AbstractBackgroundAnaplasma phagocytophilum, the causative agent of granulocytic anaplasmosis, affects several species of wild and domesticated mammals, including horses. We used direct and indirect methods to compare and evaluate exposure toA. phagocytophilumin horses in northern Tunisia.MethodsSerum from 60 horses was tested by IFA for antibodies toA. phagocytophilum, and whole blood was tested forA. phagocytophilum16S rRNA gene using a nested-PCR. To examine the risk ofA. phagocytophilumtransmission, 154 ticks that had been collected from horses were examined for the presence ofA. phagocytophilumby nested-PCR targeting 16S rRNA gene.ResultsThis is the first time thatA. phagocytophilumhas been detected in horses in Tunisia, with an overall seroprevalence of 40/60 (67%). Six of the seroreactive samples (10%) had an IFA titer of 1:80, 14 (23%) of 1:160, 8 (13%) of 1:320 and 12 (20%) a titer 1 ≥ 640. The seroprevalence revealed no significant regional and sex differences. In contrast, a significant difference was observed between breeds. Eight (13%) of the horses were positive forA. phagocytophilumin the PCR, with no significant breed and age differences.Hyalomma marginatumwas a predominant tick species (130/154),and 3 were infected byA. phagocytophilum(a prevalence of 2.3%). The concordance rate ofA. phagocytophilumdetection between IFA and PCR had akvalue of −0.07.ConclusionsThe results presented in this study suggest that horses infested by ticks in Tunisia are exposed toA. phagocytophilum.