Borrelia burgdorferi is transmitted from animals to humans by ticks belonging primarily to the Ixodes ricinus complex. Transmission occurs during feeding, either by salivation, regurgitation, or by both processes. Two Eurasian (Ixodes ricinus and I. persulcatus) and three North American (I. dammini, I. scapularis and I. pacificus) species vector B. burgdorferi to humans. All ticks feed on three different host animals during their lives, and each species parasitizes a relatively large number of different host animals. For example, Ixodes ricinus is known to feed on more than 300 different kinds of mammals, birds, and reptiles. Borrelia burgdorferi has been isolated or detected in 24 different species of mammals or birds. Rodents belonging to the genera Peromyscus and Apodemus are important reservoirs in North America and Europe, respectively. Birds are a natural means of distributing infected ticks long distances during their migratory flights. Reptiles may not support the growth of borreliae. The B31 strain of B. burgdorferi is predominant in north-eastern United States. It is the only strain known to cause disease in humans, and the only one thus far isolated from rodents. In Europe, variants with distinctly different proteins from those found in the B31 strain also are infectious to humans, through they have not been identified with any specific clinical syndrome of Lyme borreliosis. Variants of the B31 strain have been isolated from lagomorphs in the United States.