Ehrlichiosis--a disease of animals and humans.

Ehrlichiae are one of several kinds of obligate intracellular bacteria. Taxonomically, they are grouped with rickettsiae, but they can be distinguished by their unique tropism for circulating leukocytes. Ehrlichia canis causes a pancytopenia in dogs that becomes chronic if untreated. Certain breeds develop severe infections, characterized by fever, anorexia, dramatic weight loss, marked pancytopenia, anemia, peripheral edema, and hemorrhage. Ehrlichia risticii, a recently discovered species, is the cause of a serious diarrheal disease of horses. Other species of ehrlichiae have been documented as being veterinary pathogens. Recent data indicate that E. canis or a closely related species causes an acute febrile illness in humans. Clinically, the disease is similar to Rocky Mountain spotted fever, except that most patients do not have a rash. Human ehrlichiosis appears to be tickborne and is prevalent primarily in the southern Atlantic and south-central states. A mild from of ehrlichiosis has also been documented.