Ehrlichiosis

Lone star tick
Ehrlichiosis

Ehrlichiosis is a tick-borne disease caused by the bacteria Ehrlichia chaffeensis (most common) and Ehrlichia ewingii (less common). In Tennessee, it is transmitted by Amblyomma americanum, also known as the lone star tick.

Reports of ehrlichiosis in Tennessee have been rising since 2015. Unlike other reportable tick-borne diseases in the state, ehrlichiosis hospitalizes a significant number of Tennesseeans who become infected. Per the surveillance data from 2021, approximately 50% of all probable and confirmed ehrlichiosis cases were hospitalized for their illness.

For more information about this disease, see the CDC's Ehrlichiosis Home Page.

On the map below, each dot represents one case of ehrlichiosis that occurred in 2021. Each dot is placed randomly in the patient's county of residence. The gradient refers to ehrlichiosis incidence rates for each county in Tennesee from 2011-2021, which was calculated using county population data from the 2020 U.S. Census.

Davidson and Williamson counties accounted for 30% of all ehrlichiosis cases reported in the state in 2021.

Ehrlichiosis case map across Tennessee by county

In Tennessee, illness onset for cases of ehrlichiosis is mainly reported in the warmer months of the year. The number of reported ehrlichiosis cases is lowest in the late autumn through winter (November-February), slightly increases in the early spring (March-April), peaks in the late spring to mid summer (May-July), and then gradually declines late summer through early autumn (August-October).

Signs and symptoms of ehrlichiosis typically begin 5-14 days after being bitten by an infected lone star tick. Therefore, late spring through early summer (April-June) is the time of year when Tennesseans are most at risk for contracting ehrlichiosis.

Ehrlichiosis cases by onset week
Note: The n=757 in this chart refers to reported cases of ehrlichiosis from 2011-2021 with recorded illness onset dates. Thus, this figure only accounts for a portion of the total number of ehrlichiosis cases reported during this time period.

Anyone who spends time outdoors in Tennessee is at risk of contracting ehrlichiosis, because the lone star ticks that spread this disease are most often found in wooded, brushy areas. People holding outdoors-based occupations, such as farmers and landscapers, may be at increased risk of getting bitten by an infected tick, as are people who regularly hike, camp, hunt, or garden in or around wooded areas.

Lone star ticks may also dwell in the property surrounding Tennessee homes, especially if your yard is either next to a brushy area or has tall grass and/or leaf litter. Even outside of our state's forests, it is important to take the necessary steps to prevent tick bites in any environment habitable by these ticks.

Individuals of any reported gender, sex, and age group in Tennessee can get sick with ehrlichiosis. However, most cases of ehrlichiosis are reported in older residents over 50. Additionally, within almost every age group, ehrlichiosis was reported more in males than females.

Ehrlichiosis cases by age group and reported sex

The number of ehrlichiosis cases reported in 2021 was up by 70% from the previous year. Although 2020 saw a significant decrease in case counts, likely due to less human-tick exposure during the start of the COVID-19 pandemic, reports of ehrlichiosis in Tennessee have been rising since 2015.

Unlike other reportable tick-borne diseases in the state, ehrlichiosis results in hospitalization for 35-65% of Tennesseeans who become infected. In 2021, approximately 50% of all probable and confirmed ehrlichiosis cases were hospitalized for their illness.

Ehrlichiosis cases by year and hospitalization status

Signs and symptoms of ehrlichiosis typically develop within 1-2 weeks after being bitten by an infected tick. Early signs and symptoms (days 1-5) are usually mild or moderate, and they may include:

  • Fever and/or chills
  • Severe headache
  • Muscle aches
  • Nausea, vomiting, diarrhea, or loss of appetite
  • Confusion
  • Rash that looks like red splotches or pinpoint dots (more common in children than adults)

If antibiotic treatment is delayed, ehrlichiosis can sometimes cause severe illness. Early treatment can reduce your risk of developing severe illness. Late stage signs and symptoms can include:

  • Damage to the brain or nervous system (e.g. inflammation of the brain and surrounding tissue)
  • Respiratory failure
  • Uncontrolled bleeding
  • Organ failure
  • Death

For more information, see the CDC's Ehrlichiosis Signs and Symptoms web page.

Ehrlichiosis is diagnosed by a healthcare provider, who can order blood tests to look for evidence of Ehrlichia chaffeensis or Ehrlichia ewingii infection. It may take several weeks for test results to return. If your provider is concerned, they may prescribe you with antibiotics while you wait for the results.

For more information, see the CDC's Ehrlichiosis Diagnosis and Testing web page.