Acute clinical, hematologic, serologic, and polymerase chain reaction findings in horses experimentally infected with a European strain of Anaplasma phagocytophilum

Six horses were experimentally infected by administration of horse blood containing a Swedish strain of Anaplasma phagocytophilum. The polymerase chain reaction (PCR) signal was consistently detected 2-3 days before appearance of clinical signs and persisted 4-9 days beyond abatement of clinical signs, whereas diagnostic inclusion bodies were 1st noted on average 2.6 +/- 1.5 (SD) days after onset of fever. Clinical signs and hematologic changes were largely indistinguishable from those previously reported for diseases caused by A phagocytophilum (formerly Ehrlichia equi--"Californian agent") and the human-derived human granulocytic ehrlichiosis agent. Horses 1st demonstrated antibody response 12-16 days after inoculation, 2 cases of which were still febrile, and serotiters rapidly peaked within 3-7 days of clinical illness. One horse died during the acute stage of disease, but initial clinical signs and hematologic changes were similar to those of other infected horses. This report shows that, despite minor genetic differences, a European equine-derived strain of A. phagocytophilum may be similar in pathogenicity to the Californian agent. The PCR used holds promise to widen the diagnostic window and would also be diagnostic during the initial days of clinical disease when inclusions in neutrophils in blood smears are not yet apparent.