Fine structural evidence for the penetration of the Lyme disease spirochete Borrelia burgdorferi through the gut and salivary tissues of Ixodes dammini

The route followed by the Lyme disease spirochete Borrelia burgdorferi as it passes from vector to host has been the subject of controversy over whether the spirochete is transmitted through the saliva of the tick or through regurgitation during feeding. In the event that the spirochete's presence in the salivary tissues is transient we employed a detailed electron microscopic study spanning the period of nymphal attachment to the host to determine whether B. burgdorferi invades the salivary acini and ducts. In addition we examined other tissues of the tick to determine the route and mode of migration. Two different groups of nymphs were used in this study. After feeding, spirochetes were found in both groups in the gut lumen, epithelium, and within the salivary glands and ducts. Borrelia is able to pass both inter- and intra-cellularly through these tissues. In one group of unfed nymphs Borrelia was limited to the gut lumen, whereas the second group demonstrated a disseminated infection. This difference might be due to transovarial transmission, the geographic origin, and (or) age of the ticks. The finding of Borrelia within the salivary glands and ducts provides strong evidence for the salivary transmission of the Lyme disease spirochete.