Life cycle of Borrelia burgdorferi sensu lato and transmission to humans.

Lyme borreliosis is a zoonosis: its causative agent, Borrelia burgdorferi sensu lato, circulates between Ixodes ricinus ticks and a large variety of vertebrates. I. ricinus has a wide geographical distribution throughout Europe within the latitudes of 65 degrees and 39 degrees and from Portugal into Russia. Enzootic cycles in Europe involve at least 7 Borrelia species. Apparently, associations exist in nature between Borrelia species and hosts. B. afzelii and B. burgdorferi sensu stricto are associated with rodents, and B. garinii and B. valaisiana with birds. B. lusitaniae may be transmitted to ticks by some lizard species and birds. B. spielmanii appears to be associated with dormice and hedgehogs. Less strict associations also exist. Transmission of Borrelia infection by I. ricinus to their hosts, including humans, does not occur immediately when ticks attach to host skin. A delay is observed, which may depend on the Borrelia species infecting the tick. B. afzelii can be transmitted during the first 24 h, whereas B. burgdorferi needs 48 h of tick attachment before its transmission begins. Nothing is known about the other Borrelia species; however, success of transmission always increases with tick attachment duration. Therefore, careful visual examinations of the body for at least 2 successive days are recommended after visiting an endemic area.