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. It is estimated that 30-50% of apparently healthy or convalescent pigs harbour ER in their tonsils and other lymphoid tissues. This study aimed to determine the prevalence of ER in the 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 (n = 180) were sampled during hunting, primarily in Östergötland County, in 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 76/167 (45.5%) of wild boar tonsils. Whole-genome sequencing revealed a high genetic diversity among the isolates with no dominant clones. Overall, these results indicate that Swedish pig husbandry, characterized by indoor rearing of fattening pigs, age-segregated rearing, sow vaccination, enhanced biosecurity, and restricted straw access largely prevents tonsillar colonization by ER aligning with the low occurrence of clinically diagnosed erysipelas in such herds. For wild boars, the high isolation rate suggests that wild boar could act as a reservoir and potential source of infection for domestic pigs. The potential zoonotic risk should also be considered.