Cyclic thrombocytopenia induced by a Rickettsia-like agent in dogs.

Hematologic manifestations and the ultrastructure of a platelet-specific microorganism isolated from a dog in Florida were studied. The agent was readily transmitted experimentally to adult dogs by intravenous inoculation with infected blood. Parasitemias and concomitant thrombocytopenias were cyclic in that both recurred within relatively constant periods of one to two weeks following experimental infections. Hemorrhage was not a manifestation of the disease even though thrombocytopenias were severe. Microorganisms were visualized by light and electron microscopy. They were observed only in platelets and were composed of single or multiple subunits (morula forms). The microorganisms were ultrastructurally very similar to those reported in Ehrlichia canis infections of dogs and Anaplasma marginale infections of cattle. Microorganisms were surrounded by single membranes which more or less conformed to the external surfaces of subunits that were surrounded by double membranes. From electron microscopic studies, it is suggested that these organisms be classified in the order Rickettsiales.