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Key Features

Essentials of Diagnosis

  • Infection of monocyte or granulocyte by tick-borne gram-negative bacteria

  • Nine-day incubation period; clinical disease ranges from asymptomatic to life-threatening

  • Malaise, nausea, fever, and headaches

General Considerations

  • Human ehrlichiosis and anaplasmosis are endemic in the United States

  • Ehrlichiosis typically occurs in men aged 60–69 years; anaplasmosis typically occurs in men over 40 years old

  • Ehrlichia chaffeensis

    • Most common species infecting humans

    • Seen primarily in Arkansas, Missouri, Oklahoma and New York

  • Ehrlichia ewingii

    • Causes human granulocytic ehrlichiosis similar to anaplasmosis

    • Constitutes almost 10% of ehrlichiosis cases

    • Most cases in the United States are reported from the Midwest and Southeast

  • Ehrlichia muris eauclairiensis is seen in the upper Midwestern United States

  • Anaplasma phagocytophilum

    • Causes human granulocytic anaplasmosis

    • Most cases in the United States are reported from New England, New York, Minnesota, and Wisconsin

    • Increasingly being reported from Asia, South Korea, Mongolia, China (where a new species [Anaplasma capra] has been identified), and Northern Europe

  • CDC reports that the incidences of human monocytic ehrlichiosis, granulocytic ehrlichiosis and, in particular, anaplasmosis are increasing; coinfection may occur

Clinical Findings

  • Human monocytic ehrlichiosis

    • Clinical disease ranges from mild to life-threatening

    • Incubation period: 1–2 weeks

    • Prodrome then develops, consisting of malaise, rigors, and nausea, high fever and headache

    • A pleomorphic rash may occur

    • Presentation in immunosuppressed patients and older patients tends to be more severe

  • Human granulocytic ehrlichiosis and anaplasmosis

    • Clinical manifestations are similar to those seen with human monocytic ehrlichiosis

    • However, rash is infrequent

    • If a rash is present, coinfection with other tick-borne diseases or an alternative diagnosis should be suspected

    • Persistent fever and malaise are reported to occur for 2 or more years

  • Coinfection with anaplasmosis and Lyme disease or babesiosis may occur, but the clinical manifestations (including fever and cytopenias) are more severe with anaplasmosis than with Lyme disease

Differential Diagnosis

  • Infection with Borrelia miyamotoi may mimic anaplasmosis in its clinical manifestations


  • 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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