If a tick can infect a human with the bacteria that causes Lyme disease as soon as it bites

A common held belief is that there is no risk of infection if a tick is removed within 24 hours, but infection can occur if the tick feeds for more than 24 hours. This information is still controversial.

Several studies support the advice that you should remove the tick as soon as possible to decrease the risk of infection, but 24 hours is not a fixed magic time period when infection suddenly begins.

A literature review completed on this subject identified that laboratory studies report that transmission can occur in less than 16 hours but that the minimum attachment time for transmission has never been established (50).

Firstly, it is important to highlight that the time needed for the bacteria to be transmitted from the tick into the host depends mainly on the tick species and the bacteria involved. There are some evident differences between studies from the U.S. and studies from Europe. In the U.S. the bacteria that causes Lyme disease is Borrelia burgdorferi s. s. and the vector is I. scapularis. In Europe there are different species of Borrelia that can cause disease and the main vector is I. ricinus (51, 52).

Piesman et al. (53) conducted experimental studies of transmission of Borrelia burgdorferi s. s. in rodents and reported that 7% of the rodents became infected in less than 24 hours and 33% in less than 48 hours.

Kahl et al. (51) reported that 57% of Mongolian gerbils used in their experiments became infected in less than 16 hours, 53% in less than 24 hours and 100% in less than 48 hours. In the study by Crippa et al. (52) the proportion of infected mice in less than 24 hours varied with the method of infection, the number of mice used and the species of Borrelia, with a maximum of 67% infected mice in less than 24 hours.

These and other studies concluded that the longer the tick is attached, the higher the risk of infection and up to 100% chance of infection after 48 hours of tick attachment (51,53,54).