Equine Granulocytic Anaplasmosis (EGA) is caused by infection with the rickettsial organism Anaplasma phagocytophilum (formerly Ehrlichia equi). Anaplasmosis is a common, seasonal, non-contagious, ticktransmitted disease of equids observed where the Ixodes tick vector is found. Ixodes scapularis is found in the eastern half of the United States, while Ixodes pacificus is found along the entire west coast. Equine anaplasmosis has been reported in the United States, Canada, South America, Europe, Asia, and North Africa.

Risk Factors

Exposure to Ixodes ticks harboring A. phagocytophilum is the primary risk factor for developing disease. Horses visiting or introduced into an endemic area are more likely to develop EGA than native horses, likely attributable to ongoing exposure and protective immune response.

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Resource Type

  • Guideline


  • Equine Granulocytic Anaplasmosis (EGA)
  • Infectious Diseases

Publish Date

August 14, 2023