Epidemiology and Statistics
- Anaplasmosis is a disease caused by the bacterium Anaplasma phagocytophilum.
- This organism was previously known by other names, including Ehrlichia equi and Ehrlichia phagocytophilum, and the disease was previously known as human granulocytic ehrlichiosis (HGE).
- However, a taxonomic change in 2001 identified that this organism belonged to the genus Anaplasma, which resulted in changing the name of the disease to anaplasmosis.
- In the United States, anaplasmosis was first recognized as human disease in the mid-1990s but did not become nationally notifiable until 1999.
- CDC compiles the number of cases reported by state and local health departments and reports national trends.
- The number of anaplasmosis cases reported to CDC has increased steadily since the disease became reportable, from 348 cases in 2000, to a peak of 5,762 in 2017. Cases reported in 2018 were substantially lower, but increased to near 2017 numbers in 2019 with 5,655 cases.
- The case fatality rate (i.e., the proportion of anaplasmosis patients that reportedly died as a result of infection) has remained low, at less than 1%.
Figure 1 – Number of reported cases of anaplasmosis –United States, 2000–2019
|Year of report||Number of cases|
- Although cases of anaplasmosis can occur during any month of the year, the majority of cases reported to the CDC have an illness onset during the summer months and a peak in cases typically occurs in June and July.
- This period is the season for nymphal blacklegged ticks. Nymphal blacklegged ticks bite people and can spread the pathogen.
- A second, smaller peak occurs in October and November when adult blacklegged ticks are most active.
Figure 2 – Number of reported cases of anaplasmosis, by month of onset–United States, 2000–2019
|Month of onset||Number of cases|
- Anaplasmosis is most frequently reported from the upper midwestern and northeastern United States.
- These areas correspond with the known geographic distribution of the blacklegged tick (Ixodes scapularis), the primary tick vector of A. phagocytophilum.
- This tick also transmits the bacteria that causes Lyme disease (including Borrelia burgdorferi) and other human pathogens. Co-infections with these organisms have occasionally been reported.
- The geographic range of anaplasmosis appears to be increasing, which is consistent with the blacklegged tick’s expanding range.
- Increasing ranges for the blacklegged tick have been documented along the Hudson River Valley, Michigan, and Virginia.
- Eight states (New York, Massachusetts, Maine, Vermont, Minnesota, Wisconsin, New Hampshire, and Conneticut) account for nearly 9 in 10 of all reported cases of anaplasmosis.
- Occasionally, anaplasmosis cases are reported in other parts of the United States, including southeastern and south-central states where the pathogen has not been commonly found.
- Some of these cases might be due to patient travel to states with higher levels of disease, or misdiagnosis of anaplasmosis in patients actually infected with another closely related tickborne disease, ehrlichiosis.
Figure 3 – Annual incidence (per million population) of reported anaplasmosis–United States, 2019. (NN= Not notifiable)
|State of Residence||Cases per Million|
|District of Columbia||0|
- The frequency of reported cases of anaplasmosis is highest among males and people over 40 years of age.
- People with weakened immune systems (such as those occurring due to cancer treatments, advanced HIV infection, prior organ transplants, or some medications) might be at increased risk of severe outcome.
- People who live near or spend time in known tick habitats might be at increased risk for infection.
Page last reviewed: August 6, 2021