What is ehrlichiosis?

Updated: Jun 22, 2021
  • Author: Chinelo N Animalu, MD, MPH; Chief Editor: Michael Stuart Bronze, MD  more...
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Ehrlichiosis is an infection of white blood cells that affects various mammals, including mice, cattle, dogs, deer, horses, sheep, goats, and humans. [1, 2]  

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.

Ehrlichia/Anaplasma are tiny (0.2-2 µm) obligate, intracytoplasmic, gram-negative bacteria that resemble Rickettsia; divide by binary fission; and multiply within the cytoplasm of infected white blood cells. Clusters of Ehrlichia multiply in host monocyte vacuoles (phagosomes) to form large, mulberry-shaped aggregates called morulae.

Ehrlichia inclusion bodies, such as morulae, are visible in the cytoplasm of infected mononuclear phagocytic cells after 5-7 days. The type of ehrlichiosis that develops varies and depends on the infecting species and the type of leukocyte infected. Human granulocytic anaplasmosis (HGA), formerly known as human granulocytic ehrlichiosis (HGE), is caused by Anaplasma phagocytophilum, which infect granulocytes. In contrast, human monocytic ehrlichiosis (HME) is caused by Ehrlichia chaffeensis, which infects monocytes.

HGA and HME cause the same clinical manifestations. Therefore, the term ehrlichiosis is used for both types of infections. The total duration of illness for HME and HGA is unknown. No chronic cases have been reported at this time. 

Table. Characteristics of HME Versus HGA (Open Table in a new window)


Human monocytic ehrlichiosis (HME)

Human granulocytic anaplasmosis (HGA)

Cell type Affected




E chaffeensis

A phagocytophilum


Amblyomma americanum (Lone Star tick)

Ixodes scapularis (black-legged tick), Ixodes pacificus (Western black-legged tick) in California, Ixodes ricinus in Europe, and probably Ixodes persulcatus in parts of Asia


Southeastern and south-central United States

Wisconsin and Minnesota, less active in New York and Connecticut, also California


30% of adults, 60% of children



~3% mortality

< 1% mortality


In 2008, the ehrlichiosis case definition was divided into four classifications:

  1. Ehrlichia chaffeensis infection
  2. Ehrlichia ewingii infection
  3. Anaplasma phagocytophilum infection
  4. Undetermined ehrlichiosis/anaplasmosis

In 2009, a new Ehrlichia species was identified in patients in the upper Midwest area.

  • This species was previously called  “ E. muris-like agent” (EMLA),  but is now referred to as  E. muris eauclairensis. No death has been reported with infection caused by this species. [3]

Because the tick vector and geographic range for HGA is the same as that for Lyme disease, rarely the 2 may coexist in the same patient; doxycycline is effective therapy for both. 

In October 2008, a report was made of an apparent nosocomial infection with A phagocytophilum that was transmitted from blood donated by an infected woman who had spent time in Minnesota just prior to donating.

The major antigenic determinants of Ehrlichia are surface membrane proteins. These antigenic proteins are complex and consist of thermolabile and thermostable components. In terms of kilodalton (kd) molecular weight, the key protein bands associated with HME are the 27-, 29-, and 44-kd bands. The major antigenic determinants associated with HGA include the 40-, 44-, and 65-kd bands.

In 1999, Buller et al reported 4 incidents of ehrlichiosis in Missouri due to Ehrlichia ewingii. [4] The associated disease may be clinically indistinguishable from infection caused by E chaffeensis or A phagocytophilum; however, laboratory testing can distinguish these incidents from HGA and HME. 

Go to Tick Removal and Tick-Borne Diseases for complete information on these topics.

See 7 Bug Bites You Need to Know This Summer, a Critical Images slideshow, for helpful images and information on various bug bites.

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