Clinical, histopathological, and immunological responses of ponies to Ehrlichia sennetsu and subsequent Ehrlichia risticii challenge

Abstract

Ehrlichia risticii has a close antigenic relationship to E. sennetsu. Sera of ponies experimentally infected with E. risticii, the etiologic agent of Potomac horse fever, consistently reacted with E. sennetsu, a human pathogen, in indirect fluorescent-antibody (IFA) testing, while human E. sennetsu convalescent serum reacted with E. risticii by IFA testing and immunoferritin labeling of cells infected in vitro. Two ponies injected intravenously with live E. sennetsu did not develop clinical illness. Subsequent injection with live E. risticii also did not induce any disease, in contrast to two control ponies given E. risticii without prior exposure to E. sennetsu. Both controls developed fever, anorexia, depression, dehydration, and diarrhea, which are typical clinical signs of Potomac horse fever, and had characteristic lesions of enteritis and lymph node histiocytosis at postmortem examination. E. sennetsu-exposed ponies had normal gastrointestinal morphologies and lymph node hyperplasia. Ponies primed with E. sennetsu before E. risticii challenge developedhigh titers of immunoglobulin G antibody which reacted against both E. sennetsu and E. risticii antigens by IFA testing. The most prominent antigenic polypeptide in Western (immuno-) blot analysis of sera collected from ponies primed with E. sennetsu before subsequent challenge with E. risticii was present in lysates of both Ehrlichia species and had an apparent molecular mass of 44 kilodaltons. This band was not prominent in Western blots performed with sera of ponies injected with E. risticii alone. Thus, injection of E. sennetsu protects ponies from clinical and pathological manifestations of the disease induced by injection with E. risticii. Immunologic cross-reactivity of the two organisms with IFA testing and strong immunologic recognition by ponies of the 44-kilodalton antigen common to the two organisms may be related to the development of protective immunity against E. risticii.

Detection ofEhrlichia risticii, the Agent of Potomac Horse Fever, in Freshwater Stream Snails (Pleuroceridae:Jugaspp.) from Northern California

“…Human disease caused by E. sennetsu has been associated with the consumption of gray mullet (15,16). The SF agent causes mild clinical signs of fever in dogs (16), while E. sennetsu can establish infections in horses but evidently is not pathogenic in this species (36).…”

“…Research has revealed a close phylogenetic relationship between E. risticii and the following three rickettsiae associated with aquatic environments: Neorickettsia helminthoeca, the agent of "salmon poisoning," a frequently fatal systemic, metacercariaborne disease of canids; the SF agent, isolated in Japan from trematode metacercariae (Stellantchasmus falcatus) parasitic on gray mullet fish; and Ehrlichia sennetsu, the agent of human sennetsu ehrlichiosis in Japan and Malaysia (15,16,31,33,43). Based on 16S rRNA gene sequences, antigenic cross-reactivity, and the proven or suspected mechanism of transmission, these four agents form a distinct cluster or genogroup distinct from other rickettsiae (11,33,36,43).…”

Comparison of PCR and culture to the indirect fluorescent-antibody test for diagnosis of Potomac horse fever

“…Since the pony did not have diarrhea at day 1 p.i. and E. risticii does not cause bloody diarrhea (18,19), E. risticii DNA in the feces is unlikely derived from the blood but is derived from infected cells shed into the gut lumen. We have shown previously by electron microscopy that ehrlichial organisms are present in the intestinal epithelial cells, macrophages, and mast cells in the intestinal wall of affected horses and that infected cells containing ehrlichiae are shed into the gut lumen (22).…”

“…(17). The E. risticii 16S rRNA gene PCR product was detected by agarose gel electrophoresis by a nested PCR amplification of DNA extracted from blood mononuclear cell fractions and feces of the pony on days 1,6,8,11,13,15,18,20,22,25,28, and 32 p.i. (Table 1; Fig.…”

Molecular cloning and analysis of recombinant major antigens of Ehrlichia risticii

“…From our studies of the immune response in horses (4), six major antigens (70, 55, 51, 44, 33, and 28 kDa) were considered as potential immunogens on the basis of their surface predilection (8), early recognition in the antibody response (4), and protection of horses from challenge infection during this early antibody response phase (unpublished data). A recent finding that E. sennetsuimmunized horses were protected from E. risticii challenge along with high reactivity to the 44-kDa protein of E. risticii (18) further suggests the protective importance of the 44-kDa antigen. Furthermore, single proteins have been demonstrated to be protective in other rickettsial species (13).…”