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Ehrlichiosis (formerly called human monocytic ehrlichiosis) is used to describe diseases caused by the bacteria Ehrlichia chaffeensis, E. ewingii, or E. muris eauclairensis in the United States. These bacteria are spread to people primarily through the bite of infected ticks including the lone star tick and the black-legged tick. In rare cases, Ehrlichia species have been spread through blood transfusion and organ transplant. People with ehrlichiosis will often have fever, chills, headache, muscle aches, and sometimes upset stomach. Treatment is available; however, if it is delayed, or there are underlying medical conditions, symptoms can become more severe. Doxycycline is the treatment of choice for adults and children of all ages. The best way to prevent Ehrlichiosis is to prevent tick bites.


Anaplasmosis typically begins within 1–2 weeks after the bite of an infected tick. Tick bites are usually painless, and many people do not remember being bitten.

Early Illness

Early signs and symptoms (days 1-5) are usually mild or moderate and may include:

  • Fever, chills
  • Severe headache
  • Muscle aches
  • Nausea, vomiting, diarrhea, loss of appetite

Late Illness

Rarely, if treatment is delayed or if there are other medical conditions present, anaplasmosis can cause severe illness. Prompt treatment can reduce your risk of developing severe illness.

Signs and symptoms of severe (late-stage) illness can include:

  • Respiratory failure
  • Bleeding problems
  • Organ failure
  • Death

Risk factors for severe illness:

  • Delayed treatment
  • Age: being older puts you at risk
  • Weakened immune system: People with weakened immune systems (such as those receiving some cancer treatments, individuals with advanced HIV infection, prior organ transplants, or people taking some medications) are at risk for severe illness


Early recognition and treatment of ehrlichiosis is crucial to prevent complications. If you suspect you have been exposed to ticks or are experiencing symptoms consistent with the disease, it is important to seek medical attention promptly. Early treatment with the antibiotic doxycycline can prevent death and severe illness. Doxycycline is the recommended antibiotic treatment for ehrlichiosis in adults and children of all ages.


Prevention begins with you! Limiting exposure to ticks is the most effective method of prevention of tick-transmitted diseases because there is no vaccine. In persons exposed to tick-infested habitats, prompt, careful inspection, and removal of crawling or attached ticks is an important method of preventing disease. It may take several hours of attachment before the organisms are transmitted from the tick to the host.


CDC Information on Ehrlichiosis

Page last updated: November 14, 2023