CHARACTERIZATION OF A SPOTTED-FEVER GROUP RICKETTSIA FROM IXODES-RICINUS TICKS IN SWEDEN

A spotted fever group rickettsia isolated from the common tick, Ixodes ricinus, was genetically characterized by PCR and genomic sequencing. This study was performed with nymphal and adult ticks collected in so uthern and central Sweden. I. ricinus is the only North European tick species of medical importance which is regularly collected from humans . No species of the genus Rickettsia has previously been found in Scan dinavian ticks, nor has any case of domestic rickettsial infection in humans or animals been reported, According to the nucleotide sequencin g, the present Rickettsia sp. belongs to the spotted fever group of ri ckettsiae. Ticks are the most common arthropod reservoirs and vectors of the rickettsiae of this group. Among 748 ticks investigated, 13 (1. 7%) were positive for a Rickettsia sp. Borrelia burgdorferi was detect ed in 52 (7%) of the ticks, a prevalence similar to or somewhat lower than that previously been recorded in other Swedish studies. There was no evidence of ehrlichial or chlamydial DNA in these ticks. The Ricke ttsia sp. was further characterized by 16S ribosomal DNA (rDNA) sequen cing and restriction fragment length polymorphism (RFLP). The 168 rDNA sequencing resulted in a sequence identical to that described for Ric kettsia helvetica, but the pattern obtained with RFLP of the citrate s ynthetase gene diverged from previously known patterns. The rickettsia l agent of one tick which was positive by PCR was confirmed by transmi ssion electron microscopy. The morphology of this rickettsia was simil ar to that of the spotted fever and typhus group rickettsiae, This rep resents the first documented isolate of a Rickettsia sp. from Swedish ticks.