Erythema migrans (EM) is a rash that develops in response to infection by Borrelia spirochetes. It is the primary symptom required by the Infectious Diseases Society of America (IDSA) and the Centers for Disease Control (CDC) surveillance criteria to diagnose and report a case of Lyme disease.
Appearance of the Rash
The most common presentation of erythema migrans is as a round or oval rash that becomes larger over several days and develops an inner clear area surrounding a reddish center: the bull's-eye rash. In about half the patients, however, the rash is either absent or presents a different appearance:
- It may be fiery red with a raised or eroded center.
- Many people have a dark blue and red rash resembling a recent bruise.
- Some people develop multiple rashes of varying sizes.
According to the Canadian Lyme Disease Association (CanLyme), approximately 10% of rashes are of these atypical forms. When a patient presents an atypical rash, even after removing a tick one to four weeks earlier, some doctors will not associate the rash with the tick bite and rule out Lyme disease because it does not fit the EM pattern.
Size of Rashes
Typically, the rash is between two inches (5 cm) and five inches (25 cm) in diameter. But it may be as small as a quarter or cover the greater part of a leg or the torso. A person might have a single rash or several to many rashes at the same time.
Histological and Molecular Analysis of Erythemas
In the Journal of the American Academy of Dermatology, March 2008, Dr. Carmen Moreno and her colleagues at Universidad Autnoma, Madrid, report EM rashes have similar microscopic structure:
- There are higher than normal accumulations of histiocytes (immune system bacteria-eating, phagocytic cells) in the deeper levels of the skin.
- These histiocytes form rosettes by clustering in radiating constellations surrounding thickened collagen bundles.
- Polymerase chain reaction (PCR) biopsies from the outer margins of the rashes of eleven patients more consistently showed Borrelia DNA than do biopsies from the centers of the same rashes. ELISA tests were positive for Borrelia in samples taken from only five of these patients, while IgM and IgG ELISA tests of the other six patients were negative or inconclusive. CanLyme suggests that up to 70% of patients are ELISA negative.
A positive ELISA response is the CDC's second surveillance criterion. If the patient does not display a rash to the doctor and has a negative ELISA test, that patient does not meet the CDC or IDSA criteria for Lyme disease – regardless of the outcome of a histological examination or PCR test.
Absence of a Rash
When someone goes to a physician suspecting Lyme disease, the first two questions asked are usually:
Did you see a tick?
Did you have a rash?
Marcus A. Cohen, author of “Lyme Disease Update: Science, Policy, and Law” states that 20% to 50% of people do not see a rash and many of these also do not see ticks. When the answers are “No,” many doctors rule out Lyme disease as a possible diagnosis because of the lack of this symptom.
Erythema Migrans Is Characteristic of Lyme Disease
The appearance of erythema migrans is considered by CDC to be diagnostic of Lyme disease. The interpretation of what is EM and what is not is, however, subjectively inconsistent. When EM is present as the typical bull's-eye rash, doctors are convinced, but when it is absent or atypical, many are hesitant or even refuse to diagnose Lyme disease. Much of this appears to be because the diagnostic guidelines of the IDSA require and surveillance criteria of the CDC suggest another diagnosis if the rash or ELISA test are negative - no matter what the clinical presentations are.
Disclaimer
The information in this article is believed to be accurate and is presented for the sole purpose of informing community members of information pertaining to Lyme Borreliosis. Any and all liability for the content or any omissions, inaccuracies, errors, or misstatements in such information is expressly disclaimed. Because the symptoms of Lyme disease may vary from person to person, if Lyme Disease is suspected, consult a qualified, Lyme disease literate doctor to discuss your symptoms and for medical advice. The author and Suite101.com are not liable for any direct or indirect damages or any damages whatsoever resulting from loss of function, use, data, or profits arising from or in connection with the application of information presented in this article.
