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The enzootic cycle of Borrelia burgdorferi, the etiologic agent of
Lyme disease, involves Ixodes spp. ticks and vertebrates. Resident tick Borrelia, harbored inside the midgut, are eventually expelled with the tick’s saliva into the vertebrate host when a tick consumes a blood meal. During this 4- to 5-day feeding period I. scapularis will defecate onto the host’s skin. Previously we detected borrelial DNA in tick feces throughout engorgement. In this study we report the microscopic examination for B. burgdorferi in nymphal excrement. Using immunofluorescence assays, we observed Borrelia in all mouse skin and capsule fecal swabs tested, although we could not culture the spirochetes. These results update our previous analysis by revealing that spirochetes can also be visualized in tick excrement. Furthermore, the results emphasize that borrelial contamination by defecation is a possibility, and that caution should be exercised by researchers investigating pathogen/host/vector interactions. The biological significance of the presence of non-culturable Borrelia in tick feces during engorgement is unclear.