Comparison of Brachyspira hyodysenteriae Isolates Recovered from Pigs in Apparently Healthy Multiplier Herds with Isolates from Herds with Swine Dysentery

Swine dysentery (SD) is a mucohaemorrhagic colitis of grower/finisher pigs classically resulting from infection by the anaerobic intestinal spirochaete Brachyspira hyodysenteriae. This study aimed to determine whether B. hyodysenteriae isolates from pigs in three healthy German multiplier herds supplying gilts to other farms differed from isolates from nine German production herds with SD. Isolates were subjected to whole genomic sequencing, and in silico multilocus sequence typing showed that those from the three multiplier herds were of previously undescribed sequence types (ST132, ST133 and ST134), with all isolates from the same herd having the same ST. All isolates were examined for the presence of 332 genes encoding predicted virulence or virulence lifestyle associated factors, and these were well conserved. Isolates from one multiplier herd were atypical in being weakly haemolytic: they had 10 amino acid substitutions in the haemolysin III protein and five in the haemolysin activation protein compared to reference strain WA1, and had a disruption in the promoter site of the hlyA gene. These changes likely contribute to the weakly haemolytic phenotype and putative lack of virulence. These same isolates also had nine base pair insertions in the iron metabolism genes bitB and bitC and lacked five of six plasmid genes that previously have been associated with colonisation. Other overall differences between isolates from the different herds were in genes from three of five outer membrane proteins, which were not found in all the isolates, and in members of a block of six plasmid genes. Isolates from three herds with SD had all six plasmid genes, while isolates lacking some of these genes were found in the three healthy herds-but also in isolates from six herds with SD. Other differences in genes of unknown function or in gene expression may contribute to variation in virulence; alternatively, superior husbandry and better general health may have made pigs in the two multiplier herds colonised by "typical" strongly haemolytic isolates less susceptible to disease expression.