L yme disease, or Lyme borreliosis, is the most common tickborne disease in both the United States and Europe; an estimated ≈476,000 cases are diagnosed and treated per year in the United States and >200,000 cases per year in western Europe (1-3). The principal tick vector in the United States is Ixodes scapularis, followed by I. pacifi cus; in Europe, most cases are transmitted by I. ricinus, followed by I. persulcatus ticks (Table 1). The etiologic agent, Borrelia burgdorferi, was discovered in 1982 in the United States. Later, it became recognized that strains of B. burgdorferi in Europe were more heterogenous than strains in North America. B. burgdorferi sensu lato was then classifi ed into 3 main genospecies. The originally discovered genospecies was named B. burgdorferi sensu stricto.The second genospecies was named Borrelia garinii sp. nov., and the third was named Borrelia afzelii sp. nov. Recently, the taxonomy of the family Borreliaceae (and the genus Borrelia) has been revised into 2 main genera, Borrelia and Borreliella (4). The spirochetes that cause relapsing fever retained the genus name Borrelia, and spirochetes that cause Lyme disease have been renamed Borreliella (hereafter referred to as Lyme borrelia). However, these changes have been challenged (5).Most cases of Lyme disease in the United States occur in the mid-Atlantic, Northeast, and Upper Midwest regions. B. burgdorferi s.s., which also is found in Europe, causes most human infections in the United States (1,2); the newly recognized species B. mayonii (which is not known to exist in Europe) is an infrequent cause of human illness in the Upper Midwest region of the United States (6). The incidence of Lyme disease in Europe is highest in the Scandinavian and Baltic states in northern Europe and in Austria, the Czech Republic, Germany, and Slovenia in central Europe. B. afzelii and B. garinii are the genospecies most frequently detected in I. ricinus and I. persulcatus ticks and cause most cases of Lyme disease in Europe (1,2). Neither genospecies is found in the United States. Transmission of B. burgdorferi s.s. by I. scapularis or by I. pacifi cus ticks is very infrequent during the fi rst 36 hours after tick attachment; in contrast, transmission of B. afzelii by I. ricinus ticks may occur within 24 hours (Table 1) (7).
Erythema Migrans and Other Skin ManifestationsAfter Lyme borrelia are deposited in the skin by the bite of an infected Ixodes tick, an infection is typically established at that site, which causes the characteristic skin lesion, erythema migrans (Figure 1). Erythema migrans is the most common clinical manifestation of Lyme disease in the United States and Europe, occurring in >80% of patients in both geographic areas (2). Overall, US patients with erythema migrans caused by B. burgdorferi s.s. are less likely than patients in Europe with erythema migrans caused by B. afzelii or B. garinii to remember a tick bite at the site of the lesion (25% vs. 60% for B. afzelii or 64% for B. garinii) but more likely to have concomitant ...