Anaplasma phagocytophilum is a tick-borne pathogen of domestic horses and the causative agent of equine granulocytic anaplasmosis. This case series describes three confirmed cases of clinical anaplasmosis, and a fourth case of presumptive anaplasmosis in Przewalski's horses ( Equus ferus przewalskii) housed at the Smithsonian Conservation Biology Institute from 2008 to 2014. Clinical signs varied among individuals with affected horses exhibiting lethargy, weakness, pyrexia, hypophagia, reluctance to move, or ataxia. Anaplasmosis cases were confirmed with a combination of identification of neutrophilic inclusions (morulae) on peripheral blood smear, positive polymerase chain reaction (PCR) testing of whole blood, or convalescent titers. All animals recovered after antimicrobial therapy with oxytetracycline. Diagnosis should be made by a combination of clinical signs plus identification of morulae or positive A. phagocytophilum PCR. Treatment is curative using oxytetracyline intramuscularly or intravenously followed by daily therapy with oxytetracyline or minocycline for 14-30 days. The authors recommend that A. phagocytophilum infection be included on any differential list for Przewalski's horses presenting with fever or ataxia within or near an enzootic area.

Anaplasma phagocytophilum (formerly Ehrlichia equi ) is a tickborne pathogen of domestic horses and the causative agent of equine granulocytic anaplasmosis. After the occurrence of clinical anaplasmosis in a Przewalski's horse ( Equus ferus przewalskii) housed at the Smithsonian Conservation Biology Institute in 2008, opportunistic serosurveillance of the herd was initiated. From 2008 to 2014, 57 serum samples were collected from 27 individuals (10 males; 17 females). Using indirect immunofluorescent antibody assays for anti- Anaplasma phagocytophilum antibodies, it was determined that prevalence was 53%. No significant sex differences were identified. A statistical association between increasing age and seropositive status suggests cumulative risk of exposure to Anaplasma phagocytophilum . After exclusion of four clinical cases of anaplasmosis, it was found that 22-57% of those sampled each year were seropositive and clinically normal, suggesting that the majority of Przewalski's horses develop subclinical or self-limiting anaplasmosis after exposure to A. phagocytophilum .

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