About Ehrlichiosis

Ehrlichiosis is an illness spread by ticks. In Wisconsin, it is spread by the deer tick, also known as the black-legged tick. Ehrlichiosis can also be spread by the lone-star tick, but these ticks are rare in Wisconsin.

Ehrlichiosis can be caused by several species of Ehrlichia bacteria, specifically Ehrlichia chaffeensis, Ehrlichia ewingii, and Ehrlichia muris eauclairensis.

Anyone can get ehrlichiosis, but people who spend more time outdoors are at a higher risk of being bitten by an infected tick. Ticks can be found in areas with woods, brush, or tall grass. Ticks are most active from May to September, but it is important to use caution year-round.

Map of Ehrlichiosis cases per 100,000 residents 2018


 How is ehrlichiosis spread to humans?

In Wisconsin, ehrlichiosis is spread to humans through the bite of an infected deer tick.

Tick crawling on skin

  • It is unknown how long a tick must be attached before it is able to spread ehrlichiosis to a person. It is important to remove ticks as soon as they are found to help prevent illness.
  • Most humans are infected by immature ticks, called nymphs.
    • Nymphs are very small, about the size of a poppy seed. They are difficult to see, and most people may not feel their bite. This makes them harder to remove promptly.
    • Nymphs are most active during the spring and early summer.
  • Adult ticks also spread ehrlichiosis.
    • Adult ticks are much larger than nymphs, and are more likely to be found and removed before the bacteria are spread to the person.
    • Adult ticks are most active during the cooler months, but can be out looking to feed any time temperatures are above freezing.
  • Ticks can attach to any part of the body but are often found in hard-to-see areas, such as:
    • Behind the knees
    • Armpits
    • Scalp
    • In and around the ears
    • Inside the belly button
    • Groin

Ehrlichiosis is preventable and treatable. Visit our Tick Bite Prevention page to learn how to prevent tick bites, and how to properly remove a tick if you are bitten.

 What are the signs and symptoms of ehrlichiosis?

Symptoms can show up one to two weeks after being bitten by an infected tick.

The following symptoms can be seen with ehrlichiosis. However, it is important to keep in mind that few people will have all symptoms listed, and the combination of symptoms varies greatly depending on the person. Early signs and symptoms are usually mild. However, if antibiotic treatment is delayed, more severe symptoms can occur. It is important to get treatment as soon as possible to avoid more serious illness.

Early signs and symptoms:

  • Fever
  • Chills
  • Severe headache
  • Muscle aches
  • Tiredness
  • Nausea, vomiting, diarrhea
  • Joint pain
  • Confusion
  • Rash (in up to 60% of children, less than 30% of adults)

Late signs and symptoms:

  • Damage to the brain or nervous system
  • Respiratory failure
  • Uncontrolled bleeding
  • Organ failure
  • Death

 How is ehrlichiosis treated?

Ehrlichiosis can be treated with antibiotics. Doxycycline is the antibiotic of choice. Treatment should be started whenever ehrlichiosis is suspected. Most people treated with oral antibiotics during the early stages of ehrlichiosis recover completely.

It is important to get treatment as soon as possible after symptoms start. Delay in treatment can be extremely dangerous. The following table shows the current treatment recommendations from the Centers for Disease Control and Prevention (CDC) for ehrlichiosis in adults and children.

Treatment for Ehrlichiosis
Age Category Drug Dosage Maximum Duration, Days
Adults Doxycycline 100 mg, twice per day 100 mg/dose Typically 5–7 days*
Children under 45 kg (100 lbs) Doxycycline 2.2 mg/kg body weight, twice per day 100 mg/dose Typically 5–7 days*

*Treatment should continue for at least three days after fever has gone away and patient shows improvement. The minimum course of antibiotics is five days.

Antibiotic treatment following a tick bite is not recommended to prevent ehrlichiosis. There is no evidence this practice is effective, and this may only delay onset of disease. Instead, if you get bitten by a tick, be alert for symptoms listed above and call your doctor if fever, rash, or other symptoms develop.


DHS resources
CDC resources
Partner resources
Ehrlichiosis is preventable and treatable. Visit our Tick Bite Prevention page to learn how to protect yourself from illnesses spread by ticks.

Questions about illnesses spread by ticks? Contact us!
Phone: 608-267-9003 | Fax: 608-261-4976

The recommendations in this webpage were developed in accordance with Wis. Stat. ch. 252 and Wis. Admin. Code ch. DHS 145.

Last Revised: June 29, 2020