Swine dysentery (SD), or bloody scours, has historically been associated with the presence of the strongly beta-hemolytic spirochaete, Brachyspira hyodysenteriae. However, Brachyspira spp. not identified as B. hyodysenteriae by polymerase chain reaction assays (PCR) have recently been recovered from pigs with clinical SD.

The goal was to identify more consistent and accurate methods to confirm a clinical SD diagnosis. The study, reported by Pork Checkoff, addressed this by:

1. utilising a mouse model to assess the virulence potential of 21 Brachyspira strains, including multiple atypical isolates from pigs with features consistent with SD, as well as nonpathogenic isolates and multiple isolates of B. hyodysenteriae
2. using the mouse experiment results, eight isolates, including typical and atypical pathogenic strains as well as non-pathogenic strains, were inoculated into pigs to confirm virulence and clinical manifestations
3. based upon combined results of the mouse and pig experiments, the eight isolates received a well-defined pathotype (virulent versus avirulent) and can be compared by genetic analyses to identify targets in the pathogenic strains to develop improved PCR assays for diagnostic testing.

In both the mouse and pig experiments, Brachyspira spp. that impart characteristic strong beta-haemolysis were associated with the greatest degree of virulence as determined by the caecal and colonic inflammation in infected animals. In pigs, clinical SD was observed only following infection with strongly beta-haemolytic spirochaetes, with disease also occurring following infection by strongly beta-haemolytic isolates not identified as B. hyodysenteriae by PCR.

These data suggest that the isolation of a strongly beta-haemolytic spirochaete from faeces or colonic tissues of pigs with bloody scours should be interpreted as a clinical diagnosis of SD even if the isolate is not identified as B. hyodysenteriae.

Also, the cultural characteristics of Brachyspira spp. are a more sensitive indicator of the potential to induce SD than the molecular identification of the isolate alone.

May 2013