A man, aged 64 years, presents for evaluation of an asymptomatic rash that developed on his left medial mid-thigh 3 days ago. He spends a substantial amount of time outdoors but denies a history of antecedent tick or spider bite or bee sting. He denies fever, malaise, muscle aches, and joint pain. He has a history of eczema and has been self-medicating the site with clobetasol cream since onset. Current medications include hydrochlorothiazide and propranolol for control of hypertension. Having Googled the rash, he is convinced that he has contracted Lyme disease.
Submit your diagnosis to see full explanation.
This patient was indeed afflicted with Lyme disease, a multisystem disorder transmitted by ticks. The disorder is the most commonly reported vector-borne illness in the United States and incidence is rising; in 2013, it was the fifth most common nationally notifiable disease.1
The characteristic rash is termed erythema migrans, and it arises early in the disease. However, this classic manifestation is present in only approximately 50% of cases among adults, and the majority are unaware of a prior tick bite.2,3
The rash itself is asymptomatic but may be accompanied by fever, malaise, and arthralgias. It begins as an erythematous papule that enlarges centrifugally and may attain a diameter greater than 15 cm.4 Differential diagnosis includes hypersensitivity reactions to insect bites, drug eruption, urticaria, and cellulitis.
In endemic areas, therapy should be initiated without serology in patients with classic erythema migrans and a history of tick exposure. Disease dissemination may be heralded by the presence of additional skin lesions, but these are usually of lesser diameter and may be atypical in appearance.5 Doxycycline at a dose of 100 mg twice daily is the recommended therapy for patients with first-stage disease (except for pregnant females and children aged less than 8 years); the therapy should be administered for a minimum of 2 weeks.6
Megha D. Patel is a student at the Commonwealth Medical College, Scranton, Pennsylvania.
Stephen Schleicher, MD, is an associate professor of medicine at the Commonwealth Medical College and an adjunct assistant professor of dermatology at the University of Pennsylvania Medical College. He practices dermatology in Hazleton, Pennsylvania.
- Lyme disease: Data and statistics. Centers for Disease Control and Prevention Web site. Updated March 4, 2015. Available at www.cdc.gov/lyme/stats
- Bacon RM, Kugeler KJ, Mead PS; Centers for Disease Control and Prevention. Surveillance for Lyme disease—United States, 1992-2006. MMWR Surveill Summ. 2008;57(10):1-9.
- Bhate C, Schwartz RA. Lyme disease: Part I. Advances and perspectives. J Am Acad Dermatol. 2011;64(4):619-636.
- Müllegger RR. Dermatological manifestations of Lyme borreliosis. Eur J Dermatol. 2004;14(5):296-309.
- Steere AC, Bartenhagen NH, Craft JE, et al. The early clinical manifestations of Lyme disease. Ann Intern Med. 1983;99(1):76-78.
- Bhate C, Schwartz RA. Lyme disease: Part II. Management and prevention. J Am Acad Dermatol. 2011;64(4):639-653.