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)
- Tick bites
- Most people get ehrlichiosis from the bite of an infected tick.
- Ehrlichia chaffeensis and E. ewingii are transmitted by the lone star tick (Ambylomma americanum) Cdc-pdf[PDF – 2 pages], found primarily in the southcentral and eastern U.S.
- Ehrlichia muris eauclairensis is spread by the blacklegged tick (I. scapularis) Cdc-pdf[PDF – 2 pages] which is widely distributed in the eastern United States, although cases have only been reported in Minnesota and Wisconsin.
- Blood transfusion and organ transplant
- In rare cases, Ehrlichia species have been spread through blood transfusion and organ transplant.
- E. chaffeensis has been shown to survive for more than a week in refrigerated blood.
- Blood products are not routinely screened for the presence of Ehrlichia species.
- Leukoreduction might reduce the risk of Ehrlichia transmission, but it does not eliminate it.
- Two instances of E. chaffeensis transmission through renal transplant from a common donor have been reported.
- Patients who develop ehrlichiosis within a month of receiving a blood transfusion or solid organ transplant should be reported to state health officials for prompt investigation.
- For more information about ehrlichiosis and blood transfusions see the Special Considerations section of Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis — United States: A Practical Guide for Health Care and Public Health Professionals (2016) Cdc-pdf[PDF – 48 pages]
- 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.
In addition to E. muris eauclairensis, the blacklegged tick also transmits several other pathogens in certain geographic areas, including:
- Anaplasma phagocytophilum (the causative agent of anaplasmosis),
- Borrelia burgdorferi (the causative agent of Lyme disease),
- Babesia microti (the primary cause of human babesiosis),
- B. mayonii (a newly discovered cause of borreliosis),
- B. miyamotoi (a cause of relapsing fever), and
- Powassan virus (cause of Powassan virus disease)