Immunodiagnosis of Ehrlichia canis infection with recombinant proteins.

Ehrlichia canis causes a potentially fatal rickettsial disease of dogs that requires rapid and accurate diagnosis in order to initiate appropriate therapy leading to a favorable prognosis. We recently reported the cloning of two immunoreactive E. canis proteins, P28 and P140, that were applicable for serodiagnosis of the disease. In the present study we cloned a new immunoreactive E. canis surface protein gene of 1,170 bp, which encodes a protein with a predicted molecular mass of 42.6 kDa (P43). The P43 gene was not detected in E. chaffeensis DNA by Southern blot, and antisera against recombinant P43 (rP43) did not react with E. chaffeensis as detected by indirect fluorescent antibody (IFA) assay. Forty-two dogs exhibiting signs and/or hematologic abnormalities associated with canine ehrlichiosis were tested by IFA assay and by recombinant Western immunoblot. Among the 22 samples that were IFA positive for E. canis, 100% reacted with rP43, 96% reacted with rP28, and 96% reacted with rP140. The specificity of the recombinant proteins compared to the IFAs was 96% for rP28, 88% for P43 and 63% for P140. The results of this study demonstrate that the rP43 and rP28 are sensitive and reliable serodiagnostic antigens for E. canis infections.