Ehrlichia are small gram-negative pleomorphic coccobacilli that primarily infect circulating leukocytes and other cells derived from the hematopoietic system. ^ There are two species of ehrlichia that cause human disease. Ehrlichia chaffeensis is the causative organism of human monocytic ehrlichiosis, and an organism that is closely related to or nearly identical to E. equi and E. phagocytophila is the causative agent of granulocytic ehrlichiosis.    E. chaffeensis infects mononuclear phagocytes in the blood and tissues, and the granulocytic Ehrlichia species infects granulocytic phagocytes in blood and tissues.  The infectious agents of human ehrlichiosis and Lyme disease are both transmitted by tick bites. Most cases of human granulocytic ehrlichiosis have been identified in Wisconsin, Minnesota, New York, Connecticut, and Massachusetts, and most cases of human monocytic ehrlichiosis have been identified in the southeastern and south-central United States. 
The pathogenesis of the diseases caused by ehrlichiae are not well understood at this time. It is clear, however, that the ehrlichiae damage and lyse host phagocytic cells once they have attached to these cells resulting in leukopenia and thrombocytopenia.  Patients with human granulocytic ehrlichiosis present with a febrile illness associated with myalgia, headache, malaise and occasionally nausea, vomiting, or a cough. The clinical presentation may be helpful in distinguishing Lyme disease from human granulocytic ehrlichiosis. Nausea, vomiting, and cough are very infrequent in Lyme disease. Fatigue is more often a complaint with Lyme disease than with human granulocytic ehrlichiosis. The incubation period is approximately 7 days following a tick bite, and approximately 20 percent of patients with ehrlichiosis have a rash that consists of a maculopapular eruption or petechiae. [w] Despite the different causative agents of monocytic and granulocytic ehrlichioses, the clinical and laboratory manifestations of the infections are similar. Neurological manifestations of both include severe headache, lethargy, confusion, coma, and seizure activity. The most common CSF abnormalities are lymphocytic pleocytosis with an elevated protein concentration and a borderline low CSF glucose concentration. Laboratory evidence of leukopenia, thrombocytopenia, and anemia, and mild to moderate elevations in levels of aspartate and alanine aminotransferases, alkaline phosphatase, and lactate dehydrogenase are present. 
Monocytic and granulocytic ehrlichiosis are separate diseases caused by different etiological agents; therefore, laboratory diagnosis requires different tests for each. The current CDC recommendation for the serological diagnosis of monocytic ehrlichiosis is a fourfold increase in E. chaffeensis antibody titer (a minimum titer of 64) or a single high-serum antibody titer (greater than or equal to 128) in
patients with a clinically compatible history. The current CDC recommendation for serological confirmation of human granulocytic ehrlichiosis requires a serological reaction or a four-fold increase in titer to E. equi antigen. A minimum titer of 80 is required.^ Diagnosis during the acute phase of the illness can also be made by PCR amplification of the agent of human granulocytic ehrlichiosis DNA from blood. ^08 Co-infections with B. burgdorferi and the agent of human granulocytic ehrlichiosis can occur.
Ehrlichioses must be treated with doxycycline. E. chaffeensis is susceptible only to doxycycline. Doxycycline is also the recommended agent for human granulocytic ehrlichiosis. The recommended dose in children and adults is 100 mg twice daily for adults or 3 mg/kg/d in two divided doses for children. Doxycycline can be given by either the oral or intravenous route. 
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