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To determine by xenodiagnosis length and concentrations of spirochetemias produced by Borrelia burgdorferi in white-footed mice (Peromyscus leucopus), laboratory reared mice were inoculated with either spirochete-containing tick suspensions or BSK II spirochete culture and were exposed for as long as three months to larval Ixodes dammini. Upon development to the nymphal stage, ticks were evaluated for spirochetal infections by direct immunofluorescence. All mice were found to circulate spirochetes for at least three months in concentrations sufficient to infect ticks. The percentage of infected ticks alternated from low to high, suggesting occurrence of episodes of mild and heavy spirochetemias. The results suggest that B. burgdorferi in its animal hosts and possibly also in humans causes prolonged spirochetemias characterized by episodes of alternating high and low concentrations of spirochetes as reflected by similar percentages of infected ticks. The long persistence of spirochetes in the peripheral blood stream and the cyclical form of
Lyme borreliosis appear to be related, as in relapsing fevers, to the capacity of B. burgdorferi to undergo antigenic variations.