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HME is a systemic disease characterized by fever,
headache, myalgia, anorexia, and chills, and frequently accompanied with
leukopenia, thrombocytopenia, anemia, and elevations in serum hepatic
aminotransferases. The severity of the disease varies from asymptomatic
seroconversion to death, and severe morbidity is frequently documented.
More than 1,500 probable or confirmed cases of HME have been reported
primarily in the southeastern and south central regions since the original
discovery of the disease in 1986. In certain geographic regions, the
incidence exceeds that of Rocky Mountain spotted fever, most known
rickettsial (rickettsia is the closest relative of Ehrlichia spp. and
vector-borne) disease in the U.S. HME has been also reported in Europe and
Africa. E. chaffeensis has been most commonly identified in the Lone Star
tick (Amblyomma americanum), and white-tailed deer are considered to be the
major reservoir of E. chaffeensis.
HGE is characterized by similar clinical signs and laboratory
findings as HME. To date more than 500 cases of HGE have been confirmed in
the upper Midwest, northeastern, and Pacific states. These areas correspond
to the Lyme disease-endemic foci. HGE has been more frequently reported in Europe than HME. The
causative agent, Anaplasma phagocytophilum (formerly the HGE agent), is closely related to two previously known
veterinary pathogens, Ehrlichia equi and E. phagocytophilum (they are now all belong to the same species: A. phagocytophilum). A. phagocytophilum has been
found in the deer tick (Ixodes scapularis) and white-footed mice are
considered to be the major reservoir of A. phagocytophilum. Human coinfection with A. phagocytophilum and
Borrelia burgdorferi occurs, presenting a new diagnostic and treatment
problem. Clinical signs caused
by infection with another granulocytotropic ehrlichia, E. ewingii are very
similar to those of HGE and HME and patients were discovered due to serologic
crossreactivity between E. chaffeensis and E. ewingii.
Neorickettsia sennetsu (formerlyE. sennetsu) causes Sennetsu ehrlichiosis, an infectious
mononucleosis-like disease (fever, fatigue, general malaise, and
lymphadenopathy). Although the mode of transmission of N. sennetsu is still
unproven, N. sennetsu is genetically and antigenically closely related to
known trematode-borne agents. Therefore, the trematode is considered to be
the reservoir and vector, and the mode of transmission may be oral, as
suspected based on association of cases with the consumption of raw or
under-cooked gray mullet fish which often infected with the metacercaria. Another
potential human ehrlichiosis which is yet to be discovered in the U.S. is
human E. canis infection. E. canis is monocytotropic ehrlichia closely
related to E. chaffeensis. One year after the initial serologic diagnosis our
laboratory isolated an E. canis-like agent from an asymptomatic person in
Venezuela who had slight thrombocytopenia and lymphocytosis, suggesting
possible persistent infection as is commonly seen in dogs with E. canis
infection. E. canis infection of dogs is very common in the U.S. and
throughout the world.
For any of human ehrlichioses, no vaccine exists and
the diagnosis is made based on retrospective seroconversions or PCR analysis.
Although doxycycline is generally found effective in treating ehrlichiosis,
delayed initiation of therapy, the presence of underlying illness, and
immunosuppression often lead to severe complications, chronic illness, or
death.
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