Epidemiology and Statistics

• 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.
• Ehrlichiosis, previously referred to as human monocytic ehrlichiosis (HME), was first recognized as a human disease in the United States in the late 1980s but did not become a reportable disease until 1999, with the first data reported in 2000.
• CDC compiles the number of cases reported by state and local health departments and reports national trends. Disease caused by E. ewingii was not considered separately reportable until 2008.

In 2008, the ehrlichiosis case definition was split into four categories:

1. Ehrlichia chaffeensis infection
2. Ehrlichia ewingii infection
3. Anaplasma phagocytophilum infection
4. Undetermined ehrlichiosis/anaplasmosis

Infections from the recently discovered E. muris eauclairensis are still reported under the undetermined ehrlichiosis/anaplasmosis category. Due to low reporting numbers of other ehrlichioses, national surveillance data shown below only relate to cases of E. chaffeensis ehrlichiosis.

At a Glance: E. chaffeensis reporting

• The number of ehrlichiosis cases due to E. chaffeensis reported to CDC has increased steadily since the first year of disease reporting.
• In the year 2000, only 200 cases of ehrlichiosis were reported. In 2018, the number of reported cases continues to rise to 1,799 cases.
• Notably, while cases and incidence rose, the case fatality rate (i.e., the proportion of ehrlichiosis patients that died as a result of infection) has declined since 2000. The case fatality rate in recent publications is still roughly 1% of cases.

Figure 1 – Number of U.S. ehrlichiosis cases caused by Ehrlichia chaffeensis and reported to CDC, 2000–2018

Seasonality

• Although cases of ehrlichiosis can occur during any month of the year, 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.
• This period coincides with the season for increased numbers of adult and nymphal lone star ticks. All stages of this tick feed on humans, however, only adult and nymphal ticks spread E. chaffeensis to humans.

Figure 2 – Number of reported ehrlichiosis cases caused by Ehrlichia chaffeensis by month of onset, 2000–2018

Geography

• Ehrlichiosis is most frequently reported from the southeastern and south-central United States, from the East Coast extending westward to Texas.
  • These areas overlap with the known geographic distribution of the lone star tick (Amblyomma americanum), the primary tick vector of E. chaffeensis and E. ewingii.
• In 2018, four states (Missouri, Arkansas, New York, and Virginia) accounted for more than half of all reported cases of ehrlichiosis.
• Ehrlichiosis caused by E. muris eauclairensis has been found in patients living in Minnesota and Wisconsin.
  • The primary tick vector for E. muris eauclairensis is Ixodes scapularis, the tick that spreads Lyme disease, anaplasmosis, Powassan virus infection, and babesiosis.

Figure 3 – Annual reported incidence (per million population) for E. chaffeensis in the United States for 2018. (NN= Not notifiable)

People at Risk

• From a review of the national surveillance data, cases of ehrlichiosis are more frequently reported in men than women.
• People between 60 to 69 years of age account for the highest number of cases.
• People with compromised immune systems (e.g., resulting from cancer treatments, advanced HIV infection, prior organ transplants, or some medications) might be at increased risk for severe disease.

E. ewingii Surveillance

• E. ewingii infections are impossible to distinguish from E. chaffeensis infections based on clinical signs alone. As a result, some proportion of cases currently reported as E. chaffeensis infection actually may be due to E. ewingii.
• Currently, there is no available serologic test that can distinguish between diseases caused by these agents, and surveillance for E. ewingii ehrlichiosis is based on detection of the organism using molecular-based diagnostic tests.
• In total, 218 cases of E. ewingii ehrlichiosis were reported to CDC from 2008–2018. No fatal cases of E. ewingii ehrlichiosis have been reported.

Undetermined ehrlichiosis/anaplasmosis

• This reporting category reflects cases that showed clinical and laboratory signs consistent with either an ehrlichiosis or anaplasmosis infection, but which could not be listed as a specific organism due to the limitations of diagnostic test results.
• This category is also used to report cases of ehrlichiosis not caused by E. chaffeensis or E. ewingii (such as cases resulting from infection with the newly identified E. muris eauclairensis).

E. muris eauclairensis, formerly E. muris-like agent (EMLA)

• In 2009, a new Ehrlichia species was identified in patients in the upper Midwest.
• Formerly referred to as the “E. muris-like agent” (EMLA), this bacteria has now been formally named E. muris eauclairensis.
• Since its discovery, more than 115 cases of ehrlichiosis caused by E. muris eauclairensis have been reported. No deaths have been described.