Ehrlichia

 
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Ehrlichia species are intracellular bacteria that are within the family of Anaplasmataceae of the order Rickettsiales. The family of Anaplasmataceae now includes the genera of Ehrlichia, Anaplasma, and Neoricketssia, after renaming of organisms (notably Ehrlichia equi and Ehrlicia platys as Anaplasma sp.) and reclassification in 2001 based on 16S RNA and groESL gene sequences in the bacteria  (Allison and Little, 2013). The names of some species, however, remains unchanged. The genus now includes those organisms that infect leukocytes (e.g, Ehrlichia canis).

Old name New name Species affected Infected blood cell
Ehrlichia canis Ehrlichia canis Dogs, cats Monocyte, lymphocyte (canine ehrlichiosis)
Ehrlichia ewingii Erhlichia ewingii Dogs Neutrophils, eosinophils (polyarthritis)
Ehrlichia equi Anaplasma phagocytophilum Horses, dogs, cats, camelids, humans Neutrophils, eosinophils (pathogenic)
Ehlrichia platys Anaplasma platyus Dogs Platelets (cyclic thrombocytopenia)

Ehrlichia canis

Disease in dogs

  • Vector: Rhipicephalus sanguineus, Dermacenter variabilis.
  • Cells: Monocytes, macrophages, lymphocytes. Morulae rarely seen in infected dogs and usually only in acute infection (Allison and Little 2013).
  • Clinical signs: Fever, lethargy, anorexia, excess hemorrhage (attributed to platelet dysfunction as well as thrombocytopenia – thrombocytopenia is usually not severe enough to induce hemorrhage alone – platelet counts are usually moderately decreased, i.e. 50-150,000/uL). Some dogs have neurologic disease and may have increased numbers of granular lymphocytes in their cerebrospinal fluid (this finding is not specific for this organism).
  • Bloodwork: Thrombocytopenia (characteristic, multiple mechanisms), non-regenerative anemia, pancytopenia (ineffective hematopoiesis in most dogs and bone marrow aplasia in chronic infections, particularly in German Shepherd dogs). May induce a lymphocytosis of granular lymphocytes, comprised of cytotoxic T cells (CD3+, CD8+), and can have a clonal T cell expansion on polymerase chain reaction for antigen receptor rearrangement testing (PARR), that mimics a chronic lymphocytic leukemia.  All dogs with this lymphocytosis in blood should be tested for Ehrlichia canis, as should dogs with pancytopenia and ineffective hematopoiesis or bone marrow aplasia. Can also stimulate a restricted oligoclonal gammopathy that mimics a monoclonal gammopathy (due to multiple myeloma or other B cell neoplasia). The involved immunoglobulin is usually IgG.
  • Diagnosis: Serologic testing, PCR.

Ehrlichia ewingii

  • VectorAmblyomma americanum.
  • Cells: Neutrophils, eosinophils. Morulae frequently seen in infected dogs, with numbers of infected cells corresponding to severity of infection (Allison and Little, 2013). Morulae can also be seen in neutrophils within joint fluid, but may be only seen in <1% of cells in the fluid.
  • Clinical signs: Fever, lethargy, anorexia, polyarthritis (main differential diagnosis is Anaplasma phagocytophilum, which produces a similar disease).
  • Bloodwork: Thrombocytopenia is the most characteristic finding in infected dogs.
  • Diagnosis: Morulae in neutrophils in blood or synovial fluid. Serologic testing, but cross reactivity seen with other bacteria, including Anaplasma phagocytophilium and Ehrlichia chafeensis (latter usually not clinical in dogs). May get negative results with 4DX SNAP test in our experience, 4DX plus SNAP test usually positive. PCR.
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