Two Anaplasma phagocytophilum Strains in Ixodes

We developed PCR-based assays to distinguish a hu-man pathogenic strain of Anaplasma phagocytophilum, Ap-ha, from Ap-variant 1, a strain not associated with human infection. The assays were validated on A. phagocytophilum– infected black-legged ticks (Ixodes scapularis) collected in Canada. The relative prevalence of these 2 strains in I. scapularis ticks differed among geographic regions. The gram-negative bacterium Anaplasma phagocytophi-lum is the causative agent of human granulocytic ana-plasmosis (HGA) in the United States (1). More than 90% of HGA cases occur in the Upper Midwest and Northeast (2). In these regions, black-legged ticks (Ixodes scapularis) are the vectors of a human pathogenic strain (Ap-ha) and a variant strain (Ap-variant 1) of A. phagocytophilum (3–7), the latter of which appears not to be associated with hu-man infection (1,3). HGA represents an emerging diseas