First Detection of Salmonella enterica Serovar Choleraesuis in Free Ranging European Wild Boar in Sweden

Abstract

Following the first detection of Salmonella enterica subsp. enterica, serovar Choleraesuis (S. Choleraesuis) in a Swedish pig herd for more than 40 years and subsequent detection of the same serotype in an enclosure with kept wild boar, a national surveillance for S. Choleraesuis in free living wild boar was launched. A total of 633 wild boar sampled within the active and the enhanced passive surveillance were examined for Salmonella enterica serovars by culture. Of these, 80 animals were culture positive for S. Choleraesuis var. Kunzendorf. All positive animals, including those in the original outbreaks, originated from counties located in the southern and eastern parts of Sweden. Fifty-eight isolates were selected for sequence typing, revealing a relatively homogenous population of S. Choleraesuis with two distinct genetic clusters containing isolates from the southern counties in one and the counties further northeast in the other. Sequenced isolates from domestic pig farms all clustered with wild boar in the same region. S. Choleraesuis appears highly contagious in dense wild boar populations, making it a relevant model for other infectious diseases that may be transmitted to pigs. The many potential routes of introduction and spread of S. Choleraesuis warrant further investigations in order to prepare for other disease threats.

Keywords: Salmonella Choleraesuis; Sus scrofa; surveillance; wild boar; wildlife/livestock interface.

Figure 1

(a–c) Geographic distribution of Swedish pig holdings (a); wild boar population, based on hunting bags (b) and sampled wild boar in this study (c). The square indicates the area shown in higher resolution in Figure 2.

Figure 2

Geographic location of sampled wild boar and results of sample analyses in the surveillance of Swedish wild boar for Salmonella enterica serovars in 2020–2022.

Figure 3

NeighBor net visualization of whole-genome SNP variation among all sequenced isolates of S. Choleraesuis in 2020–2022. Isolates are labelled by hunting district and year and colored according to the county of the hunting district.