Potential value of major antigenic protein 2 for serological diagnosis of heartwater and related ehrlichial infections

Cowdria ruminantium is the etiologic agent of heartwater, a disease causing major economic loss in ruminants in sub-Saharan Africa and the Caribbean, Development of a serodiagnostic test is essential for determining the carri er status of animals from regions where heartwater is endemic, but most ava ilable tests give false-positive reactions with sera against related Erhlic hia species. Current approaches rely on molecular methods to define protein s and epitopes that may allow specific diagnosis. Two major antigenic prote ins (MAPs), MAP1 and MAP2, have been examined for their use as antigens in the serodiagnosis of heartwater, The objectives of this study were (i) to d etermine if MAP2 is conserved among five geographically divergent strains o f C. ruminantium and (ii) to determine if MAP2 homologs are present in Ehrl ichia canis, the causative agent of canine ehrlichiosis, and Ehrlichia chaf feensis, the organism responsible for human monocytic ehrlichiosis. These t wo agents are closely related to C. ruminantium. The map2 gene from four st rains of C, ruminantium was cloned, sequenced, and compared with the previo usly reported map2 gene from the Crystal Springs strain. Only 10 nucleic ac id differences between the strains were identified, and they translate to o nly 3 amino acid changes, indicating that MAP2 is highly conserved. Genes e ncoding MAP2 homologs from E. canis and E. chaffeensis also were cloned and sequenced. Amino acid analysis of MAP2 homologs of E, chaffeensis and E. c anis with MAP2 of C, ruminantium revealed 83.4 and 84.4% identities, respec tively. Further analysis of MAP2 and its homologs revealed that the whole p rotein lacks specificity for heartwater diagnosis. The development of epito pe-specific assays using this sequence information may produce diagnostic t ests suitable for C. ruminantium and also other related rickettsiae.