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  • 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.

  • US cases of ehrlichiosis typically occur in men aged 60–69 years; US cases of anaplasmosis typically occur in men aged over 40 years; both occur in the summer, with different geographic areas of prevalence.

  • Excellent response to therapy with tetracyclines.

General Considerations

Human ehrlichiosis and anaplasmosis are endemic in the United States.

Ehrlichia chaffeensis (Table 32–3), the most common Ehrlichia species infecting humans, is seen primarily in the south-central United States (especially Arkansas, Missouri, and Oklahoma) (eFigure 32–8). Ehrlichia ewingii causes human granulocytic ehrlichiosis similar to anaplasmosis and constitutes almost 10% of ehrlichiosis cases; most cases in the United States are reported from the Midwest and Southeast. Ehrlichia muris eauclairensis, isolated in 2009, is seen in the upper Midwestern United States. Human granulocytic anaplasmosis is caused by Anaplasma phagocytophilum; most cases in the United States are reported from New England, New York, Minnesota, and Wisconsin (eFigure 32–9). Increasingly, anaplasmosis is being reported from Asia, South Korea, Mongolia, China (where a new species is identified, Anaplasma capra), and Northern Europe.

eFigure 32–8.

Annual reported incidence (per million population) for Erhlichia chaffeensis in the United States for 2018. (

eFigure 32–9.

Annual reported incidence (per million population) for anaplasmosis—United States, 2018.

In North America, the major tick-borne rickettsial disease vectors for these pathogens are (1) the Lone Star tick (Amblyomma americanus), which is the vector for E chaffeensis and E ewingii; (2) the black-legged tick (Ixodes scapularis), which is a vector for B burgdorferi (Lyme disease), Babesia microti (babesiosis), and A phagocytophilum (anaplasmosis), and a possible vector for Ehrlichia muris eauclairensis; and (3) the western black-legged tick (Ixodes pacificus), which is a vector for A phagocytophilum along the Pacific coast of the United States. Vectors for European and Asian cases are not reported to date. The principal reservoirs for human monocytic ehrlichiosis and human granulocytic anaplasmosis are the white tail deer and the white-footed mouse, respectively. Other mammals are implicated as well. Transfusion-transmitted anaplasmosis has been reported.

CDC reports indicate that the incidences of human monocytic ehrlichiosis, granulocytic ehrlichiosis and, in particular, anaplasmosis are increasing; cases are reportable to local and state health departments. Because more than one agent may coexist in the same area, cases of human ehrlichiosis and anaplasmosis may be reported as “human ehrlichiosis/anaplasmosis undetermined” in the absence of species identification.

Neorickettsia sennetsu is a related rickettsial pathogen that causes rare infection in Southeast Asia among ...

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