Genetic structure of Ehrlichia ruminantium based on Multilocus Sequence Typing: particularities of strains from West-Africa

Ehrlichia ruminantium is the tick-borne rickettsia causing heartwater to wild and domestic ruminants. Previously, Multilocus Sequence Typing of this pathogen (MLST) highlighted the co-circulation of two genotypic groups within nearby villages from Burkina Faso where  vaccination assays had been previously recurrently performed. In the present study the laboratory strains that had been involved in  vaccination assays, which evidenced their close relationships with strains that were later involved in local outbursts, were genotyped. Characterization of the bacterium genetic diversity was extended to four distant Beninese localities where no vaccination assays had ever  been performed, through sampling of N=500 specimens of the local tick vector, Amblyomma variegatum. This led evidencing an  even higher frequency of new multilocus MLST genotypes in Benin than in Burkina Faso, 93% of the Beninese genotypes included at least  one new private allele (i.e., no detected elsewhere), while this was only the case of 80% bacteria genotypes sampled in Burkina Faso. Investigating the genetic structure of the bacterium across the Africa continent revealed a reduction of its diversity in the eastern and  southern part of the continent relatively to West Africa. As the bacterial genetic diversity currently remains highest in West Africa than  anywhere else in the continent, this area remains optimal for the development and/or testing of vaccines against heartwater.