MICROBIOLOGY AND EPIDEMIOLOGY

Borrelia burgdorferi, the causative agent of Lyme disease, is a fastidious microaerophilic spirochete. The human infection Lyme borreliosis is caused primarily by three pathogenic genospecies: B. burgdorferi sensu stricto (hereafter referred to as B. burgdorferi), Borrelia garinii, and Borrelia afzelii.

• B. burgdorferi is the sole cause of Lyme borreliosis in the United States; all three genospecies are found in Europe, and the latter two species occur in Asia.
• Lyme disease is the most common vector-borne illness in the United States, with ∼300,000 cases each year.
  • Ixodes ticks transmit the disease.
  • I. scapularis, which also transmits babesiosis and anaplasmosis, is found in northeastern and midwestern states; I. pacificus is found in western states.
• The white-footed mouse is the preferred host for larval and nymphal I. scapularis. Adult ticks prefer the white-tailed deer as host.
• Nymphal ticks transmit the disease to humans during the early summer months after feeding for ≥24 h.