A 20-year-old college student presents with a rash on her flank, which she first noticed approximately 2 weeks ago. The rash was bright red initially but has faded in color over the past several days. She is an avid hiker but denies antecedent tick bite at the site. However, several weeks ago, she removed a tick from her scalp. One day after onset of the rash, she visited student health services and was tested for Lyme disease, but the titer was negative. She received a prescription for doxycycline but now questions why she was placed on this antibiotic and why another Lyme titer has not been obtained. On examination, a similar patch is noted on her mid back.
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The earliest manifestation of Lyme disease is referred to as erythema migrans, and it occurs in up to 80% of people with the disease.1 The initial target-like rash is >5 cm in size and arises at the site of the tick bite within 2 weeks of attachment. Over time, other associated findings may include neurologic manifestations (including Bell palsy),2 myocarditis and heart block,3 and migratory arthritis.4
The cause of Lyme disease is the parasite Borrelia burgdorferi, and the disease is transmitted by the Ixodes scapularis tick. More than 3 million serologic tests are performed each year for Lyme disease, at an approximate cost of $500 million.5 Nonetheless, early diagnosis is best made based on the characteristic rash, especially when it is associated with a tick bite. Serology tests are positive in fewer than 40% of people during the initial phase of the disease while the host is mounting an immune response.6
Given the clinical appearance and history of tick exposure, this patient’s current treatment of at least a 10-day course of doxycycline is appropriate, and follow-up serology is not required.
Stephen Schleicher, MD, is an associate professor of medicine at the Commonwealth Medical College in Scranton, Pennsylvania, and an adjunct assistant professor of dermatology at the Perelman School of Medicine at the University of Pennsylvania in Philadelphia. He practices dermatology in Hazleton, Pennsylvania.
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