Lyme borreliosis is caused by a spirochete, Borrelia burgdorferi sensu lato, that is transmitted by ticks of the Ixodes ricinus complex. The infection usually begins with a characteristic expanding skin lesion, erythema migrans (EM; stage 1, localized infection). After several days or weeks, the spirochete may spread to many different sites (stage 2, disseminated infection). Possible manifestations of disseminated infection include secondary annular skin lesions, meningitis, cranial neuritis, radiculoneuritis, peripheral neuritis, carditis, atrioventricular nodal block, or migratory musculoskeletal pain. Months or years later (usually after periods of latent infection), intermittent or persistent arthritis, chronic encephalopathy or polyneuropathy, or acrodermatitis may develop (stage 3, persistent infection). Most patients experience early symptoms of the illness during the summer, but the infection may not become symptomatic until it progresses to stage 2 or 3.
Lyme disease was recognized as a separate entity in 1976 because of a geographic cluster of children in Lyme, Connecticut, who were thought to have juvenile rheumatoid arthritis. It became apparent that Lyme disease was a multisystemic illness that affected primarily the skin, nervous system, heart, and joints. Epidemiologic studies of patients with EM implicated certain Ixodes ticks as vectors of the disease. Early in the twentieth century, EM had been described in Europe and attributed to I. ricinus tick bites. In 1982, a previously unrecognized spirochete, now called Borrelia burgdorferi, was recovered from Ixodes scapularis ticks and then from patients with Lyme disease. The entity is now called Lyme disease or Lyme borreliosis.
B. burgdorferi, the causative agent of Lyme disease, is a fastidious microaerophilic bacterium. The spirochete’s genome is quite small (~1.5 Mb) and consists of a highly unusual linear chromosome of 950 kb as well as 17–21 linear and circular plasmids. The most remarkable aspect of the B. burgdorferi genome is that there are sequences for more than 100 known or predicted lipoproteins—a larger number than in any other organism. The spirochete has few proteins with biosynthetic activity and depends on its host for most of its nutritional requirements. It has no sequences for recognizable toxins.
Currently, 13 closely related borrelial species are collectively referred to as Borrelia burgdorferi sensu lato (i.e., “B. burgdorferi in the general sense”). The human infection Lyme borreliosis is caused primarily by three pathogenic genospecies: B. burgdorferi sensu stricto (“B. burgdorferi in the strict sense,” hereafter referred to simply as B. burgdorferi), Borrelia garinii, and Borrelia afzelii. B. burgdorferi is the sole cause of the infection in the United States; all three genospecies are found in Europe, and the latter two species occur in Asia.
Strains of B. burgdorferi have been subdivided according to several typing schemes: one based on sequence variation of outer-surface protein C (OspC), a second based on differences in the 16S–23S rRNA intergenic spacer region (RST or IGS), and a third ...