Swine dysentery (SD) is a severe colitis of pigs caused by Brachyspira hyodysenteriae (Harris, 1999). The control of SD is still mainly based on the use of few antimicrobials, such as pleuromutilins, macrolides and lincosamides.

However more than half of the Italian isolates of B. hyodysenteriaewere shown to be resistant to pleuromutilins, with a significant increasing trend in the last 10 years (Rugna, 2015).

The aim of this study was to compare the susceptibility of Brachyspira hyodysenteriae versus tiamulin, valnemulin, doxycycline, aivlosin, lincomycin and tylosin.

The antimicrobial susceptibility of 206 B.hyodysenteriae isolates from Italian herds from 2005-2013 was evaluated by a broth microdilution test. The MIC values were interpreted using clinical breakpoints (Ronne and Szancer, 1990) and epidemiological cut-offs (Pringle, 2012). The vast majority of the isolates (above 80%, approximately) showed MIC values above the epidemiological cut offs, indicating a previous exposure to antimicrobials.

Moreover, most isolates (98%) were clinically resistant to tylosin, about half (56%) to lincomycin and roughly a third (34%) to tiamulin.

Furthermore, a significant increase of non-susceptible isolates (resistant or intermediate) was seen for both lincomycin and tiamulin from 2008 to 2013. A non-negligible proportion of isolates (54%) showed no susceptibility to tylosin, lincomycin and tiamulin, with a significant increase from 2010 to 2013 compared to 2008. Finally, a correlation was found between the non-wild type status of isolates to aivlosin and tylosin (p=0.025) and between non-wild type status of isolates to tylosin and lincomycin (p=0.016).

In conclusion, the decreased susceptibility of Italian isolates of B. hyodysenteriae is not restricted to pleuromutilins, but involves most of the antibiotics used for the control of SD. An increase of multiresistant isolates was recorded in the last eight years, posing a significant threat to pig industry.