Anaplasmosis

Statistics and Epidemiology

Annual Cases of Anaplasmosis in the United States

Anaplasmosis was first recognized as a disease of humans in the United States in the mid-1990’s, but did not become a reportable disease until 1999. CDC compiles the number of cases reported by state health departments.

Anaplasmosis is caused by the bacterium Anaplasma phagocytophilum. This organism used to be known by other names, including Ehrlichia equi and Ehrlichia phagocytophilum, and the disease caused by this organism has been previously described 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.

The number of anaplasmosis cases reported to CDC has increased steadily since the disease became reportable, from 348 cases in 2000, to 1006 cases in 2008. 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 4.2 cases per million persons 2008. 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%.

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Figure 1 - Number of U.S. anaplasmosis cases^* reported to CDC, 1994-2008.

^*Numbers presented here may differ from numbers presented in the MMWR Annual Summary of Notifiable Diseases. Data presented here are based on year of illness onset rather than reporting year.

Annual Anaplasmosis Cases, 1994 - 2008

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Figure 2 Proportion of U.S. anaplasmosis cases reported to CDC with fatal outcome (case fatality rate), 2000-2008.

Anaplasmosis Fatality Rate 2000 - 2008

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Geography

Anaplasmosis is most frequently reported from the upper midwestern and northeastern United States. A fewer number of cases are also reported in northern California. The areas from which cases are reported correspond with the known geographic distribution of Lyme disease. The tick responsible for transmission of A. phagocytophilum in the upper Midwest and northeastern U.S. is the black-legged tick (Ixodes scapularis). Along the West Coast, the western black-legged tick (I. pacificus) may transmit the organism. These tick species also transmit the agents of Lyme disease (Borrelia burgdorferi) and babesiosis (Babesia species), and human co-infections with these organisms have occasionally been reported.

Six states (New York, Connecticut, Massachusetts, Rhode Island, Minnesota, and Wisconsin) account for 88% of all reported cases of anaplasmosis. Occasionally, A. phagocytophilum infections may be reported in other parts of the United States, including southeastern and south-central states where the organism is not commonly found. Some of these cases may be due to patient travel to states with higher levels of disease, or the misdiagnosis of anaplasmosis in patients actually infected with another tickborne disease, such as ehrlichiosis or Rocky Mountain Spotted Fever.

Figure 3 - Annual reported incidence (per million population) for anaplasmosis in the United States for 2008. (NN= Not notifiable)

Annual reported incidence (per million population) for anaplasmosis in the U.S. for 2008

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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 the months of June and July. This period is the season for increased numbers of nymphal black-legged ticks, which is the primary life stage of this tick that bites humans and may transmit the pathogen.

Figure 4 - Proportion of anaplasmosis cases reported to CDC by month of onset, 2008.

Anaplasmois cases reported to CDC by month of onset 2008

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Persons at Risk

The frequency of reported cases of anaplasmosis is highest among males and people over 50 years of age. A compromised immune system (such as may occur through cancer treatments, advanced human immunodeficiency virus infection, prior organ transplants, or some medications) may increase the risk of severe outcome. Individuals who reside near or spend time in known tick habitats may be at increased risk for infection.

Figure 5 - Average annual incidence of anaplasmosis by age group, 2008

Average annual incidence of anaplasmosis by age group, 2008

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Other Ehrlichiosis, Undetermined

This reporting category reflects cases that showed clinical and laboratory signs consistent with either an ehrlichiosis or anaplasmosis infection, but which could not be attributed to a specific organism due to the limitations of diagnostic test results. This category is also used to report new cases of human illness attributed to other Ehrlichia or Anaplasma species.