Equis ISSN 2398-2977


Synonym(s): Equine granulocytic ehrlichiosis

Contributor(s): Rachael Conwell, Melissa Kennedy, Alex Morrow, Graham Munroe


  • A tick-borne, gram-negative obligatory intracellular Rickettsia bacterial infection of horses.
  • Cause: Anaplasma phagocytophilum (formerly Ehrlichia equi) similar or identical to the causal agent of human granulocytic anaplasmosis (formerly human granulocytic ehrlichiosis) and closely related to the organism causing tick-borne fever in ruminants.
  • Signs: fever, which later in infection may be fluctuating, mild depression, inappetence, icterus and limb edema.
  • Diagnosis: identification of the organisms in granulocytes.
  • Treatment: intravenous oxytetracycline (6.6-7 mg/kg SID) for 7 days.
  • Prognosis: excellent if treatment started early; guarded if secondary infections develop.
Print off the Owner factsheet on Anaplasma phagocytophilum infection Anaplasma phagocytophilum infection to give to your clients.



Predisposing factors


  • Immunologic status of the host.
  • Stress including malnutrition and pregnancy.
  • Virulence of the strain of the organism.


  • Infestation with infected Ixodes spp ticks.


  • A tick-transmitted infection in which granulocytes are parasitized by the rickettsial bacteria A. phagocytophilum.
  • A. phagocytophilum is an obligate intracellular bacteria which multiplies inside cytoplasmic vacuoles in granulocytes, neutrophils, eosinophils and monocytes.
  • The bacteria is able to evade the bacteriocidal effects of the granulocytes. Up to 30% of granulocytes may be infected.
  • Bacteremia occurs 4-7 days after the tick bite. The primary site of replication is not certain, but may include the lungs and spleen. Bacteremia is accompanied by an initial high fever (>41°C/105.8°F). 
  • Vasculitis and interstitial inflammation develops affecting various organs which contributes to the clinical manifestations:
    • Behavior: lethargy.
    • Hemopoietic system: lymphadenopathy, edema, mucosal hemorrhages, hemolytic anemia.
    • Musculoskeletal system: limb edema, lameness, reluctance to move.
    • Nervous system: ataxia.
    • Cardiac system: myocarditis, arrhythmias.
  • Thrombocytopenia appears to result from bone marrow suppression of platelet maturation or release and the presence of anti-platelet antibodies.
  • Concurrent infections may be exacerbated during the period of leukopenia.
  • Chronically infected animals may show more vague signs.


  • Incubation period ranges from 8-25 days following experimental inoculation and is believed to be less than 14 days following natural exposure.
  • Clinical signs evolve over several days and last for 7-14 days - initially there is 1-3 days high fever and depression followed by 7 days of lower grade fever and progression to jaundice, edema of the limbs, formation of petechiae, ataxia and reluctance to move.
  • A secondary fever may occur 2-4 weeks after the first.


  • A. phagocytophilum is transmitted by species of Ixodes ticks including Ixodes pacificus (Pacific coast), I. scapularis (primary vector in eastern and mid-western USA) andI. ricinusin Europe. The bacteria can survive in host ticks for over a year.
  • Free-living rodents, white-tailed deer, grey squirrels, white-footed moose and some species of bird have been shown to act as reservoir hosts.
  • Occurrence tends to be seasonal, related to tick activity.
  • In endemic areas many horses appear to develop subclinical infections.
  • Cell-mediated and humoral immune responses develop following natural infection and recovery.
  • A carrier state does not appear to occur in horses, although has been reported in sheep.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Woldehiwet Z (2010) The natural history of Anaplasma phagocytophilumVet Parasitol 167 (2-4), 108-122 PubMed.
  • Lewis S R (2009) Equine granulocytic anaplasmosis: A case report and review.  J Equine Vet Sci 29 (3), 160-166 VetMedResource.
  • Walston R D et al (2004) Acute recumbency associated with Anaplasma phatocytophilum infection in a horse. JAVMA 224 (12), 1964-1966 PubMed.
  • Le Blond A, Pradier S et al (2005) An epidemiological survey of equine anaplasmosis (Anaplasma phagocytophilum) in southern France. Rev Sci Tech 24 (3), 899-908 PubMed.
  • Madigan J E (1993) Equine ehrlichiosis. Vet Clin North Am Equine Pract 9 (2), 423-429 (comprehensive review) PubMed.