Host exchange of Lyme disease borreliae during tick feeding

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Lyme disease is a newly recognized systemic infection caused by spirochetes of the genus Borrelia. In this study, the host exchange of borreliae during tick feeding process was examined histologically by using Borrelia garinii, Ixodes persulcatus ticks, and mice. In infected murine skin, the number of borreliae was extremely low ; however, they seemed to be wrapped around collagen fibers in dermis. The number of borreliae in the murine skin was estimated at<1000 per 10 mg by the nested PCR analysis using flagellin gene-based primers. During tick feeding process, borreliae in murine skin were sucked into larval ticks and transstadially transmitted to nymphal ticks through the molting process. In unfed infected nymphs, borreliae were localized in their midgut lumen. The dynamics of borrelial migration in feeding ticks was unclear ; however, borreliae gradually increased in the salivary glands, supporting a hypothesis of salivary transmission of borreliae. The number of borreliae in salivary glands of 48 hours-fed nymphs was estimated at<10 cells per tick by the fluorescent antibody technique. This small number of borreliae was sufficient to induce the murine infection. The mechanism of midgut penetration and salivary invasion of borreliae in ticks still remains to be elucidated. In the future, the transformation of borreliae from the silent form to the infective form during the feeding process of ticks should be examined.