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 |
Monocytes |
Granulocytes |
Organism |
E chaffeensis |
A phagocytophilum |
Vector |
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 |
Location |
Southeastern and south-central United States |
Wisconsin and Minnesota, less active in New York and Connecticut, also California |
Rash |
30% of adults, 60% of children |
Rare |
Prognosis |
~3% mortality |
< 1% mortality |
In 2008, the ehrlichiosis case definition was divided into four classifications:
- Ehrlichia chaffeensis infection
- Ehrlichia ewingii infection
- Anaplasma phagocytophilum infection
- Undetermined ehrlichiosis/anaplasmosis
In 2009, a new Ehrlichia species was identified in patients in the upper Midwest area.
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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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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.
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Map of the United States showing the distribution of the Lone Star Tick, which is the principle vector for ehrlichiosis.
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Established and reported distribution of anaplasmosis vectors Ixodes scapularis and Ixodes pacificus, by county, in the United States from 1907-1996. Courtesy of the Division of Vector-Borne Infectious Diseases at the Centers for Disease Control and Prevention.
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Anaplasmosis incidence by age. Courtesy of the Centers for Disease Control and Prevention.
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Ehrlichiosis incidence by age. Courtesy of the Centers for Disease Control and Prevention.
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This graph displays the number of human cases of ehrlichiosis caused by Ehrlichia chaffeensis reported to the Centers for Disease Control and Prevention (CDC) annually from 2000 through 2016. *From 2000 to 2008, ehrlichiosis was included in the reporting category “human monocytic ehrlichiosis” in reports to the National Notifiable Diseases Surveillance System (NNDSS). **Since 2008, ehrlichiosis has been reported to the NNDSS under the categories “Ehrlichia chaffeensis infections,” “Ehrlichia ewingii infections,” and “Undetermined ehrlichiosis/anaplasmosis infections”, which include infections caused by Ehrlichia muris eauclairensis. Only E chaffeensis infections are shown above. Courtesy of the CDC (https://www.cdc.gov/ehrlichiosis/stats/index.html).
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This graph shows the number of ehrlichiosis cases caused by Ehrlichia chaffeensis reported from 2000 through 2016 by month of onset to illustrate the seasonal trends. Cases are reported in each month of the year, although most are reported in June and July. Courtesy of the CDC (https://www.cdc.gov/ehrlichiosis/stats/index.html).
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This graph shows the number of US ehrlichiosis cases caused by Ehrlichia chaffeensis and reported to the CDC from 2000 to 2018. Courtesy of the CDC (https://www.cdc.gov/ehrlichiosis/stats/).
Tables
|
Human monocytic ehrlichiosis (HME) |
Human granulocytic anaplasmosis (HGA) |
Cell type Affected |
Monocytes |
Granulocytes |
Organism |
E chaffeensis |
A phagocytophilum |
Vector |
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 |
Location |
Southeastern and south-central United States |
Wisconsin and Minnesota, less active in New York and Connecticut, also California |
Rash |
30% of adults, 60% of children |
Rare |
Prognosis |
~3% mortality |
< 1% mortality |
