The Lyme disease causing spirochaete Borrelia burgdorferi sensu lato is transmitted by ticks within the genus Ixodes. These ticks are liberal host seekers and parasitise mammals, birds and reptiles. Prior to this study, the distribution of I. ricinus ticks and Lyme disease was thought to be restricted to the southern half of Sweden. On the island Norrbyskär, located in the Bothnian Gulf, there were reports of a high incidence of tick infestation on humans. To investigate the occurrence of B. burgdorferi s.l. in these ticks and to characterise presumptive isolates at the molecular level we sampled a number of I. ricinus ticks. Three different isolates were obtained from two different ticks, NBS16 from a nymph and NBS23a and NBS23b from an adult female tick. The seabird associated tick I. uriae is circumpolar distributed in both hemispheres. On the island Bonden, which house one of the largest seabird colonies in the Baltic Sea, I. uriae were collected and surveyed for spirochaetes. One isolate of B. burgdorferi s.l. was obtained. This B. burgdorferi s.l. isolate is identical to the Lyme disease Borrelia strain NBS16 isolated from Norrbyskär. To investigate the role of seabirds in the epidemiology of B. burgdorferi s.l., I. uriae were collected from seabird colonies in the southern and northern hemispheres. Borrelia DNA was extracted from the ticks and from cultured spirochaetes. Sequence analysis of the flagellin gene revealed that the DNA obtained was from B. garinii, regardless of the geographical origin of the sample. Identical fla gene fragments in ticks collected in both hemispheres indicate a transhemispheric exchange of B. garinii. A marine ecological niche and epidemiological route for Lyme disease Borrelia are proposed. The prevalence of B. burgdorferi s.l. infected ticks on migrating passerine birds was studied. A total of 22, 998 birds were caught and examined for ticks. The presence of spirochaetes in the 967 collected ticks was determined by DNA amplification by PCR on all ticks. To determine which B. burgdorferi s.l. species were present, classification was performed by DNA amplification using species-specific 16S rDNA primers and by DNA sequencing. Flagellin gene sequences of all species of B. burgdorferi s.l. previously recorded in Europe were found. B. garinii was the most prevalent. These data support the notion that passerine birds are at least partly responsible for the distribution of Lyme disease Borrelia spirochaetes in Europe. To elucidate the distribution of B. burgdorferi s.l. in subarctic regions, strains isolated from I. ricinus and I. uriae ticks found on islands in the northern Atlantic and Baltic Sea were characterised molecularly. All isolates were verified as B. garinii by 16S-rRNA gene analysis and immunoblotting using monoclonal antibodies specific for the outer surface proteins A and C. Three ribotypes (RT's) of B. garinii were found. The I. ricinus associated RT1 is phenotypically the most heterogeneous. RT2 is restricted to the islands in the northern Baltic Sea, whereas RT3 was also recovered from ticks found on islands in the North Atlantic. The heterogeneity of the B. garinii population in the Baltic Sea might be influenced by two geographically opposite directions, North Atlantic (RT3) and Euroasia (RT1). / digitalisering@umu.se