Experimental reproduction of Potomac horse fever in horses with a newly isolated Ehrlichia organism.

Potomac horse fever, a recently recognized disease of equines, characterized by high fever, leukopenia, and a profuse diarrhea, was studied for its etiology. An Ehrlichia organism was isolated in equine macrophage-fibroblast cell cultures and mouse macrophage cell cultures from the mononuclear cells of blood of infected horses. The agent was continuously propagated in mouse macrophage cell cultures. The organism multiplied in the cytoplasm of mouse macrophage cells and was identified by Giemsa staining, acridine orange staining, and by indirect immunofluorescence with convalescent sera from infected horses. The disease was experimentally reproduced in horses inoculated with Ehrlichia-infected cell culture material. The Ehrlichia organism was reisolated from the blood of these infected horses during the course of the disease. Antibody against the organism was detected in the sera of experimentally infected horses. This study confirmed that the new Ehrlichia organism is the etiological agent of Potomac horse fever.