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Background: Trypanosomes are protozoan parasites of vertebrates that are of medical and veterinary concern. A variety of blood-feeding invertebrates have been identified as vectors, but the role of ticks in trypanosome transmission remains unclear. Methods: In this study, we undertook extensive molecular screening for the presence and genetic diversity of trypanosomes in field ticks. Results: Examination of 1089 specimens belonging to 28 tick species from Europe and South America led to the identification of two new trypanosome strains. The prevalence may be as high as 4% in tick species such as the castor bean tick Ixodes ricinus, but we found no evidence of transovarial transmission. Further phylogenetic analyses based on 18S rRNA, EF1-α, hsp60 and hsp85 gene sequences revealed that different tick species, originating from different continents, often harbour phylogenetically related trypanosome strains and species. Most tick-associated trypanosomes cluster in a monophyletic clade, the Trypanosoma pestanai clade, distinct from clades of trypanosomes associated with transmission by other blood-feeding invertebrates. Conclusions: These observations suggest that ticks may be specific arthropod hosts for trypanosomes of the T. pestanai clade. Phylogenetic analyses provide further evidence that ticks may transmit these trypanosomes to a diversity of mammal species (including placental and marsupial species) on most continents. [ABSTRACT FROM AUTHOR]