What causes ehrlichiosis?

Updated: Jun 22, 2021
  • Author: Chinelo N Animalu, MD, MPH; Chief Editor: Michael Stuart Bronze, MD  more...
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Ehrlichia and Anaplasma species, members of the family Rickettsiae, are gram-negative, obligate, intracellular coccobacilli that resemble Rickettsia species. All 3 are forms of Alphaproteobacteria.

Like RickettsiaEhrlichia organisms gain access to the blood via a bite from an infected tick. A americanum (Lone Star tick, seen in the image below) is the principle tick vector of E chaffeensis and is the primary vector of human monocytic ehrlichiosis (HME). A phagocytophilum may be transmitted from Ixodes persulcatus ticks and possibly Dermacentor variabilis (dog tick/wood tick).

Female Lone Star tick, Amblyomma americanum, found Female Lone Star tick, Amblyomma americanum, found in the southeastern and Midatlantic United States. It is a vector of several zoonotic diseases, including human monocytic ehrlichiosis and Rocky Mountain spotted fever. Courtesy of the CDC/Michael L. Levin, PhD.

The primary target cell for HME is the macrophage, and the primary target for human granulocytic anaplasmosis (HGA) is the granulocyte. Intracellular infection is established within phagosomes, most often found in macrophages in the liver, spleen, lymph nodes, bone marrow, lung, kidney, and CNS.

HME and HGA are more severe in those with impaired splenic function.

Individuals considered to be at risk for ehrlichiosis include the following:

  • From a review of the national surveillance data, cases of ehrlichiosis are more frequently reported in men than in women.
  • People aged 60-69 years account for the highest number of cases.

People with compromised immune systems (eg, resulting from cancer treatments, advanced HIV infection, prior organ transplants, or some medications) might be at increased risk for severe disease. [5]

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