Lyme disease is the most common vector-borne infection in the United States. In 1997, there were 12,801 cases reported in the United States. It is transmitted by ticks from the genus Ixodes. The Ixodes tick goes through a 2-year life cycle that is composed of three stages: larva, nymph, and adult. Tick larvae acquire the spirochete via a blood meal from an infected host. Both the nymph and female adult infect humans.
A tick must be attached for at least 24 h to transmit the spirochete. Tick engorgement and attachment for 72 h are predictors of subsequent human infection. Ixodes ticks in the northeastern and midwestern United States belong to the Ixodes dammini (scapularis) species, in the western United States to Ixodes pacificus, in Europe to Ixodes ricinus, and in Asia to Ixodes persulcatus. Rodents and small mammals are the natural hosts of the larval and nymphal stages.
The incidence of Lyme disease reflects a changing dynamic between the principal reservoir, the white-footed mouse, its food supply, and the suitability of its local habitat. Deer, horses, dogs, and other larger mammals and birds may be occasional hosts to the adult ticks. Most cases have their onset during summer and occur in association with hiking, camping, and residence in wooded, rural, or coastal areas.