Ehrlichia Ewingii—Another Human Ehrlichia Identified
ABSTRACT & COMMENTARY
Synopsis: DNA sequences of Ehrlichia ewingii were identified in leukocytes from four of 413 persons evaluated for possible ehrlichiosis. This is the fourth Ehrlichia species to be recognized to cause human disease.
Source: Buller RS, et al. Ehrlichia ewingii, a newly recognized agent of human ehrlichiosi. N Engl J Med 1999; 341:148-155.
Peripheral blood leukocytes from 413 patients with fever, headache, and thrombocytopenia with or without leukopenia were tested by PCR for Ehrlichia species. DNA of E. ewingii, an agent of granulocytic ehrlichiosis in dogs, was found in four patients. All had been exposed to ticks, and three were receiving immunosuppressive therapy. The disease appears indistinguishable from ehrlichiosis caused by Ehrlichia chaffeensis or the agent of human granulocytic ehrlichiosis.
Comment by Hal B. Jenson, MD, FAAP
The story of Ehrlichia as a cause of human disease is expanding. The first case of human ehrlichiosis was reported in 1987. The cause was initially thought to be due to the agent of ehrlichiosis in dogs, E. canis, but was later shown to be a distinct species and was subsequently named E. chaffeensis. In 1994, human granulocytic ehrlichisosis (HGE) was described, caused by the unnamed Ehrlichia species that is still known as "the agent of human granulocytic ehrlichiosis," which is closely related to E. equi and E. phagocytophila. E. chaffeensis infection is now distinguished by the name human monocytic ehrlichiosis (HME). Another species, E. sennetsu, causes a mononucleosis-like illness in Japan and Malaysia. From 1986-1997 there were 742 cases of HME reported in the United States. These infections are spread by specific ticks: Amblyomma americanum (the Lone Star tick) for E. chaffeensis, and Ixodes scapularis, the same tick that transmits Lyme disease, for HGE. A. americanum may also be the vector for E. ewingii. Coinfection with Lyme disease and HGE has been reported, and it is possible that coinfection with E. chaffeensis and E. ewingii may also occur. An interesting aspect of E. ewingii is that three of these patients were receiving prednisone, azathioprine, or methotrexate, suggesting that this infection is more likely, or more likely to be symptomatic, in immunocompromised persons. The majority of human Ehrlichia infections resolve without specific treatment and without sequelae, although infections can be severe and deaths have been reported. Human ehrlichiosis should be considered in any symptomatic person with a history of recent tick exposure—a thorough history for possible exposure is probably the most sensitive diagnostic test. The presence of thrombocytopenia or leukopenia also suggests ehrlichiosis. Diagnosis by PCR or by serologic testing would take too long to be clinically useful. Doxycycline or tetracycline should be given for a patient with a compatible history and signs, especially if the illness is severe or has not responded to other antibiotics. Maybe instead of telling patients that "It’s just a virus" for an unexplained febrile illness, we may have to say—at least for those persons with recent tick exposure—"Of course, it could be an Ehrlichia."
Human infections and diseases with Ehrlichia organisms:
a. are universally a result of infection by one species of Ehrlichia.
b. are often associated with a history of a tick bite.
c. have high mortality even when treated.
d. should not be treated with tetracyclines.
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