Biologic and Pathogenic Properties of Ehrlichia Risticii: The Etiologic Agent of Equine Monocytic Ehrlichiosis

Abstract

The etiologic agent of the recently recognized disease, equine monocytic ehrlichiosis (EME, synonymous with Potomac horse fever), was isolated in 1984. Antigenic analyses confirmed this microorganism to be a new species of the genus Ehrlichia. The agent was subsequently named Ehrlichia risticii in honor of Miodrag Ristic for his numerous scientific contributions to the study of rickettsiae and rickettsial diseases. Ehrlichia risticii is a gram-negative staining, pleomorphic bacterium, with a preference for growth in monocytes, and ranging in size from 0.4 to 0.75 μm long. The microorganisms occur within vacuoles as single or multiple intracytoplasmic inclusion bodies. Ehrlichia risticii has been successfully cultured in-vitro by using primary equine and canine blood monocytes as well as the continuous cell lines P388D1 (murine macrophage) and U937 (human histiocyte). L-glutamine is required for growth in tissue culture. The microorganism is strongly inhibited in-vitro by antibiotics. Antigenically, E. risticii is most closely related to the human pathogen, E. sennetsu. Experimentally, dogs, cats, mice, and non-human primates have been shown to be susceptible to infection with E. risticii. Clinically, EME is a noncontagious, infectious equine disease often characterized by fever, depression, anorexia, leukopenia, colic, mild to severe diarrhea, and with a mortality rate as high as 30%. Fetal infections may occur. The suspected arthropod vector has not been identified. Equine monocytic ehrlichiosis is widespread, with cases being confirmed in 38 states of the United States, in Canada, and in France. The indirect immunofluores-cent antibody test is used for serodiagnosis. Although a newly developed vaccine for prevention of EME has greatly reduced the number of clinical cases, the disease remains a serious threat to the equine industry.