Multiple-locus variable-number tandem-repeat analysis of the swine dysentery pathogen, Brachyspira hyodysenteriae

Abstract

The spirochete Brachyspira hyodysenteriae is the causative agent of swine dysentery, a severe colonic infection of pigs that has a considerable economic impact in many swine-producing countries. In spite of its importance, knowledge about the global epidemiology and population structure of B. hyodysenteriae is limited. Progress in this area has been hampered by the lack of a low-cost, portable, and discriminatory method for strain typing. The aim of the current study was to develop and test a multiple-locus variable-number tandem-repeat analysis (MLVA) method that could be used in basic veterinary diagnostic microbiology laboratories equipped with PCR technology or in more advanced laboratories with access to capillary electrophoresis. Based on eight loci, and when performed on isolates from different farms in different countries, as well as type and reference strains, the MLVA technique developed was highly discriminatory (Hunter and Gaston discriminatory index, 0.938 [95% confidence interval, 0.9175 to 0.9584]) while retaining a high phylogenetic value. Using the technique, the species was shown to be diverse (44 MLVA types from 172 isolates and strains), although isolates were stable in herds over time. The population structure appeared to be clonal. The finding of B. hyodysenteriae MLVA type 3 in piggeries in three European countries, as well as other, related, strains in different countries, suggests that spreading of the pathogen via carrier pigs is likely. MLVA overcame drawbacks associated with previous typing techniques for B. hyodysenteriae and was a powerful method for epidemiologic and population structure studies on this important pathogenic spirochete.

FIG. 1.

Dendrogram of the 44 B. hyodysenteriae MLVA types found in the present study and clustered using UPGMA. Roman numerals I to VI indicate clonal complexes defined at the single-locus variant level. The scale bar represents genetic distance as the absolute number of differences in marker alleles among genotypes. Bootstrap values of ≥40% are shown.

FIG. 2.

MLVA types (circled) and relationships found among them according to the goeBURST algorithm. Solid lines show the single-locus variant level, dashed lines show the double-locus variant level, and dotted lines show the triple-locus variant level. Groups at the single-locus variant level are indicated by roman numerals I to VI.