Transmission and Epidemiology

Ehrlichiosis is the general name used to describe diseases caused by the bacteria Ehrlichia chaffeensis, E. ewingii, or E. muris eauclairensis in the United States. The majority of reported cases are due to infection by E. chaffeensis.

Figure 1: Estimated geographic distribution of lone star ticks (above) and blacklegged ticks (below)

Map of the United States showing the approximate distribution of the Lone Star tick.  The area affected is the eatern half of the country.
Approximate distribution of the Blacklegged tick in the United States of America



  • Geographic range: The geographic range of ehrlichiosis cases depends highly on the species of Ehrlichia causing illness.
    • E. chaffeensis and E. ewingii infections occur primarily in south-central, southeastern, and mid-Atlantic states.
    • E. muris eauclairensis infections have only been reported from Wisconsin and Minnesota and travelers to those states.
  • Peak transmission: The majority of cases reported to CDC have an illness onset during the summer months with a peak in cases typically occurring in June and July.

For more information, see: Seasonality

Other Tickborne Pathogens Spread by the Same Tick Vector

In addition to E. chaffeensis and E. ewingii, the lone star tick also transmits several other pathogens in certain geographic areas, including:

  • Heartland virus (the cause of Heartland virus disease),
  • Francisella tularnesis (the causative agent of tularemia),
  • Southern tick-associated rash illness (STARI)
  • Bourbon virus (the cause of Bourbon virus disease)

Allergic reactions associated with consumption of red (mammalian) meat and tick paralysis have also been associated with lone star tick bites. For more information on red meat allergies see NIAID’s websiteexternal icon.

In addition to E. muris eauclairensis, the blacklegged tick also transmits several other pathogens in certain geographic areas, including: