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People infected with Lyme Disease often present with an erythema migrans ("migrating redness") rash. Most often, these rashes are in the shape of a bulls-eye.

Rash image.

Presumably, this is a reaction that many of our bodies have to the Borrelia burgdorferi pathogen.

Lyme disease also sometimes presents with different-looking rashes, and other pathogens also sometimes produce a bulls-eye-looking rash. CDC Link on Lyme Disease Rashes and Look-alikes.

What causes the bulls-eye shape of these rashes?

Thought process: A gradient of redness from the site of the tick bite makes sense--an immune response might be strongest at the bite site--but I'm still stumped on why there would be a spot of redness surrounded by a no-rash-ring surrounded by a rash-ring.


Note: If I remember correctly, external links to images are usually frowned upon. Although the images linked in this post aren't super graphic, I decided to make them external links for the sake of anyone who is sensitive to medical images.

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The spreading of the rash is caused by dissemination (migration) of the Lyme spirochetes (Borrelia) through the skin out from the initial site of infection.

The spirochetes trigger a immune reaction that involves macrophages entering the skin and releasing pro-inflammatory factors.

The clear areas (when present) apparently represent areas where macrophages have been cleared from the skin and thus are no longer triggering a inflammatory response.

The different patterns associated with the rashes can be explained by differences in how fast macrophages are cleared from the skin.

Reference: Vig, D. K., & Wolgemuth, C. W. (2014). Spatiotemporal evolution of erythema migrans, the hallmark rash of Lyme disease. Biophysical journal, 106(3), 763-768.

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