The Etiology of Potomac Horse Fever (Ehrlichia Risticii)
Holland, Cynthia Jane
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https://hdl.handle.net/2142/71335
Description
Title
The Etiology of Potomac Horse Fever (Ehrlichia Risticii)
Author(s)
Holland, Cynthia Jane
Issue Date
1986
Department of Study
Veterinary Medical Science
Discipline
Veterinary Medical Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Veterinary Science
Abstract
Potomac horse fever (PHF) was clinically characterized in 1979. It is fatal in approximately 30 percent of cases. The finding of antibodies to Ehrlichia sennetsu, causative agent of human sennetsu rickettsiosis, in the serum of horses recovering from PHF implicated a rickettsia as the potential causative agent of PHF. A rickettsial agent was subsequently isolated in cultured blood monocytes derived from a pony experimentally inoculated with blood from an acute case of PHF. This isolate was successfully adapted to continuous in vitro propagation in primary canine blood monocytes and the murine macrophage cell line P388D1. Two healthy ponies inoculated with organisms cultured in primary canine blood monocytes developed typical signs of PHF and the organism was reisolated in culture from the blood of both animals, thereby fulfilling Koch's postulates. Light and electron microscopy demonstrated that the morphology and ultrastructure of the organism was consistent with the genus Ehrlichia. The organisms were observed to be contained within a cytoplasmic vacuole and were intermediate in size between E. sennetsu and E. canis, ranging from 0.4 to 0.75 (mu)m in width and from 0.5 to 1.5 (mu)m in length. There was evidence that the organism divided by binary fission.
Utilizing culture-derived organisms, an indirect fluorescent antibody (IFA) test was developed and used to monitor the humoral response of experimentally infected animals and for the confirmatory diagnosis of field cases of the disease. In the past year, PHF has been confirmed in 16 states of the U.S. and the province of Ontario, Canada.
Using the IFA test, serologic cross-reaction was demonstrated between the Potomac horse fever (PHF) agent, E. sennetsu (strong), and E. canis (moderate to weak), but not between the PHF agent and E. equi nor with 12 species of the genus Rickettsia.
By immunoadsorption and Western blot analysis, unique antigenic determinants were demonstrated in the PHF agent in addition to those shared with E. sennetsu. Antigenic, morphologic and ultrastructural studies warranted the classification of the PHF agent as a new species of Ehrlichia. The agent was named Ehrlichia risticii and accepted by the International Committee on Taxonomic Nomenclature. The agent was deposited with the American Type Culture Collection (strain HRC-IL, ATCC VR-986).
Finally, Rhesus (Macaca mulatta) monkeys were found susceptible to infection by E. risticii thus underscoring the potential zoonotic nature of the organism.
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