Prevalence of Erysipelothrix rhusiopathiae in tonsils of domestic pigs and wild boars in Sweden

Erysipelothrix rhusiopathiae (ER) causes erysipelas in multiple animal species and may persist in the environment or be carried asymptomatically. Tonsils are a recognized reservoir in domestic pigs. This study aimed to determine the prevalence of ER in tonsils of healthy Swedish fattening pigs and wild boars. Tonsils were collected from 200 fattening pigs at slaughter from ten abattoirs across Sweden in 2017, with one pig per herd sampled. Wild boars were sampled during hunting in the county of Östergötland during the year 2018. Cultures were performed using selective media and isolates were confirmed as ER by MALDI-TOF MS. ER was recovered from 6/200 pig tonsils (3.0%), all originating from three abattoirs in southern Sweden. ER was isolated from 45.5% of tonsils from wild boar. Whole-genome sequencing revealed a high diversity among the isolates with no dominant clones. Overall, these results indicate that that Swedish breeding practices, including indoor rearing of fattening pigs, age-segregated rearing, sow vaccination, enhanced biosecurity, and restricted straw access largely prevent colonization by ER aligning with the low occurrence of clinically diagnosed erysipelas in such herds. For wild boars, the high isolation rate may suggest that wild boar could act as a reservoir and potential source of the infection for domestic animals. The potential zoonotic risk should also be considered.