Origins and global context of Brucella abortus in Italy

Abstract

Background: Brucellosis is a common and chronic disease of cattle and other bovids that often causes reproductive disorders. Natural infection in cattle is caused by Brucella abortus and transmission typically occurs during abortions, calving, or nursing. Brucellosis is also a major zoonotic disease due to contamination of dairy products or contact with the tissues of infected animals. Brucellosis has been eradicated from most of the developed world in the last 40 years but persists in many regions-the disease remains prevalent in portions of Africa, the Middle East, Asia, and Central and South America, as well as in the Mediterranean basin. In Italy, B. abortus has persisted in southern regions in both cattle and water buffalo. Previous attempts at analyzing the phylogenetics of B. abortus in Italy have been challenging due to limited genetic variability and unresolved global population genetic structure of this pathogen.

Results: We conducted genome-wide phylogenetic analyses on 11 representative strains of B. abortus from Italy, and compared these sequences to a worldwide collection of publically available genomes. Italian isolates belong to three clades that are basal to the main and global B. abortus lineage. Using six SNP-based assays designed to identify substructure within the Italian clades, we surveyed a collection of 261 isolates and found that one clade predominates throughout endemic districts in the country, while the other two clades are more geographically restricted to portions of southern Italy.

Conclusions: Although related strains exist worldwide, B. abortus isolates from Italy are substantially different than those found in much of the rest of Europe and North America, and are more closely related to strains from the Middle East and Asia. Our assays targeting genetic substructure within Italy allowed us to identify the major lineages quickly and inexpensively, without having to generate whole genome sequences for a large isolate collection. These findings highlight the importance of genetic studies to assess the status and the history of pathogens.

Fig. 1

Maximum parsimony tree for Brucella abortus, with genomes from Italian isolates highlighted in red. Bootstrap support values were 100 for all major branches, with only the shallowest clades having less support (data not shown). Numbered branches indicate the number of SNPs identified for the clade (left most) and subclade (right most) level groupings for the West Italia (top), Trans Italia (middle) and East Italia (bottom) subclades. Samples are identified by the strain name and where known, the biovar (bv) number, animal source, country of origin (3 letter ISO codes), and year of isolation. Top most clade containing biovars 1, 2, 4 is compressed in larger tree to fit into image, true branch lengths are represented in smaller figure in gray box

Fig. 2

Geographical mapping of the Italian isolates of Brucella abortus according to Melt-MAMA genotyping. GIS coordinates separated by groups are shown as follow: green for the West Italia group (a), yellow for the East Italia (b) and red for the Trans Italia (c)