This thesis focuses on the emergence and establishment of equine tick-borne infections in the Netherlands, with particular attention to their diagnosis, clinical relevance and treatment. Four tick-borne agents (Borrelia burgdorferi, Theileria equi, Babesia caballi and Anaplasma phagocytophilum) appeared to be present in horses in the Netherlands, and a study was performed to examine whether they were present in horses with fever of unknown origin (FUO). Sixty-one horses were tested for the presence of the tick-borne agents by stained blood-smear examination and PCR-RLB, after ruling out other possible causes of FUO. It transpired that six horses (10%) were acutely infected with Anaplasma phagocytophilum, as evidenced by a positive PCR-RLB in 6/6 and a positive stained blood smear in 5/6 of the horses. This study also indicated that stained blood-smear examination is a sensitive and practical tool for diagnosing Anaplasma infection in pyrexic horses. In an attempt to prospectively investigate whether horses in the Netherlands were at risk of developing tick-related disease after a tick-bite, 47 recreational horses were examined after removal of a feeding tick, with follow-ups over 9-23 months, using serological and molecular tests for the presence of tick-borne agents. Although many horses were infected with tick-borne agents, associated clinical disease was rare, with the exception of the low thrombocyte counts that were associated with Anaplasma seropositivity. It was concluded that most tick-borne infections in horses in the Netherlands run a subclinical course, at least in recreational horses. When assessing the prevalence of Babesia caballi and Theileira equi infections (Piroplasmosis) in horses, it was shown that the risk of this so-called “exotic disease” is increasing in the Netherlands. This is presumably associated with the recent establishment of its vector, Dermacentor reticulatus, as an indigenous tick, and with the unrestricted importation of horses from surrounding piroplasmosis endemic areas. ...