired when you are bitten by an infected tick, usually a deer tick. Other tick-borne diseases can also cause a rash similar to erythema migrans. For example, a bite from a lone star tick may lead to a rash associated with southern tick-associated rash illness (STARI).

Diagnosis

See a healthcare provider when you experience erythema migrans, since all tick-borne illnesses need medical care. A healthcare provider will ask about your symptoms and the tick that bit you. Usually, healthcare providers make a diagnosis and treatment plan based on your symptoms alone.

However, they may also order a Lyme disease test. Some of these tests check your blood for antibodies that are made in response to the bacterium that causes Lyme disease, while others analyze the type of bacteria causing your symptoms. These tests can give a false negative (incorrect result showing no disease present), so they’re not always used. 

Treatment

If you have erythema migrans, healthcare providers will immediately start antibiotic treatment, even if you haven’t been tested for Lyme disease. Lyme disease must be treated promptly to avoid chronic disease. 

The treatment for early-stage Lyme disease is a two- to three-week course of antibiotics, such as doxycycline or amoxicillin.

Once you start antibiotics, you may notice your erythema migrans fade or go away. That’s a good sign, but it’s important to continue taking the medication for as long as your provider prescribes it. The antibiotics aren’t just treating erythema migrans, but also preventing other long-term symptoms like arthritis.

Prognosis

Most people with erythema migrans see their symptoms end once they start antibiotics. However, other Lyme disease symptoms, including arthritis and neurological problems, can last longer than the rash. Yet, they’re rarely permanent. 

Some people with Lyme disease will develop post-treatment Lyme disease syndrome (PTLDS), sometimes known as chronic Lyme disease. This occurs in about 10% of people and includes symptoms like brain fog, muscle aches and hearing loss. If you continue to have symptoms even after your erythema migrans has cleared, work with a healthcare provider who is experienced treating post-treatment Lyme disease. 

Summary

Erythema migrans is a red, often circular rash that’s closely associated with early-stage Lyme disease. Often, it’s one of the first symptoms that people experience. If you have erythema migrans, it’s important to see a healthcare provider for proper diagnosis and to start antibiotics, which can prevent a long-term infection.