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, and resulted in a change in 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.

At a glance

• The number of anaplasmosis cases reported to CDC has increased steadily since the disease became reportable, from 348 cases in 2000, to 4,151 cases in 2016.
• The incidence (the number of cases for every million persons) of anaplasmosis has also increased, from 1.4 cases per million persons in 2000 to 6.1 cases per million persons in 2010.
• 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 U.S. anaplasmosis cases reported to CDC, 2000–2016

The graph displays the number of human cases of anaplasmosis reported to CDC annually from 2000 through 2016. *From 2000 to 2008, anaplasmosis was included in the reporting category “human granulocytic ehrlichiosis” in reports to NNDSS. **Since 2008, anaplasmosis has been reported to NNDSS in its own reporting category called “Anaplasma phagocytophilum”. Cases of anaplasmosis have generally increased from 350 cases in 2000, when the disease became nationally notifiable, to 1,163 cases in 2009, and 4,151 cases in 2016. The number of cases increased 14% between 2015 and 2016.

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Seasonality

• 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 increased numbers of nymphal blacklegged ticks, which is the primary life stage of this tick that bites humans and can transmit the pathogen.
• A second, smaller peak occurs in October and November and corresponds with the period of adult blacklegged tick activity.

Figure 2 – Number of reported anaplasmosis cases by month of onset, 2000–2016

The figure shows the number of cases reported from 2000 through 2016 by month of onset to give the seasonality of cases. There are cases reported in each month of the year, however most are reported in June and July. More than 50% of all cases occur in June and July.

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Geography

• 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 agents of Lyme disease (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 (Vermont, Maine, Rhode Island, Minnesota, Massachusetts, Wisconsin, New Hampshire, and New York) account for 90% 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 organism 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 reported incidence (per million population) for anaplasmosis – United States, 2016. (NN= Not notifiable)

The figure shows the incidence of anaplasmosis cases by state in 2016 per million persons. Anaplasmosis was not notifiable in Alaska, Colorado, the District of Columbia, Hawaii, Idaho, or New Mexico in 2016. The incidence rate was zero for Arizona, Georgia, Indiana, Louisiana, Mississippi, Montana, Nevada, South Carolina, Utah, Washington, and West Virginia. Incidence ranged from > 0 to 3.0 cases per million persons in California, Kentucky, Oregon, Florida, Ohio, Illinois, Michigan, Oklahoma, Nebraska, Alabama, Maryland, South Dakota, Kansas, North Carolina, Wyoming, Tennessee, Virginia, Missouri, and Iowa. Incidence ranged from > 3 to 26 cases per million persons in Delaware, Pennsylvania, Arkansas, New Jersey and North Dakota. Incidence ranged from > 26 to 130 cases per million persons in Connecticut, New York, New Hampshire, Wisconsin, and Massachusetts. The highest incidence rates, greater than 130 cases per million persons, were found in Minnesota, Rhode Island, Maine, and Vermont.

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People at Risk

• 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.

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