PATHOGENIC, IMMUNOLOGICAL, AND MOLECULAR DIFFERENCES BETWEEN 2 EHRLICHIA-RISTICII STRAINS

Ehrlichia risticii is the causative agent of Potomac horse fever (PHF) , an acute infectious disease of horses. In the last few years, there have been several reports of PHF cases occurring even in vaccinated ho rses. We isolated a new strain of E. risticii (90-12 strain) from a va ccinated horse suffering from clinical PHF. The major pathogenic, immu nologic, and molecular differences between the 90-12 strain and the 25 -D strain, which was originally isolated during the outbreaks in 1984, were studied. The 90-12 strain was more pathogenic for mite and horse s compared with the 25-D strain. In enzyme-linked immunosorbent assay and immunofluorescence assay with mouse and horse antisera of both the strains, two- to fourfold differences were observed between homologou s and heterologous antigens. The differences in antigen profiles were demonstrated by Western blot (immunoblot) with mouse and horse antiser a and also with the recombinant clone-specific antibodies, Though seve ral antigens were similar in both the strains, there mere significant differences between them in the 110-, 85-, 70-, 51-, and 33-kDa antige ns, The 85-kDa antigen was present only in the 90-12 strain but cross- reacted with a 50-kDa antigen of the 25-D strain. The 51-kDa antigens of both strains had different migration patterns, Southern blot hybrid ization of the genome from both the strains with DNA probes made from the 51-, 55-, and 85-kDa recombinant clones of the 90-12 strain showed a similar pattern with probes of the 51- and 55-kDa clones for both t he strains, whereas the probe of the 85-kDa clone showed a completely different pattern. The 16S rRNA gene sequences from the two strains we re identical. Neither strain replicated in gamma interferon-treated mo use peritoneal macrophages. In in vitro neutralization assay, sera fro m the 25-D strain-infected horse neutralized the homologous strain but did not neutralize the 90-12 strain, whereas sera from the 90-12 stra in-infected horse neutralized both the strains. In mouse protection ex periments, there was complete homologous protection, But in cross-prot ection, mice immunized with the 25-D strain,were only partially protec ted against challenge with the 90-12 strain, whereas mice immunized wi th the 90-12 strain were completely protected against the 25-D strain challenge. These results clearly indicate that there are major differe nces between the 90-12 and 25-D strains which may have implications re garding the vaccine failure for PHF and the development of an efficien t vaccine.