General Information: Neorickettsia risticii str. Illinois was isolated from horse blood in Maryland, USA. Neorickettsia risticii, formerly Ehrlichia risticii, is an obligate intracellular bacterium related to the Rickettsia. It can be transmitted from flukes that infest snails, fish or aquatic insects to horses by ingestion. This organism causes Potomac horse fever, also knows as equine monocytic ehrlichiosis, an acute diarrheal disease.