Distribution, genetic heterogeneity, and antimicrobial susceptibility of Brachyspira pilosicoli in Swiss pig herds.

Brachyspira (B.) pilosicoli is a bacterium causing porcine intestinal spirochaetosis, a disease characterized by diarrhoea and depressed growth rates especially in nursery and fattening pigs. Knowledge of the epidemiology and antimicrobial susceptibility of this pathogen is limited. Therefore, the aim of this study was to analyse the distribution, genetic heterogeneity, and antimicrobial susceptibility of B. pilosicoli field isolates from Swiss pig farms. Faecal swabs of 693 animals originating from 156 herds were analysed for the presence of Brachyspira spp. using culture and polymerase chain reaction identification. Further characterisation was performed using multilocus sequence typing (MLST) and broth dilution antimicrobial susceptibility testing. With 52.6% positive herds, B. pilosicoli could be frequently isolated from herds with animals suffering from diarrhoea. In herds with animals without clinical signs of diarrhoea, detection was significantly less frequent with only 10.5% positive herds (p 0.001). Among 80 isolates used for typing, genetic heterogeneity was observed with 44 different sequence types (ST) which often differed from herd to herd. No predominant ST was observed. More than 73.0% of the 41 B. pilosicoli isolates analysed, showed minimal inhibitory concentration values above the wild type cut-off values for lincomycin, tylvalosin and/ or tylosin. For tiamulin, valnemulin and doxycycline, this was the case in 48.8%, 43.9% and 36.6%, respectively. In conclusion, a diverse population of B. pilosicoli exhibited decreased susceptibility to antimicrobials used against Brachyspira infections. Monitoring of resistance in Brachyspira spp. is highly recommended to support targeted use of antimicrobials in pigs.