Swine dysentery outbreaks, characterized by bloody mucoid diarrhea, have recently become more common in U.S. swine herds. Diagnostic and research investigations determined that this re-emergence of swine dysentery was caused by Brachyspira hyodysenteriae and a novel pathogen, “Brachyspira hampsonii”. Other species, Brachyspira pilosicoli and Brachyspira murdochii, have been isolated from clinical cases of mild diarrhea in grower-finisher pigs, but their role in disease is unclear.

Due to the absence of any commercial vaccines, Brachyspira-associated diarrheal diseases are currently controlled by strict biosecurity measures and antimicrobial therapy. However, many countries in Europe and Asia have reported Brachyspira isolates with low susceptibilities to commonly used antimicrobials, thus complicating disease control strategies. Despite the increasing interest, little is known about the susceptibilities of U.S. Brachyspira isolates toward commonly used antimicrobials.

We evaluated the antimicrobial susceptibility patterns of 100 Brachyspira isolates (including B. hyodysenteriae, “B. hampsonii”, B. pilosicoli and B. murdochii), originating from 89 farms spanning 39 systems and 13 U.S. states in 2009-14. The in vitro (laboratory) susceptibilities were determined by measuring the lowest concentration of an antimicrobial that inhibits visible growth of the Brachyspira isolate, known as the minimum inhibitory concentration. These susceptibilities were determined toward commonly used antimicrobials such as carbadox, tiamulin, valnemulin, lincomycin, tylosin and doxycycline. The MICs were categorized into low MICs (high susceptibility) or high MICs (low susceptibility) for each antimicrobial as per previously proposed epidemiological cut-offs for Brachyspira.

In general, most in vitro MICs were low for carbadox, tiamulin and valnemulin, evenly distributed between low and high for doxycycline and high for lincomycin and tylosin (Figure 1). Furthermore, the general trend of MIC values by Brachyspira species (in decreasing order) was determined to be: B. pilosicoli > B. murdochii > B. hyodysenteriae > “B. hampsonii”. These results indicate that, in general, the clinically important pathogens “B. hampsonii” and B. hyodysenteriae are more susceptible to commonly used antimicrobials than other evaluated Brachyspira species. A surprising finding was the significant association between highly susceptible antimicrobial MIC patterns and sow farms and has possible implications in swine husbandry practices.

Further, we found an association between low antimicrobial susceptibility of Brachyspira and strains prevalent in certain production systems, which might reflect the influence of antimicrobial use within those systems. Therefore, strain typing of isolates with such unusual low antimicrobial susceptibilities will enable the tracking of the potential spread of such strains.

Although not indicative of clinical efficacy, the use of epidemiological cut-offs enables the early detection of isolates with low in vitro antimicrobial susceptibility profiles. Veterinarians and pork producers dealing with swine dysentery outbreaks in their herds would benefit from the ability to test the susceptibilities of their Brachyspira isolates to identify the most appropriate choice and dose of antimicrobial to control the disease.

For more information, contact Dr. Connie Gebhart at the University of Minnesota - Veterinary Diagnostic Laboratory at 612-624-3444, or [email protected].