eMedicine Specialties > Infectious Diseases > Bacterial Infections

Ehrlichiosis

Author: Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
Contributor Information and Disclosures

Updated: Feb 5, 2009

Introduction

Background

Ehrlichiosis is an infection of white blood cells that affects various mammals, including mice, cattle, dogs, deer, horses, sheep, goats, and humans.

Ehrlichia are obligate intracytoplasmic bacteria that infect mononuclear cells and granulocytes. Ehrlichia, which are tiny (0.2-2 µm) gram-negative organisms that resemble Rickettsia, divide by binary fission and multiply within the cytoplasm of infected white blood cells. Clusters of Ehrlichia multiply in host cell vacuoles 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 that infect granulocytes. In contrast, human monocytic ehrlichiosis (HME) is caused by Ehrlichia chaffeensis that infect monocytes or macrophages.

Human granulocytic anaplasmosis and human monocytic ehrlichiosis cause the same clinical manifestations. Therefore, the term ehrlichiosis is used to encompass both types of infections.

Pathophysiology

The pathophysiology of ehrlichiosis is not completely understood. Like Rickettsia species, Ehrlichia organisms gain access to the blood via a bite from an infected tick. Amblyomma americanum (Lone Star tick) is the principle tick vector of E chaffeensis and is the primary vector of human monocytic ehrlichiosis. Species that cause human granulocytic anaplasmosis may be transmitted from Ixodes persulcatus ticks and possibly Dermacentor variabilis (dog tick/wood tick).

Female Lone Star tick, <em>Amblyomma americanum,<...

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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Female Lone Star tick, <em>Amblyomma americanum,<...

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 major antigenic determinants of Ehrlichia are surface membrane proteins. These antigenic proteins are complex and consist of both thermolabile and thermostable components. In terms of kilodalton (kD) molecular weight, the key protein bands associated with human monocytic ehrlichiosis are the 27-, 29-, and 44-kD bands. The major antigenic determinants associated with human granulocytic anaplasmosis include the 40-, 44-, and 65-kD bands.

Frequency

United States

The distribution of ehrlichiosis in the United States mirrors the tick distribution and appropriate mammalian vectors (eg, white-footed mouse, white-tailed deer). Ehrlichiosis occurs where mammalian hosts are in contact with the appropriate tick vector (ie, A americanum, D variabilis, Ixodes ticks). Hundreds of cases of human monocytic ehrlichiosis and human granulocytic anaplasmosis have been reported, but ehrlichiosis is not a reportable disease; therefore, many more cases go unreported.

Most cases of ehrlichiosis in the United States occur in the states of California and Texas and the southeast and northeast regions of the country, with some cases occurring in the north central states west of the Great Lakes.

International

Ehrlichiosis occurs essentially worldwide, and the frequency parallels the distribution of the appropriate tick vectors for the transmission of Ehrlichia bacteria and the mammalian hosts.

Mortality/Morbidity

The great majority of cases of ehrlichiosis are asymptomatic. Most cases present as mild-to-moderate acute febrile illnesses. In immunocompromised hosts, ehrlichiosis may be severe, manifesting as a Rocky Mountain spotted fever (RMSF)–like illness that may be fatal.

Sex

Ehrlichiosis is more common in males, with a male-to-female ratio of 4:1.

Age

Ehrlichiosis occurs in all age groups but is most common in young adults. For information on pediatric ehrlichiosis, see the article Ehrlichiosis in eMedicine Pediatrics: General Medicine volume.

Clinical

History

  • Clinical manifestations of ehrlichiosis usually begin 5-14 days after the tick bite.
  • Patients with ehrlichiosis usually present with severe headache, myalgias, and fever. Shaking chills are often present.
  • Nausea and vomiting are common.
  • Abdominal pain is uncommon and is typically mild.
  • Skin rash due to ehrlichiosis is rare, in contrast to RMSF.

Physical

  • In contrast to RMSF, rash is rare in ehrlichiosis. When present in ehrlichiosis, the rash is maculopapular rather than petechial. Also in contrast to RMSF, ehrlichiosis does not cause vasculitis.
  • Physical findings due to ehrlichiosis are minimal. Some patients develop slight hepatomegaly.
  • Lymphadenopathy is observed in less than 25% of cases, and splenomegaly is not common.
  • Patients with severe ehrlichiosis may develop thrombocytopenia or disseminated intravascular coagulation (DIC), which can result in hemorrhage into the skin.

Causes

  • Ehrlichia are obligate intracytoplasmic bacteria that infect mononuclear cells and granulocytes.
  • Ehrlichia resemble Rickettsia and are tiny (0.2-2 µm) gram-negative organisms that divide by binary fission and multiply within the cytoplasm of infected white blood cells.
  • Clusters of Ehrlichia (called morulae) multiply in the vacuoles of cells, forming large mulberry-shaped aggregates.

More on Ehrlichiosis

Overview: Ehrlichiosis
Differential Diagnoses & Workup: Ehrlichiosis
Treatment & Medication: Ehrlichiosis
Follow-up: Ehrlichiosis
Multimedia: Ehrlichiosis
References
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Further Reading

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Keywords

ehrlichiosis, human monocytic ehrlichiosis, HME, human granulocytic anaplasmosis, HGA, human granulocytic ehrlichiosis, HGE, spotless Rocky Mountain spotted fever, Ehrlichia, Ehrlichia chaffeensis, E chaffeensis, Erlichia ewingii, E ewingii, Anaplasma phagocytophilum, A phagocytophilum, morulae

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Contributor Information and Disclosures

Author

Burke A Cunha, MD, Professor of Medicine, State University of New York School of Medicine at Stony Brook; Chief, Infectious Disease Division, Winthrop-University Hospital
Burke A Cunha, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Medical Editor

Thomas J Marrie, MD, Chair, Professor, Department of Medicine, Division of Infectious Diseases, University of Alberta College of Medicine
Thomas J Marrie, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physicians, American Society for Microbiology, Canadian Infectious Disease Society, and Royal College of Physicians and Surgeons of Canada
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: eMedicine Salary Employment

Managing Editor

Michael Stuart Bronze, MD, Professor, Stewart G Wolf Chair in Internal Medicine, Department of Medicine, University of Oklahoma Health Science Center
Michael Stuart Bronze, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physician Executives, American College of Physicians, American College of Physicians-American Society of Internal Medicine, American Federation for Clinical Research, American Medical Association, American Society for Microbiology, Association of Professors of Medicine, Association of Program Directors in Internal Medicine, Infectious Diseases Society of America, Oklahoma State Medical Association, and Southern Society for Clinical Investigation
Disclosure: Nothing to disclose.

CME Editor

Eleftherios Mylonakis, MD, Clinical and Research Fellow, Department of Internal Medicine, Division of Infectious Diseases, Massachusetts General Hospital
Eleftherios Mylonakis, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Physicians, American Society for Microbiology, and Infectious Diseases Society of America
Disclosure: Nothing to disclose.

Chief Editor

Michael Stuart Bronze, MD, Professor, Stewart G Wolf Chair in Internal Medicine, Department of Medicine, University of Oklahoma Health Science Center
Michael Stuart Bronze, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physician Executives, American College of Physicians, American College of Physicians-American Society of Internal Medicine, American Federation for Clinical Research, American Medical Association, American Society for Microbiology, Association of Professors of Medicine, Association of Program Directors in Internal Medicine, Infectious Diseases Society of America, Oklahoma State Medical Association, and Southern Society for Clinical Investigation
Disclosure: Nothing to disclose.

 
 
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