Transmission and Epidemiology

Figure 1: Estimated geographic distribution of blacklegged ticks (above) and western blacklegged ticks (below)

Approximate distribution of the Blacklegged tick in the United States of America
Map of the United States showing the approximate distribution of the Western blacklegged tick.  The area affected is the western coast.

Anaplasmosis, also known as human granulocytic anaplasmosis, is a tickborne disease caused by the bacterium Anaplasma phagocytophilum.


Tick Bites

Less commonly, through blood transfusion and organ transplant


  • Figure 2: Incidence (per million population) for anaplasmosis, United States, 2017.

    Map of the United States that shows the incidence of anaplasmosis cases by state in 2016 per million persons. Anaplasmosis was not notifiable in Alaska, Colorado, the District of Columbia, Hawaii, Idaho, or New Mexico in 2016. The incidence rate was zero for Arizona, Georgia, Indiana, Louisiana, Mississippi, Montana, Nevada, South Carolina, Utah, Washington, and West Virginia. Incidence ranged from > 0 to 3.0 cases per million persons in California, Kentucky, Oregon, Florida, Ohio, Illinois, Michigan, Oklahoma, Nebraska, Alabama, Maryland, South Dakota, Kansas, North Carolina, Wyoming, Tennessee, Virginia, Missouri, and Iowa. Incidence ranged from > 3 to 26 cases per million persons in Delaware, Pennsylvania, Arkansas, New Jersey and North Dakota. Incidence ranged from > 26 to 130 cases per million persons in Connecticut, New York, New Hampshire, Wisconsin, and Massachusetts. The highest incidence rates, greater than 130 cases per million persons, were found in Minnesota, Rhode Island, Maine, and Vermont.

    Geographic range: Anaplasmosis is most commonly reported in the Northeastern and upper Midwestern states, although the geographic range of anaplasmosis cases appears to be expanding.

  • Peak transmission: Most cases occur during June–November. Two peaks of increased case reporting usually occur, with the first peak during June–July and a smaller peak during October-November.