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Key Features

  • Tick-borne gram-negative obligate intracellular bacteria

  • Two main clinical entities

    • Human granulocytic ehrlichiosis (typically Anaplasma phagocytophilium and, rarely, Ehrlichia ewingii)

    • Human monocytic ehrlichiosis (typically Ehrlichia chaffeensis)

  • Bacteria form morulae, intracytoplasmic aggregates within leukocytes

  • In the United States, mainly found in mid-Atlantic, southeastern, and central states

  • Rare serious sequelae include acute respiratory failure and acute respiratory distress syndrome; neurologic complications, the most common being meningoencephalitis and aseptic meningitis; acute kidney disease (which may mimic thrombotic thrombocytopenic purpura); hemophagocytic syndrome, and multi-organ failure

  • May see coinfection with other tick-borne illnesses (eg, Lyme disease or babesiosis)

Clinical Findings

  • Clinical disease of human monocytic ehrlichiosis ranges from mild to life threatening

  • Incubation period is 1–2 weeks

  • Prodrome of malaise, rigors, and nausea, followed by fever, and headache

  • Respiratory failure, acute kidney disease, and encephalopathy may ensue in severe cases


  • Leukopenia, absolute lymphopenia, thrombocytopenia, and transaminitis common

  • Thrombocytopenia occurs more often than leukopenia in human granulocytic ehrlichiosis

  • Examination of peripheral blood with Giemsa stain may reveal characteristic intraleukocytic vacuoles (morulae) in up to 20% of patients

  • Polymerase chain reaction assay is most sensitive in the first week of illness and can be used as a confirmatory test

  • Indirect fluorescent antibody assay available from the CDC and requires acute and convalescent sera


  • Treatment for all forms of ehrlichiosis is with doxycycline, 100 mg twice daily (orally or intravenously) for 10–14 days or until 3 days of defervescence

  • Rifampin is an alternative in pregnant women

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