Potomac Horse Fever Research Articles

Potomac horse fever agent in mice.

Potomac horse fever agent in mice.

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

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).

Causative ehrlichial organisms in Potomac horse fever.

An ehrlichia was consistently isolated from the peripheral blood leukocyte fraction of ponies that had been experimentally infected with Potomac horse fever by whole blood transfusion from naturally infected horses. The organism was propagated in a human histiocyte cell line for 3 to 5 weeks and then inoculated intravenously or intradermally into healthy adult ponies. Clinical signs of Potomac horse fever, which varied in the degree of severity, occurred 9 to 14 days post-inoculation in all of the ponies. One pony died 20 days post-inoculation. The ehrlichial organism was reisolated in the human histiocyte cell line from the blood leukocyte fraction of all of the experimental ponies on each day that samples were examined (days 9, 10, 11, 19, and 39). These organisms were identical to those originally detected in the wall of the intestine of ponies with clinically diagnosed Potomac horse fever when compared by light and electron microscopy and an immunofluorescence labeling technique. The immunofluorescent antibody titer became positive in a pony at 20 days postinjection. These results indicate that the ehrlichial organisms is the causative agent of Potomac horse fever.

Ultrastructural study of ehrlichial organisms in the large colons of ponies infected with Potomac horse fever.

Potomac horse fever is characterized by fever, anorexia, leukopenia, profuse watery diarrhea, dehydration, and high mortality. An ultrastructural investigation was made to search for any unusual microorganisms in the digestive system, lymphatic organs, and blood cells of ponies that had developed clinical signs after transfusion with whole blood from horses naturally infected with Potomac horse fever. A consistent finding was the presence of rickettsial organisms in the wall of the intestinal tract of these ponies. The organisms were found mostly in the wall of the large colon, but fewer organisms were found in the small colon, jejunum, and cecum. The organisms were also detected in cultured blood monocytes. In the intestinal wall, many microorganisms were intracytoplasmic in deep glandular epithelial cells and mast cells. Microorganisms were also found in macrophages migrating between glandular epithelial cells in the lamina propria and submucosa. The microorganisms were round, very pleomorphic, and surrounded by a host membrane. They contained fine strands of DNA and ribosomes and were surrounded by double bileaflet membranes. Their ultrastructure was very similar to that of the genus Ehrlichia, a member of the family Rickettsiaceae. The high frequency of detection of the organism in the wall of the intestinal tract, especially in the large colon, indicates the presence of organotrophism in this organism. Infected blood monocytes may be the vehicle for transmission between organs and between animals. The characteristic severe diarrhea may be induced by the organism directly by impairing epithelial cell functions or indirectly by perturbing infected macrophages and mast cells in the intestinal wall or by both.

Experimental reproduction of Potomac horse fever in horses with a newly isolated Ehrlichia organism

Potomac horse fever, a recently recognized disease of equines, characterized by high fever, leukopenia, and a profuse diarrhea, was studied for its etiology. An Ehrlichia organism was isolated in equine macrophage-fibroblast cell cultures and mouse macrophage cell cultures from the mononuclear cells of blood of infected horses. The agent was continuously propagated in mouse macrophage cell cultures. The organism multiplied in the cytoplasm of mouse macrophage cells and was identified by Giemsa staining, acridine orange staining, and by indirect immunofluorescence with convalescent sera from infected horses. The disease was experimentally reproduced in horses inoculated with Ehrlichia-infected cell culture material. The Ehrlichia organism was reisolated from the blood of these infected horses during the course of the disease. Antibody against the organism was detected in the sera of experimentally infected horses. This study confirmed that the new Ehrlichia organism is the etiological agent of Potomac horse fever.

Isolation, experimental transmission, and characterization of causative agent of Potomac horse fever.

Potomac horse fever, a disease characterized by fever, anorexia, leukopenia, and occasional diarrhea, is fatal in approximately 30 percent of affected animals. The seasonal occurrence of the disease (June to October) and evidence of antibodies to the rickettsia Ehrlichia sennetsu in the serum of convalescing horses suggested that a related rickettsia might be the causative agent. Such an agent was isolated in cultured blood monocytes from an experimentally infected pony. This intracytoplasmic organism was adapted to growth in primary cultures of canine blood monocytes. A healthy pony inoculated with these infected monocytes also developed the disease. The organism was reisolated from this animal which, at autopsy, had pathological manifestations typical of Potomac horse fever. Cross serologic reactions between the newly isolated agent and antisera to 15 rickettsiae revealed that it is related to certain members of the genus Ehrlichia, particularly to Ehrlichia sennetsu. Since the disease occurs in other parts of the United States as well as in the vicinity of the Potomac River, and since it has also been reported in Europe, the name equine monocytic ehrlichiosis is proposed as being more descriptive.