HGA is caused by Anaplasma phagocytophilum and, in the United States, occurs mainly in northeastern and upper midwestern states.

• The geographic distribution is similar to that of Lyme disease and Babesia microti infection, given the shared Ixodes scapularis tick vector.
• HGA incidence peaks in May through July, but the disease can occur year-round.
• The epidemiology of HGA is similar to that of HME, with males (59%) and older persons (median age, 51 years) more often affected.