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1996 Case Definition

Clinical Description

A tick-borne febrile illness most commonly characterized by acute onset, accompanied by headache, myalgia, rigors and/or malaise. Clinical laboratory findings may include intracytoplasmic microcolonies (morulae) in leukocytes of peripheral smear, cerebrospinal fluid (CSF), or bone marrow aspirate or biopsy, cytopenias (especially thrombocytopenia and leukopenia), and elevated liver enzymes (especially alanine aminotransferase or aspartate aminotransferase).

There are two clinically similar yet serologically distinct forms of ehrlichiosis: a) human granulocytic ehrlichosis (HGE), caused by infection with an Ehrlichia equi-like agent and found primarily in the upper midwest and northeast, and b) human monocytic ehrlichiosis (HME) caused by Ehrlichia chaffeensis infection and found primarily in the southeastern quadrant of the United States.

Laboratory Criteria for Diagnosis

  • Fourfold or greater change in antibody titer to Ehrlichia spp. antigen by immunofluorescence antibody (IFA) test in acute- and convalescent-phase specimens ideally taken greater than or equal to 4 weeks apart. HME diagnosis requires E. chaffeensis and HGE currently requires E. equi or HGE-agent antigen, OR
  • Positive polymerase chain reaction assay. Distinct primers are used for the diagnosis of HGE and HME, OR
  • Intracytoplasmic morulae identified in blood, bone marrow, or CSF leukocytes, and an IFA antibody titer greater than or equal to 64.

Case Classification


A clinically compatible case with either a single IFA serologic titer greater than or equal to 64 or intracytoplasmic morulae identified in blood, bone marrow, or CSF leukocytes


A clinically compatible case that is laboratory confirmed


All laboratory testing should be conducted by experienced personnel with appropriate training and should include appropriate controls and reagents necessary for accurate etiologic diagnosis. States in which cases of HGE and/or HME have occurred may submit reports to CDC