Ticks have three lifecycle stages that can bite humans – larva, nymph and adult –, but bacteria are usually transmitted through bites from nymphs, which are higher in density and often go unnoticed because of their small size.
The amount of time a tick must remain attached to transmit bacteria to the vertebrate host is an essential parameter in assessing the risk of transmission and identifying measures to prevent infection. It is generally accepted that the longer a tick remains attached, the higher the risk of transmission. In Europe, it is regularly stated that there is a real risk of transmission only after 24 hours of attachment.
In this study, we used a mouse model to determine the kinetics of infection by Ixodes ricinusticks (nymphs and adult females) infected with various European and North American strains or species of Borrelia. We also compared the dissemination of various strains and species of Borrelia by different modes of inoculation (via infected ticks or by injection of bacteria).
Unlike the American strains, all the European species of B. burgdorferi that we studied were detected in the salivary glands of adult ticks before a blood meal, suggesting the possibility of rapid transmission of the bacteria following a bite. The results were consistent with this theory: infection occurred within 24 hours of a bite from an adult tick. Moreover, our analysis shows that nymphs infected by European species of B. burgdorferi are capable of transmitting these pathogens within 12 hours of attachment. Our study proves that B.burgdorferi can be transmitted more quickly than stated in the literature. It is therefore vital to remove ticks as soon as possible after being bitten to prevent infection.
Furthermore, the study shows that the tropism of Borrelia varies depending on the strain and species studied, which explains the variety of clinical manifestations of Lyme borreliosis. We also demonstrate a difference in the tropism of Borrelia following a tick bite, confirming the role of tick saliva in the efficacy of infection and dissemination in vertebrate hosts.