Human Monocytic Ehrlichiosis (HME)

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It is possible that the main title of the report Human Monocytic Ehrlichiosis (HME) is not the name you expected. Please check the synonyms listing to find the alternate name(s) and disorder subdivision(s) covered by this report.


  • Human Ehrlichial Infection, Human Monocytic Type

Disorder Subdivisions

  • None

General Discussion

Human Monocytic Ehrlichiosis (HME) is a rare infectious disease belonging to a group of diseases known as the Human Ehrlichioses. These diseases are caused by bacteria belonging to the "Ehrlichia" family. Several forms of Human Ehrlichioses have been identified, including Human Monocytic Ehrlichiosis, Sennetsu Fever, and Human Granulocytic Ehrlichiosis. Though caused by different strains of Ehrlichia bacteria, the disorders are characterized by similar symptoms.

The symptoms of Human Monocytic Ehrlichiosis may include a sudden high fever, headache, muscle aches (myalgia), chills, and a general feeling of weakness and fatigue (malaise) within a few weeks after initial infection. In addition, in many cases, laboratory findings may indicate an abnormally low number of circulating blood platelets (thrombocytopenia), a decrease in white blood cells (leukopenia), and an abnormal increase in the level of certain liver enzymes (hepatic transaminases). In some individuals, symptoms may progress to include nausea, vomiting, diarrhea, weight loss, and/or confusion. If HME is left untreated, life-threatening symptoms, such as kidney failure and respiratory insufficiency, may develop in some cases. Human Monocytic Ehrlichiosis is caused by the bacteria Ehrlichia chaffeensis (or E. chaffeensis). E. chaffeensis is carried and transmitted by certain ticks (vectors), such as the Lone Star tick (Amblyomma americanum) and the American dog tick (Dermacentor variabilis).


Human Monocytic Ehrlichiosis (HME) was the first form of Human Ehrlichial infection recognized in the United States. The onset of symptoms usually occurs about three weeks after an individual has been bitten by a tick carrying the bacterium Ehrlichia chaffeensis. Symptoms may initially include fever, chills, headaches, muscle pain (myalgia), and a general feeling of weakness and fatigue (malaise). In some cases, a rash may appear on the skin. Symptoms may then progress to include nausea, vomiting, loss of appetite (anorexia), and/or weight loss. Some affected individuals may also experience coughing, diarrhea, sore throat (pharyngitis), and pain in the abdominal area.

In most cases of HME, there is also an abnormal decrease in white blood cells (leukopenia), a low number of circulating blood platelets (thrombocytopenia), and/or an abnormal increase in the level of certain liver enzymes (hepatic transaminases). Some affected individuals may also experience inflammation of the liver (hepatitis).

In some severe cases of Human Monocytic Ehrlichiosis, if appropriate treatment is not received, symptoms may include shortness of breath (dyspnea); abnormalities in the blood's ability to clot properly (coagulopathy), potentially resulting in gastrointestinal bleeding; and/or neurologic abnormalities due to involvement of the brain and the spinal cord (central nervous system [CNS]). Affected individuals with CNS involvement may develop abnormal tissue changes (lesions) in the brain, experience inflammation of the protective membranes covering the brain and spinal cord (meningitis), and/or have abnormalities in the fluid surrounding the brain and spinal cord (cerebrospinal fluid). Neurologic symptoms and findings may include confusion, abnormal sensitivity to light (photophobia), stiffness of the neck, episodes of uncontrolled electrical disturbances in the brain (seizures), and/or coma. Additional neurologic abnormalities may include exaggerated reflex responses (hyperreflexia), impaired coordination of voluntary movements (ataxia), and/or loss of some motor function in the facial area due to impairment of one or more of the 12 nerve pairs arising from the brain (cranial nerve palsy). In severe cases, if HME is left untreated, life-threatening complications may result, such as kidney (renal) and/or respiratory failure.

Some affected individuals may have a milder form of Human Monocytic Ehrlichiosis, experiencing only some of the symptoms typically associated with the disorder. Such symptoms may include muscle aches (myalgia), joint pain (arthralgia), headache, and/or loss of appetite (anorexia). In addition, it is believed that some individuals affected with HME may demonstrate no obvious symptoms (asymptomatic).


The Human Ehrlichioses, including Human Monocytic Ehrlichiosis (HME), are caused by bacteria belonging to the "Ehrlichia" family. They are considered "gram-negative" bacteria. Bacteria may be considered "gram negative" or "gram positive," depending upon the results of "Gram's stain," a testing method in which bacteria are stained with various solutions to help identify and classify the bacteria. Such staining may be essential in identifying a specific bacterium responsible for an infectious disorder and determining appropriate, effective treatments.

In most cases, it is believed that Human Ehrlichial infection results from tick bites. Certain types of ticks serve as "vectors," carrying and then transmitting the Ehrlichia bacteria to humans. A vector is any organism that is infected with a particular disease agent (e.g., bacterium or virus), carries it, and later transmits it to another organism, which may then become infected by the disease agent in question.

Human Monocytic Ehrlichiosis is caused by a bacterium named Ehrlichia chaffeensis (or E. chaffeensis). E. chaffeensis is transmitted by tick vectors, such as the Lone Star tick (Amblyomma americanum) and the American dog tick (Dermacentor variabilis). The genetic composition of E. chaffeensis is closely related to that of two other types of Ehrlichia bacteria, i.e., Ehrlichia canis and Ehrlichia ewingii. Both of these Ehrlichial bacteria are known to cause Ehrlichiosis in dogs. In addition, the E. ewingii bacterium has been found to cause a newly recognized form of Human Ehrlichiosis. (For more information, please see the "Related Disorders" section below.)

In Human Monocytic Ehrlichiosis, the Ehrlichial bacterium (E. chaffeensis) spreads through blood and lymphatic vessels. Lymph, a body fluid, carries cells that help fight infection. E. chaffeensis then invades certain cells (monocytes and macrophages) that play an essential role in the body's immune system by engulfing and digesting microorganisms (phagocytosis), such as bacteria and other foreign materials. The invading Ehrlichial bacteria grow within membrane-bound cavities (vacuoles) in monocytes and macrophages in the blood and certain body tissues (e.g., bone marrow, lymph nodes, liver, spleen, kidneys, lungs, and the fluid that surrounds the brain and spinal cord [cerebrospinal fluid]).

Affected Populations

Human Monocytic Ehrlichiosis was first recognized in the United States in 1986. At that time, a male who was approximately 50 years of age was bitten by a tick in Arkansas. He experienced the symptoms of Ehrlichial infection approximately two weeks after tick exposure. Studies were later conducted by the Centers for Disease Control (CDC) and state public health officials, demonstrating that many individuals in the United States who had originally been diagnosed with Rocky Mountain Spotted Fever or a related disorder may have actually experienced Human Ehrlichial infection. In 1991, when a soldier stationed at an army base in Arkansas experienced similar symptoms, the bacterium responsible for the infection was isolated. The bacterium, named "Ehrlichia chaffeensis" after the army base where the soldier was stationed (Fort Chaffee, Arkansas), is the cause of Human Monocytic Ehrlichiosis (HME).

From 1986 to 1997, 742 cases of HME were reported to the CDC. Most cases have occurred in the mid-Atlantic and southeastern states in the United States, although some have been reported from other states, including Washington, Wyoming, and Utah, as well as other continents, including Europe and Africa. However, because some individuals with HME may demonstrate no obvious symptoms (asymptomatic), it may be difficult to determine the true frequency of this form of Human Ehrlichiosis in the general population.

HME infection most commonly occurs in rural areas in the months of May, June, and July. In theory, this disease affects males and females in equal numbers. However, in observed cases, approximately 80 percent of affected individuals are male, while about 20 percent are female. Infection tends to occur when individuals participate in recreational or occupational activities that expose them to tick vectors.

Researchers believe that the distinct geographic distributions of the Human Ehrlichioses may result from differences in the distribution of the various vectors (e.g., raw fish, Lone Star tick, American dog tick, deer tick) carrying and transmitting the different Ehrlichia bacterial strains.

A recent report by the Centers for Disease Control and Prevention suggests that the incidence of both HME and Human Granulocytic Ehrlichiosis is underreported.

Standard Therapies


Human Monocytic Ehrlichiosis (HME) may be diagnosed based upon a thorough clinical evaluation, characteristic findings, and specialized laboratory tests. Blood tests may reveal findings often associated with the Human Ehrlichioses such as abnormally low levels of blood platelets (thrombocytopenia), low levels of certain white blood cells (leukopenia), and/or elevated levels of certain liver enzymes (such as aspartate aminotransferase [AST] and alanine aminotransferase [ALT]). In some cases, laboratory tests may reveal abnormalities of the cerebrospinal fluid. In addition, chest X-rays may reveal abnormalities in the lungs (e.g., pulmonary infiltrates, increased fluid in the lungs).

Examination of blood smears under a microscope that uses an electron beam (electron microscopy) may reveal clusters of bacteria in membrane-bound cavities (vacuoles) within certain cells (e.g., monocytes); however, such clusters may not be apparent early in the course of infection. In some cases, additional specialized laboratory tests may then be conducted to help determine and/or to confirm a diagnosis of a specific bacterial infection.

Specialized laboratory tests may include Indirect Immunofluorescence Assays (IFA) conducted on the fluid portion of an affected individual's blood (serum). Antibodies, which are proteins manufactured by certain white blood cells, help the body fight toxins and invading microorganisms. In Indirect Immunofluorescence Assays, human antibodies are marked with special fluorescent dyes and a microscope with ultraviolet light is used, enabling researchers to observe antibody response to certain microorganisms.

IFA testing has been used in confirming a diagnosis of all known types of Human Ehrlichial infection. However, in Human Monocytic Ehrlichiosis (HME), the bacterium responsible for the infection (Ehrlichia chaffeensis) was not characterized and identified (isolated) until 1991. Therefore, for many years, HME infection was diagnosed by observing the antibody response in a patient's blood serum to the bacterium responsible for Canine Ehrlichiosis, Ehrlichia canis, a bacterium that is very genetically similar to E. chaffeensis. Since E. chaffeensis was isolated in 1991, cases of HME have been confirmed by IFA testing that measures antibody response either to E. chaffeensis itself or the closely-related E. canis.

Measurable diagnostic rises in antibody response to the Ehrlichia bacteria may not occur until approximately three weeks after the onset of Human Monocytic Ehrlichiosis. As a result, initial IFA blood serum results may be negative in some cases. Therefore, more sensitive testing techniques that can help establish early diagnosis may be used in some cases.

One such process, called Polymerase Chain Reaction (PCR), is a laboratory technique in which sequences of DNA (which contains the organism's genetic information) can be copied over and over again quickly. This enables close analysis of the DNA, aiding in the identification of the organism in question. PCR conducted on certain bacterial DNA sequences obtained from patients' blood samples may confirm Human Ehrlichial infection due to a particular strain of Ehrlichia. PCR has been used to establish an early diagnosis of Human Ehrlichial infection in some cases of HME.

The information in the medical literature indicates that because it may be difficult to differentiate Human Ehrlichial infection, such as Human Monocytic Ehrlichiosis, from other illnesses that are also characterized by high fever (febrile illnesses), Ehrlichiosis should be considered in any patient with high fever, thrombocytopenia, and leukopenia who has recently been exposed to ticks. In addition, HME should be considered in individuals with high fever, severe headache, and neurologic symptoms, particularly in areas where HME infection is known to occur and during peak seasons for such infection.

If HME is suspected, treatment should not be delayed until diagnosis has been confirmed by IFA testing, since a positive antibody response may not occur until several weeks after initial infection. Therapy should begin as soon as possible after the onset of symptoms.


The treatment of Human Monocytic Ehrlichiosis usually entails standard doses of tetracycline antibiotics. Alternatively, doxycycline therapy may be administered. In severe cases of HME infection, hospitalization may be required. Other treatment is symptomatic and supportive.


Individuals in geographic areas who risk exposure to tick vectors for Ehrlichial infection should consider taking certain steps to prevent infection. Such steps should include wearing long-sleeved shirts, long pants, and hats; wearing light-colored clothing to make ticks more visible; using insect repellents; and carefully checking their clothing and skin (particularly the scalp and the back of the neck) after being in fields or wooded areas.

Investigational Therapies

Initial laboratory (in vitro) studies have shown that the bacterium that causes Human Monocytic Ehrlichiosis (E. chaffeensis) may be susceptible to the antibacterial drug rifampin. Further research is needed to determine whether rifampin would be a safe, effective therapy in the treatment of HME.

Ongoing studies are underway to determine the geographic distribution of different forms of Human Ehrlichial infection; examine whether infection with one Ehrlichia bacterial strain may result in immunity to other strains; and determine the occurrence rates of Human Ehrlichial infection among those who do not exhibit obvious symptoms (asymptomatic infection).

Research on tropical and other infectious diseases such as the Human Ehrlichioses is ongoing. For more information about these disorders, please contact the World Health Organization (WHO) listed in the Resources section below.

Information on current clinical trials is posted on the Internet at All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222

TTY: (866) 411-1010


For information about clinical trials sponsored by private sources, contact:



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