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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My understanding of this is that the ring is caused by dissemination of the Lyme spirochetes (Borrelia) through the skin out from the initial site of infection.

I agree that the clear area seems a bit mysterious, but I have always assumed by analogy to fungal rings (e.g. Tinea corporis [ringworm]) that it represents areas where the bacteria have been suppressed or killed.

However, I have yet to find any research into this, so this may well represent lore rather than data ...


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