Diagnosis and control of heartwater, <i>Ehrlichia ruminantium</i>, infection: an update.

Heartwater, an acute and fatal tick-transmitted disease of domestic and wild ruminants caused by a rickettsial agent <i>Ehrlichia ruminantium</i>, continues to negatively impact animal health and production in sub-Saharan Africa. Its control is hampered by the absence of safe and effective vaccines. Over the last two decades in particular, major advances have occurred facilitating a better understanding of its epidemiology, vector biology and dynamics of transmission, pathogen propagation and characterization, molecular biology, antigenic repertoire and immunity. This research has facilitated development of improved diagnostic assays and strategically influenced research on development of efficacious vaccines. Heartwater serology is constrained by the detection of antigenic cross-reactions with genetically related ehrlichial agents. Serology therefore is not a reliable method for diagnosis of heartwater or for predicting its distribution or spread. Polymerase chain reaction (PCR)-based detection, especially targeting the sequences of the pCS20 gene fragment of <i>E. ruminantium</i>, is reliable and has been determined to be the most specific method for pathogen detection and hence disease surveillance. The pCS20 PCR assay readily detects infected ticks and clinically ill animals but does not always detect carrier animals. The reason for this observation will be discussed. Research on improved vaccines has focused on development of attenuated, inactivated and recombinant vaccines for heartwater. This research has led to the field evaluation of an inactivated vaccine which has provided the proof of concept for the development of recombinant vaccines for heartwater. The inactivated vaccine protects cattle, sheep and goats against natural field tick challenge, but these trials have highlighted that <i>E. ruminantium</i> antigenic disparity will be an important factor affecting vaccine deployment in the field. Progress in recombinant vaccine research has led to preliminary testing of some E. ruminantium antigens delivered as DNA vaccines. The recent publication of the genome sequence of two strains of <i>E. ruminantium</i> is likely to expedite the development of recombinant vaccines for heartwater, especially through reverse vaccinology, which has been used to successfully identify protective antigens of various other pathogens.