Genetic characterization of Brucella melitensis and Brucella abortus geographical clusters in Italy

The genetic diversity of Brucella melitensis and Brucella abortus strains isolated in 199 cattle and sheep from 156 brucellosis outbreaks which occurred in 8 regions of Southern Italy in 2011, was determined using a Multiple-Locus Variable Number of Tandem Repeats Analysis approach. The existence of possible genetic clusters was verified through a hierarchical cluster analysis based on 'single link', which is closely related to the minimum spanning tree. The Hamming weighted distance matrix was adopted in the analysis. All calculations were performed using R and the additional libraries phangorn and Cluster. For a number of clusters, ranging from 2 to 15, the average silhouette width was calculated. The number of clusters adopted was identified according to the maximum average silhouette width. For B. abortus and B. melitensis, 6 and 11 genetic clusters were identified, respectively. Three out of 6 B. abortus clusters included the 96.7% of all B. abortus isolates. Clusters were clearly geographically separated, and this highlighted the known epidemiological links among them. Brucella melitensis genotypes resulted more heterogeneous; the 3 more representative genetic clusters included 79.7% of all B. melitensis isolates. A clear geographical clusterization of genotypes is recognizable only for 1 cluster, whereas the others are more widespread across Southern Italy. The genetic characterization of Brucella strains isolated from animals may be a useful tool to better understand the epidemiology and dissemination patterns of this pathogen through host populations.