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 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
Probable: a clinically compatible
case with a single IFA serologic titer >=1:64 or intracytoplasmic morulae identified
in blood, bone marrow or CSF leukocytes
Confirmed: a clinically compatible case that meets the
laboratory criteria for diagnosis