Molecular cloning of Ehrlichia risticii and development of a gene probe for the diagnosis of Potomac horse fever.

A gene bank of Ehrlichia risticii was constructed in plasmid vector pUC13. Five clones representing discrete regions of the E. risticii genome were tested for their ability to hybridize specifically to E. risticii DNA. None of the clones cross-hybridized with Ehrlichia equi DNA, whereas four of these clones cross-hybridized with Ehrlichia canis and Ehrlichia sennetsu DNAs. However, one clone carrying a 1-kilobase HindIII fragment of E. risticii DNA failed to cross-react with the genomes of E. sennetsu, E. canis, and E. equi in dot blot hybridization assays. The sensitivity of this probe for the detection of E. risticii DNA was approximately 0.5 pg. By using this probe, the E. risticii DNA was detected in the peripheral blood mononuclear cells of 30 experimentally infected horses by 7 days postinfection (p.i.); the detection of E. risticii DNA peaked between 14 and 17 days p.i., a period immediately after the peak of the second rise in body temperature, during leukopenia and at the onset of diarrhea. E. risticii DNA was not detectable by 25 to 30 days p.i. E. risticii DNA was not detected in noninfected control horses.