• Tick-borne human infection presenting as a nonspecific febrile illness
  • Several forms of ehrlichiosis exist; 2 predominate in North America
    • Human monocytic ehrlichiosis (HME), first described in late 1980s:
      • Vector tick: Amblyomma americanum (lone star tick)
      • Geographic range: Central, southern, and mid-Atlantic states, with range expanding to parts of New England
    • Human granulocytic ehrlichiosis or human granulocytic anaplasmosis (HGE or HGA), first described in 1994:
      • Vector tick: Ixodes scapularis (deer tick)
      • Geographic range: East Coast, mid-Central States, and Pacific Northwest (same areas as Lyme disease which is more common in the U.S. than HME)
    • A third type caused by Ehrlichia ewingii was discovered in 1999
  • All are tick borne but have different vectors and geographic ranges. Other species have been reported, but at present HME and HGE are the important ehrlichial human pathogens


  • 2 distinct species of obligate intracellular organisms
  • The taxonomy of these pathogens has changed in recent years as more DNA and ribosomal RNA data become available
  • HME is caused by the organism Ehrlichia chaffeensis
  • HGE/HGA is caused by Anaplasma phagocytophila (a new name as of 2002)
  • The vasculitis found in Rocky Mountain spotted fever (RMSF) is usually not present
  • A third type may also be encountered, caused by Ehrlichia ewingii, which has the tick vector of the lone star tick. Clinically similar to HME
  • Compared with RMSF, older individuals are usually affected, commonly >40 yr of age

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