Biological support for modeling the transmission dynamics of cowdriosis

Heartwater is a disease of high mortality in domestic ruminants (cattle, sheep and goats) in sub-Saharan Africa and in the Caribbean. An intracellular rickettsia, Cowdria ruminantium, is the cause of the disease and is transmitted by ticks of the genus Amblyomma. Losses due to heartwater are high and control has historically been by treatment of livestock with acaricides. More reliable, sustainable and cost-effective methods of control, in particular vaccines, have been the focus of intense research efforts in recent years. Prior to their use, the impact of new vaccines on transmission dynamics under different disease settings and on the economics of livestock production needs to be assessed and compared with current control strategies. To do this without lengthy and expensive field trials, transmission dynamic models have been developed and used to make quantitative predictions of disease impact under different control scenarios. The models attempt to capture the results of the biological processes underlying transmission and hence require reasonable estimates of the magnitudes of these parameters. When initially constructed, these models obtained data for parameter estimation from previously published literature. Subsequently, it was recognised that certain estimates needed to be improved through prospective studies. Initial model output also identified certain parameters as key to model performance and for which improved estimates were essential. We thus investigated four aspects of the transmission biology of C. ruminantium to provide new or improved information for the models. These included (i) the rate of decline in infectivity of hosts moving from the clinical to the carrier stage of infection; (ii) the relative importance of adult and nymph vector tick instars in transmission; (iii) the prevalence of infection in free-living vector instars; and (iv) the rate of attachment of vector ticks to hosts. These areas were investigated in laboratory and field studies.