Modelling porcine reproductive and respiratory syndrome virus dissemination dynamics to quantify the contribution of multiple modes of transmission: between-farm pig and vehicle movements, farm-to-farm proximity, feed ingredients, and re-break

Summary

Accounting for multiple modes of livestock disease dissemination in epidemiological models remains a challenge. Here we developed and calibrated a mathematical model for transmission of porcine reproductive and respiratory syndrome virus (PRRSV), tailored to fit nine modes of between-farm transmission pathways including: farm-to-farm proximity (local transmission), contact network of batches of pigs transferred between farms (pig movements), four different contact networks of transportation vehicles (vehicles to transport pigs to farms, pigs to markets, feed and crew), the amount of animal by-products within feed ingredients (e.g. fat and meat and bone), and finally we also accounted for re-break probabilities for farms with previous PRRSV outbreaks. The model was calibrated on weekly PRRSV outbreaks data. We assessed the role of each transmission pathway considering the dynamics of specific types of production (i.e., sow farm, nursery). Our results estimated that the networks formed by transportation vehicles were more densely connected than the network of pigs transported between-farms. The model estimated that pig movements and farm proximity were the main PRRSV transmission routes regardless of farm types, but vehicles transporting pigs to farms explained a large proportion of infections, sow = 20.9%; nursery = 15%; and finisher = 20.6%. The animal by-products showed a limited association with PRRSV outbreaks through descriptive analysis, while our model results showed that the contribution of fat and meat and bone was 2.5% and 0.03%, respectively, of the infected sow farms. Our work demonstrated the contribution of multiple unmeasured routes of PRRSV dissemination. It also provides strong evidence to support the need for cautious, measured PRRSV control strategies for transportation vehicles and to some degree feed by-products. In addition to providing valuable information and opportunities for the swine industry to focus effort on the most relevant modes of PRRSV between-farm transmission.