Erythema chronicum migrans is the initial lesion of Lyme disease, and often appears at the site of the infecting tick bite. It is a red, enlarging rash, flat or slightly raised, and may reach from 4 to 20 inches across (the average rash is 6 inches). (Image courtesy of the Centers for Disease Control and Prevention.)
Review Date: 2/27/2012 Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.