Prevalence of avian-associated Borrelia burgdorferi s.l. genospecies in Ixodes ricinus ticks collected from blackbirds (Turdus merula) and song thrushes (T. philomelos)

Between May and September of 2002, forest passerine birds and immature Ixodes ricinus ticks infesting them were surveyed for infection with Borrelia burgdorferi s.l. in a sylvatic habitat in west-central Poland. A total of 738 feeding I. ricinus ticks were recovered from 114 birds belonging to 9 species. The majority of ticks (65.7%) were collected from two thrush species, the blackbird (Turdus merula) and the song thrush (T. philomelos). Since only the two Turdus species proved to be parasitized by Borrelia-infected ticks, this study focused mainly on these birds. Immature ticks were removed from 53 of the 54 captured thrushes. A total of 304 partially or fully engorged ticks (110 larvae and 194 nymphs), and 53 bird-derived blood samples were tested for borreliae. B. burgdorferi s.l. DNA was detected by PCR in 11% of the larval ticks and in 7.2% of the nymphs. The PCR-positive ticks were found on 8 of 53 tick-infested thrushes. B. garinii and B. valaisiana accounted for 88.5% and 11.5% of the infections detected in Borrelia-positive ticks, respectively. Only one out of 53 of Turdus blood samples were PCR-positive for borreliae. The blood-positive thrush was infected with B. burgdorferi s.s. B. afzelii was the most frequently identified genospecies in PCR-positive questing I. ricinus ticks, followed by B. valaisiana (89% and 11%, respectively). The absence of B. afzelii infection in ticks feeding on passerine birds, despite its strong predominance in local questing nymph populations, indicates that avian hosts are not reservoirs of this genospecies and consequently that the rodent-adapted B. afzelii is negatively selected in ticks feeding on these hosts. These results validate the concept of some avian-associated genospecies within the B. burgdorferi s.l. complex and demonstrate that thrush species may support the circulation of B. garinii and B. valaisiana under natural conditions.