SEROLOGIC DIAGNOSIS OF HUMAN MONOCYTIC EHRLICHIOSIS BY IMMUNOBLOT ANALYSIS

Human monocytic ehrlichiosis is caused by Ehrlichia chaffeensis, an in tracellular bacterium probably transmitted by the tick Amblyomma ameri canum in the United States. Despite its lack of specificity in discrim inating among infections by closely related Ehrlichia spp., immunofluo rescence assay (IFA) is the most frequently used serological diagnosti c method. To improve the specificity of the serological diagnosis, we compared antigenic profile of E. canis and E. chaffeensis antigen with homologous and heterologous sera, searching for the specificity of th e presence of low-molecular-weight proteins. Western immunoblot analys is of IFA-positive human sera revealed 27- and 29-kDa proteins which a re not found in E. canis IFA-positive sera from dogs. IFA-positive ser a from dogs revealed a low-molecular-weight group of proteins (20 to 2 8 kDa) which were not found in human E. chaffeensis-positive sera exce pt for a weak band at 22 kDa. The presence of antibodies directed agai nst the 27- and 29-kDa proteins on Western blots is specific for E. ch affeensis infection, and we suggest that the Western blot might comple te IFA in cases with low positive predictive value.