Clinicopathologic Characterization of Six Cases of Equine Granulocytic Anaplasmosis In a Nonendemic Area (2008-2011)

Six adult horses (four geldings and two intact mares; age range, 4-22 years) were evaluated for acute development of nonspecific malaise during a 3-year period. Clinical signs included lethargy, anorexia, fever, limb edema, and ataxia. Physical examination findings included depression, anorexia, tachycardia, fever, poor body condition, hind limb ataxia, and dehydration. Hematologic examination in these horses most commonly revealed thrombocytopenia, mild anemia, and leukopenia. Inclusion bodies consistent with the presence of Anaplasma phagocytophilum morulae were observed within circulating neutrophils in all horses. Clinical biochemistry findings were nonspecific. Infection of each horse with A. phagocytophilum was confirmed by polymerase chain reaction. All horses showed resolution of clinical signs after initiation of treatment with intravenous oxytetracycline or oral doxycycline, combined with flunixin meglumine and additional supportive care as needed. Hematologic parameters paralleled clinical recovery and returned to reference limits in patients that underwent repeated analysis. Equine granulocytic anaplasmosis is likely under-recognized in regions where it has not been previously reported; therefore, equine practitioners should be cognizant of relevant clinical signs and laboratory findings in acute infection. Additionally, horses may represent sentinels for infection in other species, including humans.