Anaplasma phagocytophilum

Paul G. Auwaerter, M.D.


  • Cause of tick-borne infection in humans.
  • Human granulocytic anaplasmosis (HGA), formerly known as human granulocytic ehrlichiosis (HGE).
    • The organism is currently known as Anaplasma phagocytophilum, transmitted by Ixodes scapularis (black-legged deer tick) on the Eastern Seaboard and in New England, the Mid-Atlantic, and the Upper Midwest; and on the West Coast by the western black-legged tick (Ixodes pacificus)—the same vectors as for Lyme disease (Borrelia burgdorferi).
      • Anaplasma marginale infects cattle.
    • An obligate, intracellular pathogen that tends to multiply in granulocytes within vacuoles that may form morulae.
      • See Ehrlichia spp. module for disease (human monocytic ehrlichiosis [HME] and others) caused by similar tick-borne pathogens.
      • Occasionally seen as a Gram-negative organism upon staining.

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Last updated: December 11, 2022