Transport of Ixodes ricinus infected with Borrelia species to Norway by northward-migrating passerine birds

Abstract

Birds are capable of transporting ticks and, consequently, tick-borne pathogens over long distances and across geographical barriers such as oceans and deserts. The purpose of this study was to assess the prevalence of Borrelia spp. in ticks transported by birds by using PCR. A total of 9768 northward-migrating passerine birds was examined for ticks at 4 bird observatories along the southern Norwegian coast during their spring migration in 2003–2005. Two of the bird observatories were located on islands where flagging revealed very few or no ticks (Akerøya and Store Færder), while the other 2 were located in areas with established dense tick populations: an island, Jomfruland (>100 ticks per hour of flagging) and a mainland locality, Lista (40 ticks in one hour of flagging). Borrelia spp. were found in 70 (13.6%) of 513 examined Ixodes ricinus nymphs (19 B. afzelii, 38 B. garinii, two B. turdi, and 11 B. valaisiana) and in 14 (8.1%) of 172 examined I. ricinus larvae (ten B. garinii, one B. turdi, and three B. valaisiana). This report is the first to identify B. turdi in Europe. Ticks collected from birds of the genus Turdus ( T. merula, T. philomelos, and T. iliacus) had a higher prevalence of Borrelia spp. than ticks from the other passerine genera. Ticks that were cofeeding with a Borrelia-infected tick had an increased probability of being infected with the same Borrelia species. Ticks collected on birds from the south-western locality Lista were less likely to have Borrelia than ticks found on birds from the other, more eastern localities. The Turdus spp. are particularly important, both because they carry many ticks per bird and because ticks carried by these species have a higher prevalence of Borrelia. This higher prevalence may be related to Borrelia infection of the birds or transmission of Borrelia through cofeeding. The prevalence of the different Borrelia species in ticks collected from migratory birds may be related to migration routes.