RésuméEast Coast fever (ECF), caused by the protozoan pathogen Theileria parva and transmitted by the ixodid tick Rhipicephalus appendiculatus, is one of the most pathogenic tick-borne diseases of cattle which hinder cattle production in the Great Lakes region of central Africa. The Great Lakes region is characterised by diversified agro-ecological conditions and extensive cattle movement, where the epidemiology of ECF was previously reported to be complex and unstable. This work aimed to assess the population genetics and biogeographical dynamics of R. appendiculatus and T. parva and to model the transmission dynamics of T. parva, as so to better understand the epidemiology of ECF in different agro-ecological zones (AEZs) of the Great Lakes region (lowlands, midlands and highlands).
Our findings highlighted the occurrence of two sympatric R. appendiculatus genetic lineages in the Great Lakes region: lineage A (equatorial lineage) and lineage B (tropical lineage). The equatorial lineage which was widely distributed, has been longer established in the Great Lakes region whereas the tropical lineage has settled a founder population from recent colonisation events and its distribution decreases with altitude. The two lineages have undergone sudden demographic and spatial population expansion associated with cattle movements in Africa. On the other hand, population genetic analyses of T. parva provided evidence of high degree of genetic variation within populations and limited agro-ecological structure due to the widespread major genotypes identical or closely similar to vaccine strains. The genetic structure and biogeographical distribution of T. parva were found to be mainly driven by gene flow facilitated by cattle movement and ecological conditions affecting tick population dynamics. The tick challenge on cattle and the transmission intensity were higher in lowlands and midlands than highlands areas.
Based on these data, different epidemiological situations were described according to AEZs, suggesting different control strategies: (i) In the highlands area, where clinical cases were previously reported in both calve and adult cattle populations, only T. parva variants identical to vaccine strains occur, associated with the presence of the equatorial tick lineage and low and constant transmission intensity of T. parva. In this area, the situation was suggested to be epidemic; (ii) In midlands and lowlands areas, where previous data reported clinical disease confined to calves, there was high genetic diversity of T. parva and R. appendiculatus. The two tick lineages were sympatric and there was high and seasonal transmission intensity of T. parva. This was an indication of endemic situation in these areas. Particularly, the coexistence of R. appendiculatus lineages, together with continuous introduction of ticks through cattle movement may disrupt the endemicity and lead to occasional epidemics reported in lowlands. The fact that ubiquitous T. parva variants were genetically identical to vaccine strains, together with the admixture of T. parva populations may justify testing the existing trivalent vaccine for cross-immunity in the region.
|la date de réponse
|26 mars 2020
|Nathalie Kirschvink (Promoteur), Tanguy Marcotty (Copromoteur), Francesco Renzi (Président), Benoit Muylkens (Jury), Jonathan Marescaux (Jury), Jean-Paul Dehoux (Jury), Luc Duchateau (Jury), Maxime Madder (Jury) & Dirk M. Geysen (Jury)