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  • Order: Rickettsiales.
  • Family: Anaplasmataceae (formerly Rickettsiaceae).
  • Genus:
    • Anaplasma.
    • Ehrlichia.
    • Neorickettsia.
    • Wolbachia.
  • The Family Anaplasmataceae now contains four genus.
  • Ehrlichiae were initially grouped according to type of blood cell most commonly infected (granulocyte, lymphocyte, monocyte, platelet), and disease classes have been termed "granulocytic (or granulocytotropic) ehrlichiosis" or "monocytic (or monocytotropic) ehrlichiosis". However, this type of classification may be misleading because some of theEhrlichiaspecies have been found in cells other than their chief target cell type. In addition, more than one species may be responsible for the broad category of "monocytic" or "granulocytic" ehrlichiosis.
  • Classification and Nomenclature changes for Ehrlichia and related species:
    • The Tribe Ehrlichieae and the genus Wolbachia are in the Family Anaplasmataceae.
    • Ehrlichia equi and E. phagocytophila become Anaplasma phagocytophilum (Ehrlichia equi).
    • Ehrlichia platys becomes Anaplasma platys.
    • Ehrlichia senetsu and E. risticii become Neorickettsia sennetsu and N. risticii respectively.
    • Cowdria ruminantium becomes Ehrlichia ruminantium.
    • Haemobartonella spp and Eperythrozoon spp become Mycoplasma spp.


  • Anaplasma: Gk. an-, without; plasma-, anything formed or molded.

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Clinical Effects



  • Reservoir hosts are mammalian cells, possibly rodents.
  • Some members of the family Anaplasmataceae may be passed trans-stadially in ticks.


  • A. phagocytophilum: ticks are vectors.
  • N. risticii: reservoir host and vector unknown, but recent evidence indicates that flukes (trematodes), water associated flies, eg caddisflies, and some species of snail may be involved.

Pathological effects

  • Antibodies are produced during infection, and recovered horses are, at least partially, immune.


  • A. phagocytophilum infects granulocytes →  vasculitis →  thrombosis and thrombocytopenia → edema and hemorrhages in distal limbs. Most severe in older horses, which may show fever, anorexia, anemia, jaundice and locomotor problems.
  • N. risticiiis the cause of Potomac Horse Fever Potomac Horse Fever. Characterized by intestinal involvement and laminitis Foot: laminitis, as well as fever, depression and anorexia. Case fatality rate 20-30% if diarrhea present.


  • Canine ehrlichiosis is a febrile disease with nasal and ocular discharges, anorexia, loss of weight and depression, usually caused by E. canis:
    • Causes pancytopenia and a raised erythrocyte sedimentation rate.
    • A severe hemorrhagic form may develop 60 days or more after infection. This is associated with aplastic anemia and is sometimes known as 'tropical canine pancytopenia'.
    • Certain breeds, eg the German Shepherd Dog (Alsation) are more susceptible to severe disease.

Other Host Effects

  • Infection may be very mild and go unrecognized, especially A. phagocytophilum infection in foals.


Control via chemotherapies


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Dumler J S et al (2001) Reorganization of Genera in the Families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales; Unification of some species of Ehrlichia with AnaplasmaCowdria with Ehrlichia, and Ehrlichia with Neorickettsia; Description of six new species combinations; and designation of Ehrlichia equi and "HGE agent" as subjective synonyms of Ehrlichia phagocytophilumInt J Syst Evol Micorbiol 51 (6)2145-2165 PubMed.
  • Madigan J E et al (2000) Transmission of Ehrlichia risticii, the agent of Potomac horse fever, using naturally infected aquatic insects and helminth vectors - preliminary report. Equine Vet J 32 (4), 275-279 PubMed.
  • Pusterla N et al (1998) Experimental infection of four horses with Ehrlichia phagocytophila. Vet Rec 143 (11), 303-305 PubMed.
  • Korbutiak E and Schneiders D (1994) Equine granulocytic ehrlichiosis in the UK. Vet Rec 135 (16), 387-388 PubMed.
  • Mulville P (1991) Equine monocytic ehrlichiosis (Potomac horse fever) - a review. Equine Vet J 23 (6), 400-404 PubMed.

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