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Human ehrlichiosis and anaplasmosis are potentially severe illnesses endemic in the United States. Several bacterial agents are known causes of these diseases: Ehrlichia chaffeensis, Ehrlichia ewingii, Ehrlichia muris-like agent, Panola Mountain Ehrlichia species, and Anaplasma phagocytophilum. Because more than one agent may be present in one area, cases of human ehrlichiosis and anaplasmosis may be reported as “human ehrlichiosis/anaplasmosis undetermined” when the available evidence does not suggest an etiology to the species level. Here, we present a brief summary of these undetermined cases with onset of symptoms from 2008 to 2012 reported to two passive surveillance systems in the United States. The reported incidence rate during this time was 0.52 cases per million person-years. Many cases (24%) had positive polymerase chain reaction results. Enhanced surveillance in an area where several of these etiologic agents are endemic may provide a better understanding of the epidemiology of ehrlichiosis and anaplasmosis in the United States.
Financial support: This study was supported by the Centers for Disease Control and Prevention as well as by an appointment to the Research Participation Program at the Centers for Disease Control and Prevention administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the Centers for Disease Control and Prevention.
Authors' addresses: F. Scott Dahlgren, Kristen Nichols Heitman, and Casey Barton Behravesh, Rickettsial Zoonoses Branch, Division of Vector-Borne Diseases, Centers for Disease Control and Prevention, Atlanta, GA, E-mails: email@example.com, firstname.lastname@example.org, and email@example.com.