The immunodominant 32-kilodalton protein of Cowdria ruminantium is conserved within the genus Ehrlichia.

Serological tests for cowdriosis are hampered by cross-reacting antibodies from animals suspected to be infected with Ehrlichia species. We have monitored infections with Ehrlichia bovis, E. ovina, E. canis and E. phagocytophila in experimental animals by competitive ELISA, Western blotting and immunofluorescence using Cowdria-infected endothelial cell culture antigens. Cross-reactions due to Ehrlichia antibodies could be attributed to the recognition of epitopes on the immunodominant Cr32 Cowdria protein. This was especially true for E. canis and E. ovina, much less for E. bovis, but not at all for E. phagocytophila. In addition, strong cross-reactivity between Cowdria and antibodies to E. Chaffeenis were demonstrated. These findings are in agreement with the phylogenetic relationships, recently reported by van Vliet et al. in 1992, between Cowdria and other members of the tribe Ehrlichieae, which showed Cowdria to be closely related to E. canis and also to E. chaffeensis. Although the tests used in this study remain valuable tools under laboratory conditions, their specificity requires improvement. It is suggested to study recombinant Cowdria antigens for the development of second generation serological tests for cowdriosis.