Review of the molecular biology of Cowdria ruminantium

Cowdria ruminantium is a rickettsial agent which causes heartwater, an economically important disease of livestock in the tropics and the Caribbean. Significant advances have been made in the molecular biology of C. ruminantium since its reproducible propagation in vitro in bovine endothelial cells. These advances have been targeted towards the development of improved vaccines and diagnostic tests. Several immunogenic proteins of C. ruminantium have been identified and monoclonal antibodies have been developed to some. The gene for the 21 kDa C. ruminantium protein has been cloned, characterized, sequenced and expressed to high levels to produce a recombinant analogue. This gene is conserved amongst all C. ruminantium isolates tested. The gene for the immunodominant 32 kDa protein has also been cloned recently. Analysis shows that this protein varies structurally between different C. ruminantium isolates. Recombinant protein analogues will have application in vaccine studies and subunit enzyme-linked immunosorbent assay (ELISA) developments. In the field of serology, a 32 kDa protein specific competitive ELISA (cELISA) is available and a 21 kDa protein specific direct and a cELISA is being developed. Detection of C. ruminantium in Amblyomma ticks and in animals is now possible using DNA and RNA probes and the sensitivity of these nucleic acid based assays is being maximized using polymerase chain reaction (PCR). Sequencing of the 16s rRNA gene has revealed the close phylogenetic relationship of C. ruminantium to Rickettsia, Anaplasma and Ehrlichia species. The implications of these studies will be discussed.