Expert workshop

A total of 24 potential disease pathways could be defined by combining the information collected during expert and farmer interviews, and were classified during the expert workshop according to the overall frequency of occurrence and the specific likelihood of disease spread for ASF, PRRS and EP.
Initial answers of the experts diverged most of the times (Supplementary Table S3). Concerning the frequency of pathway occurrence, the same answer was given by all experts in three cases, namely for the direct trade between farms (high frequency), transport performed by the farmer (moderate frequency), and visits by feed advisors (high frequency). Unanimous initial answers about disease spread likelihood were observed slightly more often: for seven pathways in the case of ASF, for eight in the case of PRRS and for six in the case of EP.
The final scores on frequency of occurrence and likelihood spread, on which the experts agreed after open discussion are presented in Table 3, together with the overall relevance for the three diseases of interest obtained by means of risk assessment matrices. Out of the ten transport-related disease pathways, four seemed to have a high frequency of occurrence, which, coupled with fair-to-high likelihood of disease transmission, resulted in high-to-very-high relevance for ASF, PRRS and/or EP spread. Another pathway with fair-to-high relevance for disease spread was self-performed pig transports by the farmer (pathway 10). On the one hand, experts believed that farmers may wash transporters less frequently after transports or may get in contact with their colleagues more often than professional lorry drivers. On the other hand, they pointed out that farmers are considerably less likely than trader companies to visit several farms on the same day, hence having a lower relevance for disease spread compared to transports performed by traders.
Out of the five pathways in the farmer encounters area, two were classified as highly relevant for PRRS transmission. These were “sharing of other farming equipment” (pathway 14) and “going to the carcass collection point” (pathway 15). “Sharing of pig farming equipment” (pathway 13) may result in a higher likelihood of pig disease transmission but happens less frequently, resulting in an overall low-to-fairly-low relevance.
In the external collaborators area, the other carcass management-related practice (pathway 16) was found to be highly relevant for the spread of PRRS and ASF. Other on-farm visits from external collaborators consist in manure trade between farms, veterinarian visits, feed advisor visits and other official farm visits. Despite the high frequency of such visits, these appear to have only low-to-moderately-low relevance for propagation of ASF, as likelihood of spread was categorized as low. However, they are considered as highly relevant for transmission of PRRS, which can be propagated much more efficiently via fomites, as well as for the transmission of EP in the case of feed advisors.
Finally, among the four environment-related contact pathways, the only one relevant for disease spread is wild boar movements from farm to farm, with relevance ranging from fairly high for ASF to high for PRRS and EP.
Table 3 List of disease pathways between Swiss pig holdings resulting from the final expert workshop. For each pathway, the frequency of occurrence, and the likelihood of disease propagation through it for ASF, PRRS and EP are reported. The three rightmost columns show the overall relevance of pathways for the three diseases of interest, as a product of frequency of occurrence and likelihood of disease spread.