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In Britain, grey squirrels (Sciurus carolinensis Gmelin) and pheasants (Phasianus colchicus Linnaeus) are important hosts of larvae and nymphs of Ixodes ricinus L., the principal European vector of the
Lyme disease spirochaete, Borrelia burgdorferi sensu lato. To test whether squirrels are competent hosts of B. burgdorferi s.l., three females were trapped in the wild and then held in captivity. Following treatment, each animal was exposed to uninfected xenodiagnostic I. ricinus ticks. Squirrel A (an adult) which was inoculated experimentally with B. burgdorferi s.l., transmitted the infection to xenodiagnostic ticks. In contrast, squirrel B (a juvenile that was not inoculated)-showed no evidence of infection. Xenodiagnostic ticks that fed on control squirrel C (an adult) became infected and subsequently transmitted the infection experimentally to an uninfected hamster. The results indicated that squirrel C had a disseminated infection acquired in the wild and which persisted for at least 11 weeks. These data clearly demonstrate that grey squirrels are amplifying and reservoir hosts of B. burgdorferi s.l. The strain associated with squirrels was related to the B. afzelii genotype. Two observations implicated pheasants in a similar role: (i) a high prevalence of infection in engorged larvae collected from trapped pheasants, and (ii) the detection of B. burgdorferi s.l. (B. garinii genotype) in the wattle of 1/10 pheasants using PCR. Xenodiagnostic experiments similar to those undertaken with the squirrels are needed to confirm the role of pheasants in the transmission cycle of
Lyme disease spirochaetes.