SEROLOGIC CROSS-REACTIONS AMONG EHRLICHIA-EQUI, EHRLICHIA-PHAGOCYTOPHILA, AND HUMAN GRANULOCYTIC EHRLICHIA

Homology in the 16S rDNAs shows that the agent of human granulocytic e hrlichiosis (HGE) is closely related to the veterinary pathogens Erlic hia equi and Erlichia phagocytophila. After HGE, patients develop anti bodies reactive with E. equi and E. phagocytophila; thus, we hypothesi zed that these species are closely related and share significant antig enicity. Antisera from humans, horses, dogs, and cattle were tested by indirect fluorescent-antibody assay (IFA) for antibodies reactive wit h E. equi and other ehrlichiae and tested by immunoblot to identify th e specific reactions with E. equi. All convalescent-phase sera from hu man patients with HGE and from animals infected or immunized dth E. eq ui or E. phagocytophila had antibodies reactive with E. equi by IFA; n o reactions with Ehrlichia chaffeensis occurred with these sera, and o nly one horse naturally infected with E. equi had a serologic reaction against Ehrlichia sennetsu. Human and animal sera obtained after infe ction or immunization with other Ehrlichia, Rickettsia, and Bartonella species did not react with E. equi by IFA. E. equi immunoblots reveal ed as many as 19 bands with equine anti-E. equi serum. All HGE agent, E. equi, and E. phagocytophila antisera tested reacted with a 44-kDa a ntigen of E. equi, while other anti-Ehrlichia spp. sera reacted with t his antigen rarely or not at all. HGE agent, E. equi, and E. phagocyto phila antisera but not other sera also reacted occasionally with 25-, 42-, and 100-MDa antigens. Most sera reacted with antigens between app roximately 56 and 75 kDa, probably heat shock proteins. The HGE agent, E. equi, and E. phagocytophila share significant antigenicity by IPA and immunoblot. Coupled with the nearly identical nucleotide sequences of 16S rRNA genes, these data indicate that E. equi, E. phagocytophil a, and the human granulocytic ehrlichia are closely related or identic al species.