An Investigation into the Etiological Agents of Swine Dysentery in Australian Pig Herds

Abstract

Swine dysentery (SD) is a mucohemorrhagic colitis, classically seen in grower/finisher pigs and caused by infection with the anaerobic intestinal spirochete Brachyspira hyodysenteriae. More recently, however, the newly described species Brachyspira hampsonii and Brachyspira suanatina have been identified as causing SD in North America and/or Europe. Furthermore, there have been occasions where strains of B. hyodysenteriae have been recovered from healthy pigs, including in multiplier herds with high health status. This study investigated whether cases of SD in Australia may be caused by the newly described species; how isolates of B. hyodysenteriae recovered from healthy herds compared to isolates from herds with disease; and how contemporary isolates compare to those recovered in previous decades, including in their plasmid gene content and antimicrobial resistance profiles. In total 1103 fecal and colon samples from pigs in 97 Australian herds were collected and tested. Of the agents of SD only B. hyodysenteriae was found, being present in 34 (35.1%) of the herds, including in 14 of 24 (58%) herds that had been considered to be free of SD. Multilocus sequence typing applied to 96 isolates from 30 herds and to 53 Australian isolates dating from the 1980s through the early 2000s showed that they were diverse, distinct from those reported in other countries, and that the 2014/16 isolates generally were different from those from earlier decades. These findings provided evidence for ongoing evolution of B. hyodysenteriae strains in Australia. In seven of the 20 herds where multiple isolates were available, two to four different sequence types (STs) were identified. Isolates with the same STs also were found in some herds with epidemiological links. Analysis of a block of six plasmid virulence-associated genes showed a lack of consistency between their presence or absence and their origin from herds currently with or without disease; however, significantly fewer isolates from the 2000s and from 2014/16 had this block of genes compared to isolates from the 1980s and 1990s. It is speculated that loss of these genes may have been responsible for the occurrence of milder disease occurring in recent years. In addition, fewer isolates from 2014/16 were susceptible to the antimicrobials lincomycin, and to a lesser extent tiamulin, than those from earlier Australian studies. Four distinct multi-drug resistant strains were identified in five herds, posing a threat to disease control.

Testing the efficacy of kitasamycin for use in the control and treatment of swine dysentery in experimentally infected pigs

“…Test plates consisted of TSA containing 5% defibrinated ovine blood and the appropriate antibiotic concentration, while control plates did not include antibiotics. The isolates were tested for susceptibility to varying concentrations of kitasamycin (0.675, 1.25, 2.5, 5.0, 10, 20, 40, 80 and 160 μg/mL), tylosin (1, 4, 25, 50 and 100 μg/mL) and lincomycin (2,4,16,36 and 72 μg/mL).…”

“…SD most commonly results from infection caused by the anaerobic intestinal spirochaete Brachyspira hyodysenteriae , although the related species Brachyspira hampsonii and Brachyspira suanatina may also cause SD in some countries . The latter two species have not been identified in Australia, but B. hyodysenteriae is present in up to a third of Australian herds – including in herds where clinical SD has not been reported …”

“…Of these, only tylosin, lincomycin and tiamulin remain registered for use in SD control in Australia. Furthermore, reduced susceptibilities to one or more of these antimicrobials increasingly have been reported among Australian isolates of B. hyodysenteriae , including identification of virulent isolates that are resistant to all the three available antimicrobials . Multidrug resistant isolates of B. hyodysenteriae have also been reported from the Czech Republic, the Netherlands, and Italy, thus emphasising the global nature of this problem.…”

“…2,3 The latter two species have not been identified in Australia, but B. hyodysenteriae is present in up to a third of Australian herdsincluding in herds where clinical SD has not been reported. 4 No effective commercial vaccines for SD are currently available, and control in infected herds is heavily reliant on prophylactic or therapeutic use of antimicrobials. The main classes of antimicrobials to which B. hyodysenteriae has been reported to be susceptible include macrolides (tylosin and tylvalosin), lincosamides (lincomycin), pleuromutilins (tiamulin and valnemulin), imidazoles (dimetridazole), quinoxalines (carbadox), streptogramins (virginiamycin) and ionophores (salinomycin and monensin).…”

“…Furthermore, reduced susceptibilities to one or more of these antimicrobials increasingly have been reported among Australian isolates of B. hyodysenteriae, [4][5][6] including identification of virulent isolates that are resistant to all the three available antimicrobials. 4 Multidrug resistant isolates of B. hyodysenteriae have also been reported from the Czech Republic, 7 the Netherlands, 8 and Italy, 9 thus emphasising the global nature of this problem. The existence of resistant isolates is causing serious difficulties for SD control and eradication, and consequently other antimicrobials and alternative treatments are being sought to achieve this.…”