Ehrlichia, Anaplasma, and related intracellular bacteria

Members of the genus Ehrlichia and Anaplasma are now recognized to be important human pathogens. They are obligate intracellular bacteria currently placed in the Proteobacteria phylum ( Alphaproteobacteria ), order Rickettsiales , and family Anaplasmataceae . Anaplasma phagocytophilum , within the genus Anaplasma , now includes Ehrlichia phagocytophila , Ehrlichia equi , and the human granulocytic ehrlichiosis (HGE) agent. Ehrlichia and Anaplasma spp. are gram-negative obligate intracellular bacteria that reside and propagate within membrane-lined vacuoles found in the cytoplasm of bone marrow-derived cells, such as granulocytes, monocytes, erythrocytes, and platelets. The causative agent of human monocytic ehrlichiosis (HME) is E. chaffeensis , a monocytotropic ehrlichia that was first identified as a human pathogen in a patient with a severe febrile illness after tick bites in 1986. The culture conditions for Ehrlichia and Anaplasma species are still being optimized. The most widely used method is polymerase chain reaction (PCR) amplification of DNA from E. chaffeensis in clinical samples using the HE1/HE3 primer set. In a prospective study, the overall sensitivity and specificity of PCR were 56% and 100%, respectively, using the 16S rRNA subunit, nadA, and 120-kDa protein genes. In this study several samples had high titers of antiehrlichial antibodies by immunofluorescence assay (IFA), suggesting that the pathogen may have already been cleared. Routine antimicrobial susceptibility testing of Ehrlichia or Anaplasma species isolates is unnecessary. The typical morphology of an Ehrlichia or Anaplasma spp. morula is observed, an assessment as to the hematopoietic lineage and the percentage of cells that contain morulae should be made and reported.