What Is Ehrlichiosis?

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Ehrlichiosis, also called human monocytic ehrlichiosis (HME), is a rare infectious disease that is transmitted by ticks that carry a specific type of rickettsial bacteria of the genus Ehrlichia. This disease is primarily seen in the Southeastern and Central U.S.

While many cases are mild with flu-like symptoms, the infection can be severe; it can even cause death. Treatment with antibiotics is needed to prevent complications.

The History of Ehrlichiosis

The first human case of ehrlichiosis was identified in 1986. From 2000 to 2011 there were hundreds of reported cases of Ehrlichia chaffeensis (the most commonly reported type of ehrlichiosis) each year. By 2012 there were more than 1,000 annual cases, and in 2019 there were 2,093 cases reported. 

Although it’s considered a nationally reportable disease, many experts believe that ehrlichiosis is underreported and that the actual number of infected people is significantly higher.

The lone star tick transmits ehrlichiosis
epantha / iStock / Getty Images

Ehrlichiosis Symptoms

The symptoms of ehrlichiosis vary, and can range from mild to severe; usually symptoms can be seen within a week or more (up to two weeks) after a person is bitten by a tick that is infected with the Ehrlichia bacterium. 

The symptoms which commonly occur early on in the disease process include:

  • Fever
  • Chills
  • Muscle aches
  • Diarrhea
  • Nausea
  • Vomiting
  • Loss of appetite
  • Fatigue
  • Headache
  • Confusion
  • Red eyes (seen more often in children)
  • A generalized rash

The rash is seen in one in three affected people, more often seen in children than adults. It develops five days after the fever begins and can either be a splotchy red rash or pinpoint dots.

In some cases, such as when a person has a weakened immune system, serious complications can occur from ehrlichiosis; these include:

  • Confusion, seizures, or coma (from brain involvement)
  • Hemorrhage (excess bleeding)
  • Heart failure
  • Acute respiratory distress syndrome (a life-threatening lung condition often requiring ventilator support for breathing)
  • Respiratory failure
  • Meningoencephalitis (an inflammation of the brain and the meninges, the protective membrane layers that cover the brain and spinal cord)
  • Kidney failure
  • Peripheral neuropathy (rarely seen in ehrlichiosis, involves damaged nerves which are outside of the brain and spinal column).
  • Septic shock (a life-threatening condition involving dangerously low blood pressure in response to an infection).

It is possible to have symptoms of ehrlichiosis that are so mild, a person may never realize they have the disease. In this instance, the body may fight off the disease without treatment. But ehrlichiosis that is left untreated could lead to serious symptoms that require hospitalization.

If you believe you may have been bitten by a tick (or if you’ve been outdoors in any of the areas where the disease-carrying tick resides) and you have symptoms (including mild symptoms), be sure to seek medical attention as soon as possible.

Serious complications are rare in younger, healthy people. When they do occur in healthy folks, it’s usually a result of delayed treatment. In a small percentage of people with serious complications, death has been known to be the outcome.

People are most likely to become infected with ehrlichiosis during the spring and summer months. This is because the number of infected ticks is usually higher during those seasons. But, it’s possible to become infected with the disease during the autumn months as well.


Ehrlichiosis is caused by bacteria that belong to the family called rickettsiae. There are several species of Ehrlichia that can cause ehrlichiosis, although the symptoms are very similar regardless of species. The main species include:

  • Ehrlichia chaffeensis
  • Ehrlichia ewingii
  • Ehrlichia muris eauclairensis

E. chaffeensis produces the majority of infections, with E. ewingii also being common.

Rickettsial bacteria cause several serious diseases including:

Each of these diseases are spread to humans by a tick, flea, lice, or mite bite or contact with their feces or other infectious fluids.

A tick must be attached to the skin for at least 24 hours before it’s able to transmit disease-causing bacteria (such as ehrlichiosis). Therefore, it’s vital to detect and remove any embedded ticks as soon as possible as a prevention measure.

A tick usually acquires the bacteria that causes ehrlichiosis from an infected deer, dog, or rodent. Then they transmit the bacteria to humans when they bite the skin.

Ehrlichiosis is most commonly spread by the lone star tick (Amblyomma americanum), which is mostly found along the East Coast and through the South Central and Southeastern states, as far west as Texas.

Ehrlichiosis can also be carried by the blacklegged tick (Ixodes scapularis), the same tick known to transmit Lyme disease to humans. The black-legged tick is found throughout the Eastern and Midwestern states, including Minnesota, Wisconsin, and neighboring states. It is the host for the E. muris euclarensis bacteria. However, infection with muris is only found in Minnesota and Wisconsin.

If you live in one of the areas of the country where one of these ticks reside, you may be at risk for getting ehrlichiosis.

The Centers for Disease Control and Prevention (CDC) reported that in 2019, Missouri, Arkansas, North Carolina, and New York accounted for nearly half of all reported cases of ehrlichiosis (E. chaffeensis) in the U.S.

On rare occasions, blood that has been infected has been known to cause ehrlichiosis through blood transfusions or organ transplants.

It's also been suggested that the disease may be transmitted from a pregnant person to the fetus, as well as through direct contact with an infected animal that is being slaughtered (such as a deer). However, more research is needed.


A diagnosis of ehrlichiosis is usually made after a thorough history (gathering information about exposure to ticks and about tick bites) and physical examination by a healthcare provider.

Blood tests may be taken to confirm the presence of the type of bacteria causing ehrlichiosis. Other types of blood tests may be performed including:

  • Polymerase chain reaction (PCR) is a lab technique used to create multiple copies of a segment of DNA, allowing small amounts of the bacteria to be more easily detected. The test is the most sensitive during the first week of ehrlichiosis.
  • Indirect immunofluorescent assay (IFA) is a test that confirms a diagnosis of ehrlichiosis. It detects antibodies. Levels are compared from an early blood sample with one drawn later in the course of the infection, so final results take several weeks.
  • A white blood cell examination under a microscope may reveal bacteria inside of the blood cells.

Ehrlichiosis is not easy to diagnose.

When a person has mild flu-like symptoms, they could be caused by many other types of infection. Sometimes, a person is unaware that they have been bitten by a tick. An infectious disease specialist may need to be consulted to pin down the cause of the disease.


The treatment of choice for ehrlichiosis is a course of antibiotics. Usually, an antibiotic called doxycycline is administered for both children and adults with ehrlichiosis. In severe cases, antibiotics may be given intravenously (IV) at a hospital until symptoms begin to subside.

Symptoms usually decline very fast (within the first few days) after antibiotics are given. Severe complications, such as organ failure, may require additional treatment like kidney dialysis or breathing support.


The prognosis (outlook) of ehrlichiosis is good when a person gets prompt treatment. Usually, the symptoms will clear up within a few days after treatment with antibiotics.

Unlike other tick-borne illnesses, such as Lyme disease, ehrlichiosis is not a chronic (long-term) disease, but rather an acute (short-term) condition that responds very well to antibiotic treatment.

However, in a very small percentage of people (around 1%) who do not receive treatment early enough in the disease process, death has been known to occur. Most often those who die from the disease are young children (under the age of 10) or adults who are aged 70 or older. This is because young children and older adults usually have a weaker immune system.

Other people at high risk of mortality (death) from ehrlichiosis are those with severe health conditions that impact the immune system (such as cancer or HIV).


The best way to prevent ehrlichiosis is to prevent tick bites. Tick bites can be prevented by implementing some common measures including:

  • Avoid areas known to be heavy with ticks (such as outdoor woods with thick ground cover).
  • Stay close to trails if you are in the woods (steer clear of bushes and other ground cover).
  • Tuck pant legs into boots.
  • Wear light-colored clothing to easily spot ticks.
  • Use tick repellent such as permethrin or DEET sprayed on skin and clothing.
  • After being in the woods or grassy areas, be sure to check your body and clothing (as well as pets if you have them) for ticks.
  • Dry clothes on high heat in the dryer to kill ticks that may be hiding in clothing.
  • Remove ticks before they are embedded for 24 hours to lower the risk of infection.
9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Columbia University Irving Medical Center. Lyme and Tick-Borne Diseases Research Center. Ehrlichiosis.

  3. Cedars-Sinai Health Library. Ehrlichiosis.

  4. Infection Prevention and You. What is sepsis?  

  5. Texas Health and Human Services. Ehrlichiosis.

  6. Centers for Disease Control and Prevention. CDC yellow book chapter 4: travel-related infectious diseases.

  7. Centers for Disease Control and Prevention. General Distribution of Blacklegged Ticks (Ixodes scapularis).

  8. Mowla SJ, Drexler NA, Cherry CC, Annambholta PD, Kracalik IT, Basavaraju SV. Ehrlichiosis and anaplasmosis among transfusion and transplant recipients in the united statesEmerg Infect Dis. 2021;27(11):2768-2775. doi:10.3201/eid2711.211127

  9. UpToDate. Human ehrlichiosis and anaplasmosis.

By Sherry Christiansen
Sherry Christiansen is a medical writer with a healthcare background. She has worked in the hospital setting and collaborated on Alzheimer's research.