Human monocytotropic ehrlichiosis
An acute infectious disease characterized by fever, chills, headache, muscle and joint pain, and variable respiratory, gastrointestinal, and systemic involvement; hematologic studies show depression of erythrocytes, leukocytes, and platelets. The finding of clumps of developing organisms, called morulae, in the cytoplasm of monocytes in a stained smear of peripheral blood establishes the diagnosis, but their detection is often difficult. Serologic testing shows antibody to Ehrlichia chaffeensis, an organism closely resembling the agent of canine ehrlichiosis, E. canis. This disease has been largely confined to the southeastern and south central United States. The Lone Star tick (Amblyomma americanum) and the American dog tick (Dermacentor variabilis) are the principal vectors. Incidence is highest from April to September, during the peak activity of these ticks.