Ehrlichia risticii sp. nov.: Etiological Agent of Equine Monocytic Ehrlichiosis (Synonym, Potomac Horse Fever)

Abstract

The etiological agent of equine monocytic ehrlichiosis (synonym, Potomac horse fever) was isolated and propagated in vitro by using techniques similar to those established for some members of the genus Ehrlichia. The organisms, which are contained within a cytoplasmic vacuole, are intermediate in size between Ehrlichia canis and Ehrlichia sennetsu, ranging from 0.4 to 0.75 μm wide and from 0.5 to 1.5 μm long. The agent is antigenically related to E. sennetsu and E. canis but also possesses unique antigenic determinants, thereby distinguishing it as a new species of the genus Ehrlichia. The name Ehrlichia risticii is proposed for this new agent. The type strain is strain HRC-IL (= ATCC VR-986).