| Summary: | <i>Anaplasma phagocytophilum</i> is the causative agent of tick-borne fever (TBF) and human granulocytic anaplasmosis (HGA) and is currently considered an emerging disease in the USA, Europe, and Asia. The increased prevalence of <i>A. phagocytophilum</i> as a human pathogen requires the detailed characterization of human isolates and the implementation of appropriate animal models. In this study, we demonstrated that the dynamics of infection with the human isolate of <i>A. phagocytophilum</i> NY-18 was variable in three different strains of mice (SCID, C3H/HeN, BALB/c). We further evaluated the ability of <i>Ixodes ricinus</i> to acquire and transmit <i>A. phagocytophilum</i> NY-18 and compared it with <i>Ixodes scapularis</i>. Larvae of both tick species effectively acquired the pathogen while feeding on infected mice. The infection rates then decreased during the development to nymphs. Interestingly, molted <i>I. ricinus</i> nymphs were unable to transmit the pathogen to naïve mice, which contrasted with <i>I. scapularis</i>. The results of our study suggest that <i>I. ricinus</i> is not a competent vector for the American human <i>Anaplasma</i> isolate. Further studies are needed to establish reliable transmission models for <i>I. ricinus</i> and European human isolate(s) of <i>A. phagocytophilum</i>.
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