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1.02 What is an Erythema Migrans (EM)?


This article is from the Lyme Disease FAQ, by Milo7 with numerous contributions by others.

1.02 What is an Erythema Migrans (EM)?

The Erythema Migrans or EM, commonly referred to as the Lyme disease
"bull's-eye" rash, is considered pathognomonic (or diagnostic) of Lyme
disease. This skin rash varies in size and shape; often has expanding rings
of varying shades, but can be uniformly discolored. It may be hot to touch,
it may even itch. It may change in color from reddish to purple to bruised
looking, and it can seldom be necrotic (crusty/oozy). Multiple rashes per
bite and multiple rashes not at the site of the bite may indicate
disseminated disease.

Lyme disease usually begins with an EM rash and flulike symptoms (headache,
stiff neck, fever, muscle aches, or fatigue). The EM can appear days to
weeks after the bite. However, only 60% of light-skinned patients notice
this rash. And recent studies indicate that as many as 40%-55% of people
never present with a rash at all, but with a flulike illness as the first
sign of onset of LD.

If you get an EM after a tickbite (and/or multiple rashes), it is advisable
to seek prompt medical attention. In addition, try to take a color
photograph of the rash. In the photo, include a ruler next to the rash for
measurement, and the date of the EM. You may need this information later on
as a visual record of the EM; for your medical records; for future health
insurance reimbursement, etc.

Currently, as part of the criteria for its case surveillance definition of
Lyme disease, the Centers for Disease Control (CDC) acknowledges only EM
rashes that are 5 cm (2 inches) or larger in size. This criterion is
designed strictly for CDC's epidemiologic purposes, and not for the
clinical diagnosis of Lyme disease. It should be emphasized that Lyme
disease rashes appear in different shapes and sizes, including rashes
smaller than 5 cm (2 inches).

For pictures of Lyme disease EM rashes:

Picture Gallery - Rashes (LDF)

Photos of Lyme disease rash (Texas Dept of Health)

The bullseye (EM, or Erythema Migrans) rash


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