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Human ehrlichiosis and anaplasmosis

Daniel J Sexton, MD
Micah T McClain, MD
Section Editors
Stephen B Calderwood, MD
Sheldon L Kaplan, MD
Deputy Editor
Jennifer Mitty, MD, MPH


The first case of human ehrlichiosis was described in 1986 when a patient became ill with fever, hypotension, confusion, acute renal failure, coagulopathy, and gastrointestinal hemorrhage [1]. The etiologic agent was identified as Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis (HME). In 1994, Anaplasma phagocytophilum was identified as the agent of human granulocytic anaplasmosis (HGA) [2]. HGA was previously called human granulocytic ehrlichiosis (HGE). Both of these tick-borne illnesses are recognized as separate disease entities, even though their clinical and laboratory manifestations are similar.

The epidemiology, clinical manifestations, diagnosis, and treatment of human ehrlichiosis will be reviewed here. The microbiology of ehrlichial organisms and the mechanisms by which they cause disease are discussed separately. (See "Biology of ehrlichiae".)


Ehrlichiae are obligate intracellular bacteria that grow within membrane bound vacuoles in human and animal leukocytes (picture 1A-B and picture 2). The taxonomy, phylogeny, and host specificity of these bacteria are discussed in detail elsewhere. (See "Biology of ehrlichiae".)

The two most important species to infect humans include E. chaffeensis, the causative agent of human monocytic ehrlichiosis (HME), and A. phagocytophilum, the agent of human granulocytic anaplasmosis (HGA). Less commonly, ehrlichiosis is caused by Ehrlichia ewingii, which was discovered in 1999 [3]. In 2011, a third species of Ehrlichia was identified in four patients from Wisconsin and Minnesota who had fever, malaise, headache, and lymphopenia [4]. Molecular methods, culture techniques, and serologic testing have demonstrated that this new species is closely related to Ehrlichia muris, which is found in Eastern Europe and Asia. These epidemiologic findings are important since ehrlichiosis has not been previously described in Wisconsin or Minnesota.

Another species less commonly seen is Neorickettsia sennetsu, which causes a mild mononucleosis-like illness. Neorickettsial illness has not been reported outside the Far East and Southeast Asia [5].

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Literature review current through: Oct 2017. | This topic last updated: Aug 15, 2016.
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