ISSN 2398-2942      

Anaplasma phagocytophilum


Leah Cohn

Severine Tasker

Synonym(s): Canine granulocytic anaplasmosis, the HGE (human granulocytic ehrlichia agent in humans




  • Phylum: Proteobacteria.
  • Order: Rickettsiales.
  • Family: Anaplasmataceae.
  • Genus: Anaplasma.
  • Species: phagocytophilum.
  • Many A. phagocytophilum strains exist; seem to group according to geographical origin in phylogenetic studies performed - these variants may vary in pathogenicity too (eg human disease due to A. phagocytophilum worse in USA compared to Europe).


  • Formerly known as Ehrlichia equi and Ehrlichia phagocytophila.
  • Gk. an-, without; plasma-, anything formed or molded; phagein-, to eat up, devour; phylum-, friend, loving. Literally: a thing (a bacterium) without form, attractive to phagocytes.

Active Forms

This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login

Clinical Effects



  • Wildlife such as rodents, and other small mammals act as the reservoir hosts, with transmission to domestic animals and man as a result of tick bites.
  • Wild ruminants such as deer may also act as reservoirs for selected strains.


  • Alternate infection of two distinct hosts, ticks and mammals.
  • A. phagocytophilum resides in the salivary glands of tick. The bacteria persists through the nymphal molts but is not passed on transovarially.


  • Transmitted by bites from Ixodes spp tick vectors, especially:
    • Ixodes pacificus Ixodes pacificus on western coast of United States (same spp that transmit Lymes disease in North America Borreliosis ).
    • Ixodes scapularis Ixodes scapularis in the north-eastern and southern coastal regions of the United States (same spp that transmit Lymes disease in North America).
    • Ixodes ricinus Ixodes ricinus throughout Europe (same spp that transmits Lymes disease and the flavivirus that causes tick borne encephalitis (TBE) in Europe) and eastern Russia.
    • Ixodes persulcatus across sections of Russia and northern Asia.
  • It has been suggested that the infection can be introduced into disease-free areas by migrating birds.
  • Time of tick bite transmission; ticks need at least 24 h feeding to transmit - usually occurs with 24-48 h of feeding.

Risk factors for canine infection

  • Lack of tick control Tick control.
  • Older dogs more likely to be infected (more cumulative exposure?).
  • No sex predisposition.
  • No breed predisposition although golden retrievers were overrepresented in one study.

Pathological effects

  • The seroprevalence of anaplasmosis is related to organism prevalence in the tick populations, animal density, and the variants of A. phagocytophilum found within the geographic area.
  • Exact mechanism of disease induction not known.
  • Preferentially infects myeloid cells, predominantly neutrophils
  • See cytokine myelosuppression, auto-antibody formation (eg versus platelets), infection of bone marrow hematopoietic cell precursors.
  • Acute infection most likely to be associated with clinical signs; chronic disease in dogs, as seen with Ehrlichia canis Ehrlichia canis , has not been reported with A. phagocytophilum.
  • Usually mild self-limiting disease in dogs.
  • Co-infections with Borrelia burgdorferi Borrelia burgdorferi may be responsible for more severe clinical signs in some cases.
  • Clinical signs reported almost exclusively with acute infection:
    • Pyrexia most frequently reported.
    • Lethargy.
    • Depression.
    • Anorexia.
    • Musculoskeletal pain; stiffness, lameness, weakness, swollen joints.
    • Occasionally see lymphadenopathy Lymphadenopathy , splenomegaly, hepatomegaly Abdominal organomegaly.
    • Gastrointestinal and respiratory signs rarely seen.
    • CNS signs rare.
    • Bleeding signs are rare compared withE. canisinfection even though see thrombocytopenia Thrombocytopenia.
  • Clinical pathology changes:


Control via animal

  • Minimize exposure to tick bites by use of topical tick repellents (eg permethrin Permethrin or flumethrin).
  • Carefully examine for and remove ticks on return from infected areas - daily tick check by owners although note that this needs to be done promptly to prevent transmission of infection as this can occur after only 24 h of tick feeding.
  • Use tick remover for complete removal of ticks.


  • Tetracyclines Tetracycline , including doxycycline (10 mg/kg/day PO) Doxycycline , are very effective at treating the infection: signs of infection (fever) usually resolve within 24-48 h of starting treatment.
  • 4 weeks of treatment has been recommended by some, although shorter courses of 2-3 weeks are generally adequate.
  • Chloramphenicol Chloramphenicol has also been used (in very young animals if tetracyclines are to be avoided).
  • Acute infection relatively easy to treat; chronic infection does not appear to be a feature of this disease.


  • No vaccine is currently available against A. phagocytophilium.


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Chomel B (2011) Tick-borne infections in dogs - an emerging infectious threat. Vet Parasitol 179 (4), 294-301 PubMed.
  • Scorpio D G, Dumler J S, Barat N C et al (2011) Comparative Strain Analysis of Anaplasma phagocytophilum Infection and Clinical Outcomes in a Canine Model of Granulocytic Anaplasmosis. Vector-Borne Zoonotic Dis 11 (3), 223-229 PubMed.
  • Smith F D, Ballantyne R, Morgan E R et al (2011) Prevalence, distribution and risk associated with tick infestation of dogs in Great Britain. Med Vet Entomol​ 25 (4), 377-384 PubMed.
  • Carrade D D, Foley J E, Borjesson D L et al (2009) Canine Granulocytic Anaplasmosis: A Review. JVIM 23 (6), 1129-1141 PubMed.
  • Stuen S (2007) Anaplasma Phagocytophilum - the most widespread tick-borne infection in animals in Europe. Vet Res Commun 31 (Suppl 1), 79-84 PubMed.

Can’t find what you’re looking for?

We have an ever growing content library on Vetlexicon so if you ever find we haven't covered something that you need please fill in the form below and let us know!


To show you are not a Bot please can you enter the number showing adjacent to this field

 Security code