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Biology of ehrlichiae

Daniel J Sexton, MD
Section Editor
Stephen B Calderwood, MD
Deputy Editor
Jennifer Mitty, MD, MPH


Although ehrlichiae have been known to cause disease of interest to veterinarians for over 50 years, their role as agents of human disease was not recognized until 1987. Since then, the number of ehrlichiae that cause human disease has rapidly expanded, as has our knowledge about the epidemiology, clinical characteristics, and treatment of the diseases caused by these organisms. (See "Human ehrlichiosis and anaplasmosis".)


Ehrlichiae are obligate intracellular bacteria that grow within membrane-bound vacuoles in human and animal leukocytes. In humans, either monocytes (human monocytic ehrlichiosis; HME) or granulocytes (human granulocytic ehrlichiosis; HGE, the organism is now called Anaplasma phagocytophilum, and the disease anaplasmosis, HGA) may be infected. Ehrlichiae replicate within the phagosomes of the host cell, unlike rickettsiae, which grow in the cytoplasm of infected cells. A microcolony of ehrlichiae within a vacuole is called a morula (picture 1A-B). (See "Biology of Rickettsia rickettsii infection".)

The cell walls of ehrlichiae contain glycoproteins but do not have peptidoglycans or lipopolysaccharides [1]. Ehrlichiae have distinct ribosomes that are homogeneously distributed in the cytoplasm and are surrounded by thin bileaflet outer and inner membranes.

Ehrlichiae are difficult to detect by light microscopy when they exist as single organisms; however, morulae are easily visualized when present within white blood cells. Ehrlichiae can be stained with Gram's, Giemsa, Wright, and silver stains, and by the Macchiavello method. Individual organisms are generally round, but may be pleomorphic when visualized in tissue culture cells [2]. Thus far, ehrlichiae have only been cultured in eukaryotic cells and in yolk sacs.

The gene for a 120-kD protein in Ehrlichiae chaffeensis contains a series of tandem repeat units, the number of which differs among isolates of E. chaffeensis [3]. As an example, the Arkansas and Jax strains contain four and the Sapulpa and St. Vincent strains contain three repeat units [4].


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Literature review current through: Sep 2016. | This topic last updated: Dec 4, 2014.
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  1. Walker DH. Ehrlichia under our noses and no one notices. Arch Virol Suppl 2005; :147.
  2. Hess PR, English RV, Hegarty BC, et al. Experimental Ehrlichia canis infection in the dog does not cause immunosuppression. Vet Immunol Immunopathol 2006; 109:117.
  3. Yu XJ, Crocquet-Valdes P, Walker DH. Cloning and sequencing of the gene for a 120-kDa immunodominant protein of Ehrlichia chaffeensis. Gene 1997; 184:149.
  4. Paddock CD, Sumner JW, Shore GM, et al. Isolation and characterization of Ehrlichia chaffeensis strains from patients with fatal ehrlichiosis. J Clin Microbiol 1997; 35:2496.
  5. Dumler JS, Barbet AF, Bekker CP, et al. Reorganization of genera in the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales: unification of some species of Ehrlichia with Anaplasma, Cowdria with Ehrlichia and Ehrlichia with Neorickettsia, descriptions of six new species combinations and designation of Ehrlichia equi and 'HGE agent' as subjective synonyms of Ehrlichia phagocytophila. Int J Syst Evol Microbiol 2001; 51:2145.
  6. Buller RS, Arens M, Hmiel SP, et al. Ehrlichia ewingii, a newly recognized agent of human ehrlichiosis. N Engl J Med 1999; 341:148.
  7. Goodman JL, Nelson CM, Klein MB, et al. Leukocyte infection by the granulocytic ehrlichiosis agent is linked to expression of a selectin ligand. J Clin Invest 1999; 103:407.
  8. Dumler JS, Choi KS, Garcia-Garcia JC, et al. Human granulocytic anaplasmosis and Anaplasma phagocytophilum. Emerg Infect Dis 2005; 11:1828.
  9. Carlyon JA, Fikrig E. Mechanisms of evasion of neutrophil killing by Anaplasma phagocytophilum. Curr Opin Hematol 2006; 13:28.
  10. Bakken JS, Dumler S. Human granulocytic anaplasmosis. Infect Dis Clin North Am 2008; 22:433.
  11. Klein MB, Hayes SF, Goodman JL. Monocytic differentiation inhibits infection and granulocytic differentiation potentiates infection by the agent of human granulocytic ehrlichiosis. Infect Immun 1998; 66:3410.
  12. Webster P, IJdo JW, Chicoine LM, Fikrig E. The agent of Human Granulocytic Ehrlichiosis resides in an endosomal compartment. J Clin Invest 1998; 101:1932.
  13. Goodman JL, Nelson C, Vitale B, et al. Direct cultivation of the causative agent of human granulocytic ehrlichiosis. N Engl J Med 1996; 334:209.
  14. Munderloh UG, Tate CM, Lynch MJ, et al. Isolation of an Anaplasma sp. organism from white-tailed deer by tick cell culture. J Clin Microbiol 2003; 41:4328.
  15. Magnarelli LA, IJdo JW, Dumler JS, et al. Reactivity of human sera to different strains of granulocytic ehrlichiae in immunodiagnostic assays. J Infect Dis 1998; 178:1835.
  16. Nyindo MB, Ristic M, Lewis GE Jr, et al. Immune response of ponies to experimental infection with Ehrlichia equi. Am J Vet Res 1978; 39:15.
  17. Yager E, Bitsaktsis C, Nandi B, et al. Essential role for humoral immunity during Ehrlichia infection in immunocompetent mice. Infect Immun 2005; 73:8009.
  18. Horowitz HW, Aguero-Rosenfeld M, Dumler JS, et al. Reinfection with the agent of human granulocytic ehrlichiosis. Ann Intern Med 1998; 129:461.
  19. Walker DH, Dumler JS. Human monocytic and granulocytic ehrlichioses. Discovery and diagnosis of emerging tick-borne infections and the critical role of the pathologist. Arch Pathol Lab Med 1997; 121:785.
  20. Allen MB, Pritt BS, Sloan LM, et al. First reported case of Ehrlichia ewingii involving human bone marrow. J Clin Microbiol 2014; 52:4102.
  21. Klein MB, Miller JS, Nelson CM, Goodman JL. Primary bone marrow progenitors of both granulocytic and monocytic lineages are susceptible to infection with the agent of human granulocytic ehrlichiosis. J Infect Dis 1997; 176:1405.
  22. Dumler JS, Barat NC, Barat CE, Bakken JS. Human granulocytic anaplasmosis and macrophage activation. Clin Infect Dis 2007; 45:199.
  23. Ismail N, Soong L, McBride JW, et al. Overproduction of TNF-alpha by CD8+ type 1 cells and down-regulation of IFN-gamma production by CD4+ Th1 cells contribute to toxic shock-like syndrome in an animal model of fatal monocytotropic ehrlichiosis. J Immunol 2004; 172:1786.
  24. Stevenson HL, Crossley EC, Thirumalapura N, et al. Regulatory roles of CD1d-restricted NKT cells in the induction of toxic shock-like syndrome in an animal model of fatal ehrlichiosis. Infect Immun 2008; 76:1434.