Phylogeography and epidemiology of Brucella suis biovar 2 in wildlife and domestic swine

Swine brucellosis due to Brucella suis biovar 2 (bv2) is enzootic in wild boar and hare in continental Europe and may cause major economic losses to the pig industry, mainly in free-ranged pig farms. The high nucleotide identity found among the B. suis biovar 2 isolates has long hindered the full understanding of the epidemiology and the phylogeography of the disease. Here, we used multilocus variable-number tandem-repeat (VNTR analysis (MLVA) and whole-genome analysis to identify single-nucleotide polymorphisms (SNPs) in order to gait insights from the largest B. suis bv2 dataset analyzed so far composed of domestic pigs and wildlife isolate collected throughout Europe since the 1970s. We found four major clades with a specific phylogeographic pattern. The Iberian Glade contains isolates exclusively from the Iberian Peninsula. The Central European clad includes most isolates from France, Northern Italy, Switzerland and an important proportion of those of Northern Spain. The Eastern European Glade clustered isolates from Croatia and Hungary mainly but also from areas of France, Germany, Italy and Poland. Finally, a separated Sardinian Glade grouped three isolates from this island At fine scale, MLVA demonstrated an endemic status of the infection in Europe and it allowed tracking a large outbreak formed by different farms from Spain linked to the same infection source. The whole genome SNPs analysis showed that the strains form genetically distinct clades, shared between wild boar and pigs, in agreement with the MLVA clades. Interestingly, all hare isolates clustered together within two groups compose exclusively of wildlife isolates. Our results support the hypothesis that maintenance and spread of B. suis bv2 in Europe is a dynamic process linked to the natural expansion of wild boar as the main wild reservoir of the infection, while spread over long distances is found largely dependent on anthropogenic activities.