Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to page options Skip directly to site content

1998 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 but serologically distinct forms of ehrlichiosis: human granulocytic ehrlichiosis (HGE) caused by infection with an Ehrlichia equi-like agent and found primarily in the upper midwest and northeast, and human monocytic ehrlichiosis (HME) caused by Ehrlichia chaffeensis infection and found primarily in the southeastern quadrant of the U.S.

Laboratory Criteria for Diagnosis

All laboratory testing must be conducted by experienced personnel with appropriate training and include appropriate controls and reagents necessary for accurate etiologic diagnosis.

  • Fourfold or greater change in antibody titer to Ehrlichia spp. antigen by immunofluorescence antibody (IFA) test in acute and convalescent specimens ideally taken four weeks or more apart. HME diagnosis requires E. chaffeensis antigen and HGE diagnosis currently requires E. equi or HGE-agent antigen; OR
  • Positive polymerase chain reaction (PCR) 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 ≥1:64.

Case Classification


A clinically compatible case with a single IFA serologic titer ≥1:64 or intracytoplasmic morulae identified in blood, bone marrow or CSF leukocytes


A clinically compatible case that meets the laboratory criteria for diagnosis