Equine granulocytic anaplasmosis (EGA) is a seasonal rickettsial disease of horses transmitted by Ixodes spp. ticks. The etiological agent is Anaplasma phagocytophilum, a coccobacillary gram-negative organism with a tropism for granulocytes. Clinical manifestations include fever, partial anorexia, depression, distal limb edema, petechiation, icterus, ataxia, and reluctance to move. Hematologic changes observed are thrombocytopenia, decreased packed-cell volume and marked leukopenia involving first lymphocytes and then granulocytes. Diagnosis is based on awareness of geographic area for infection, typical clinical signs, abnormal laboratory findings, and visualization of characteristic morulae in the cytoplasm of neutrophils and eosinophils in a peripheral blood smear. Treatment consists of the administration of tetracycline. The disease is being diagnosed with increasing frequency in the United States, Canada, Brazil and northern Europe.