ANTIGENIC, MORPHOLOGIC, AND MOLECULAR CHARACTERIZATION OF NEW EHRLICHIA-RISTICII ISOLATES

Ehrlichia risticii causes an acute infectious disease in horses called Potomac horse fever. To investigate the biological diversity of E. ri sticii organisms, nine E. risticii isolates derived from the periphera l blood monocytes of clinically sick horses in Ohio and Kentucky durin g the summers of 1991 and 1993 were compared with Illinois and Virgini a isolates originally obtained from horses in Maryland in 1984. Seven of the nine isolates (081, 606, 380, 679, As, Co, and Ov) formed large morulae (tightly packed inclusions of ehrlichial organisms). The rema ining isolates, including 1984 isolates, were individually dispersed o r formed small morulae in the cytoplasm of P388D(1) cells. In Western blot (immunoblot) analysis with four equine and one rabbit polyclonal anti-E. risticii sera, these recent E. risticii isolates showed patter ns of antigenic proteins distinct from those of the 1984 isolates and could be divided into three groups: (i) 081; (ii) 606, 022, 067, 380, and 679; and (iii) As, Co, and Ov. By indirect fluorescent antibody la beling with two panels of murine anti-E. risticii (Illinois and Maryla nd isolates) monoclonal antibodies, isolate 081 was not labeled with a ny of 20 monoclonal antibodies tested. The remaining isolates were not labeled with several monoclonal antibodies. The digestion pattern wit h one of the restriction enzymes, AvaII, of the PCR-amplified partial 16S rRNA gene of E. risticii from all Kentucky isolates (As, Co, and O v) was different from that of Illinois, Virginia, and six Ohio isolate s. These results indicate the presence of distinct variants of E. rist icii which vary significantly in morphology, antigenic composition, an d the base sequence of the 16S rRNA gene.