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Ehrlichiosis is an emerging zoonotic infection caused by obligate intracellular, gram-negative rickettsia that infect leukocytes.
Human monocytic ehrlichiosis (HME) is caused by Ehrlichia chaffeensis and is transmitted by the Lone Star tick, Amblyomma americanum. The deer is believed to be the animal reservoir and most cases of HME have been reported from the southeastern and south-central region of the United States.
Infectious forms are injected during tick bites and the organism enters the vascular system where it infects monocytes. It is sequestered in host-cell membrane-limited parasitophorous vacuoles known as morulae. These can be readily observed on Giemsa- or Wright-stained smears of peripheral blood from infected persons. Macrophages in organs of the reticuloendothelial system are also infected. Asexual reproduction occurs in WBCs and daughter cells are formed which are liberated upon cell rupture.
Most cases of ehrlichiosis are probably subclinical or mild, but the infection can be severe and life-threatening; there is a 2% to 3% mortality rate. Fever, fatigue, malaise, headache, and other "flu-like" symptoms occur most commonly. Central nervous system involvement can result in seizures and coma. Leukopenia, thrombocytopenia, and elevated hepatic transaminases are frequent laboratory findings.
As an adjunct in the diagnosis of ehrlichiosis and/or in seroepidemiological surveys of the prevalence of the infection in certain populations
Ehrlichiosis is sometimes diagnosed by observing the organisms in infected WBCs on Giemsa-stained thin blood films of smeared peripheral blood (morulae). Serology may be useful if the morulae are not seen or if the infection has cleared naturally or following treatment.
Serology may also be useful in the follow-up of documented cases of ehrlichiosis or when coinfection with other tick- transmitted organisms is suspected. In selected cases, documentation of infection may be attempted by PCR methods.
A positive immunofluorescence assay (titer >or =1:64) suggests current or previous infection with Ehrlichia chaffeensis. In general, the higher the titer, the more likely the patient has an active infection. Four-fold rises in titer also indicate active infection.
Serology for IgG may be negative during the acute phase of infection but a diagnostic titer usually appears by the third week after onset. Previous episodes of ehrlichiosis may produce a positive serology although antibody levels decline significantly during the year following infection.
Performance characteristics have not been established for hemolyzed or lipemic specimens.
Fishbein DB, Dawson JE, Robinson LE: Human ehrlichiosis in the United States, 1985 to 1990. Ann Intern Med 1994;120:736-743