Abstract

Anaplasma phagocytophilum (previously known as Ehrlichia equi) is an emerging tick-borne pathogen of domestic horses that is the causative agent of equine granulocytic anaplasmosis and affects a wide variety of mammals, including humans. This pathogen was first reported in Virginia domestic horses in 2009 and seems to have an expanding range to match that of its primary eastern North American vector, Ixodes scapularis. From 2008–2014, there were four confirmed cases of clinical anaplasmosis in three captive Przewalski's horses (Equus ferus przewalskii) at the Smithsonian Conservation Biology Institute in Virginia, USA. Affected horses exhibited lethargy, weakness, hyporexia, reluctance to move, ataxia and pyrexia. Clinicopathologic findings were varied among cases, but included leukopenia, thrombocytopenia, and anemia. Neutrophilic morulae were not an uncommon finding on peripheral blood smears. Diagnosis was confirmed with a combination of convalescent titers, neutrophilic inclusions, and PCR testing. A fourth horse is suspected to have been affected based on clinical signs, exclusion of other etiologies with CSF testing, and response to empirical therapy. All animals recovered after antimicrobial therapy with oxytetracycline (10 mg/kg IV once, then IM BID or SID) and/or minocycline (4 mg/kg PO BID). This case series reveals that A. phagocytophilum should be included on any differential list for neurologic disease in an exotic equid within or near an enzootic area.