We compared the relative reservoir competence of European wood mice for two genospecies of Lyme disease spirochetes by analyzing susceptibility, intrinsic incubation period, and degree and duration of infectivity. Borrelia afzelii, specializing in particular reservoir hosts, is better adapted to those hosts than is the more generalist genospecies B. burgdorferi sensu stricto.Diverse Lyme disease spirochetes infect European vector ticks. The local distribution of genospecies varies. In seven sites in Germany and France that we have examined, Borrelia afzelii consistently infects ticks more frequently than does B. burgdorferi sensu stricto (14,17,18). For each tick infected by B. burgdorferi sensu stricto, more than six ticks are infected by B. afzelii.The composition of reservoir hosts in a site appears to affect the local genospecies distribution. B. afzelii is said to perpetuate mainly in rodents, whereas B. garinii is said to perpetuate in birds (7). No such host association, however, has been suggested for B. burgdorferi sensu stricto. Indeed, both murine and avian hosts are reservoir competent for B. burgdorferi sensu stricto in the northeastern United States (1, 19), where it is the sole genospecies that is pathogenic for humans. White-footed mice, Peromyscus leucopus, and American robins, Turdus migratorius, readily acquire B. burgdorferi sensu stricto infection from infected nymphal ticks, maintain the infection, and infect most ticks feeding on them. If both murine and avian hosts were similarly reservoir competent for B. burgdorferi sensu stricto in Central Europe, it would seem paradoxical that this variant is less prevalent in questing ticks than is B. afzelii, which parasitizes rodents but not birds. The differential reservoir competence of rodents for B. burgdorferi sensu stricto and B. afzelii remains unknown.It may be that European rodents are more competent reservoir hosts for B. afzelii than for B. burgdorferi sensu stricto. Accordingly, we analyzed the susceptibility, the intrinsic incubation period, and the degree and duration of infectivity of these spirochetes in rodents. In particular, we compared the competence of a natural European reservoir rodent, the wood mouse Apodemus sylvaticus, to that of an experimental host, the Mongolian jird Meriones unguiculatus.(Portions of this research were conducted in partial fulfillment of the requirements for a doctoral degree [to B.K.] from

To determine whether rabbits may serve as reservoir hosts for Lyme disease spirochaetes in Europe, we compared their competence as hosts for Borrelia afzelii, one of the most prevalent European spirochaetal variants, with that of the Mongolian jird. To infect rabbits or jirds, at least 3 nymphal or adult Ixodes ricinus ticks infected with spirochaetes fed to repletion on each animal. Whereas jirds readily acquired tick-borne Lyme disease spirochaetes and subsequently infected vector ticks, rabbits exposed to tick-borne spirochaetes rarely became infectious to ticks. Only the rabbit that was infectious to ticks developed an antibody response. To the extent that I. ricinus ticks feed on European rabbits, these mammals may be zooprophylactic by diverting vector ticks from more suitable reservoir competent hosts.

To define conditions promoting inherited infection by Lyme disease spirochetes in Ixodes ticks, we variously infected ticks with Borrelia afzelii and examined their progenies by dark-field microscopy, immunofluorescence, PCR, and serial passage. No episode of inherited infection was evident, regardless of instar or gender infected or frequency of exposure. We suggest that these spirochetes rarely, if ever, are inherited by vector ticks.