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Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis (HGA), shares the same enzootic life cycle as Borrelia burgdorferi, the causative agent of
Lyme disease. Although La Crosse, WI, is a well-recognized
Lyme disease focus with an abundance of Ixodes scapularis vector ticks and the first documentation of HGA occurred in patients from northwestern Wisconsin, local transmission of A. phagocytophilum has not to date been documented. In this study, we evaluated DNA extracted from 201 ticks captured locally by a real-time PCR that targeted a unique region within msp2, and 24 samples (12%) yielded positive results. The PCR also detected A. phagocytophilum DNA in blood samples obtained from 53 patients with clinical abnormalities consistent with HGA, and sequencing confirmed that the DNA was recovered from the Ap-ha variant of A. phagocytophilum, associated exclusively with human infection. The findings therefore confirmed that the upper Midwestern focus for HGA endemicity now includes the regions immediately surrounding La Crosse, WI. The results also validated the utility of the real-time msp2 PCR test for confirming acute HGA in the clinical setting.