The rickettsial bacterium Ehrlichia ruminantium is the causative agent of heartwater, a serious tick-borne disease in ruminants. The genetic diversity of organisms in the field will have implications for cross-protective capacities of any vaccine developed, and for an effective vaccine design strategy proper genotyping and understanding of existing genetic diversity in the field is necessary. We searched for variable-number tandem-repeat (VNTR) loci for use in a multi-locus VNTR analysis (MLVA). Sequencing analysis of 30 potential VNTRs using a panel of 17 reference strains from geographically diverse origins identified 12 VNTRs with allelic profiles differing between strains. Application of MLVA to 38 E. ruminantium-infected Amblyomma variegatum collected from indigenous cattle in 6 different districts of Uganda identified 21 MLVA types. The discriminatory power of MLVA was greater than that of map1 PCR-restriction fragment length polymorphism analysis, with which only 6 genotypes were obtained. The high discriminatory power as well as cost- effective performance of MLVA provide the potential for this technique to be applied in the future with respect to optimizing vaccine trials by identifying local strain diversity, and also raise the possibility of exploring the association between E. ruminantium genotypes and phenotypes such as pathological outcome in the ruminant host.

A mathematical model of the transmission dynamics of Cowdria ruminantium by the ixodid tick Amblyomma hebraeum in the bovine host is developed and used to investigate the epidemiology of heartwater across a range of vector challenge. The processes described are supported by empirical data. The pattern of outcome measures (incidence, case-fatality and proportion of infected hosts) predicted agrees with those described anecdotally from field experience and empirical observation, and demonstrates the concept of endemic stability. The underlying theory is explored and it is shown that endemic stability may be due principally to the protection of calves against disease by either innate or maternally derived factors. The role of vertical infection in the establishment and maintenance of endemic stability is also investigated. Although increasing the vertical infection proportion results in endemic stability occurring at progressively lower levels of tick challenge, the concomitant reduction in incidence and case-fatality predictions across the range of tick challenge means the endemically stable state simultaneously becomes less discernible. Model limitations and future developments are discussed. The essential role of a transmission dynamics model in assessing the impact of new vaccines in conjunction with vector control programmes is highlighted.