Infection dynamics and incidence of wild-type porcine reproductive and respiratory syndrome virus and associated risk factors in growing pigs in Midwestern U.S herds

Abstract

Porcine reproductive and respiratory syndrome (PRRS) remains the most important swine disease in the Unites States (U.S). PRRS affects swine productivity and causes severe economic impact to the swine industry. In the U.S., PRRS virus (PRRSV) economic impact is felt mostly in growing pigs with an estimated 54% of the cost. The significance of PRRS in growing pigs cannot be underestimated given that growing pigs represent more than 90% of the US national herd inventory and are reservoirs of PRRSV infections that in turn, can affect the stability of neighboring breeding herds. Furthermore, PRRS is difficult to control and it relies on the use of modified live virus (MLV) vaccines and biosecurity practices to minimize the disease impact, and prevent the introduction and spread of the virus to other pigs, barns or farms. However, vaccination with MLV adds complexity to surveillance programs due to the need to differentiate vaccine from wild-type PRRSV (WT-PRRSV) strains. In the past years, efforts and investments to control and eliminate PRRSV have focused on breeding herds to wean PRRSV negative pigs. However, the overall national incidence of PRRSV infections in breeding herds has not changed significantly. We hypothesize that the lack of significant change in PRRSV incidence in breeding herds is in part due to PRRSV infections being prevalent in growing pigs and a risk to sow farms. Therefore, there is a need to further understand the epidemiology of PRRSV infections in growing pigs and the factors that contribute to the introduction of new WT-PRRSV into growing pigs. The overall goal is to provide recommendations to producers to prevent the introduction of PRRSV into growing pigs and enhance PRRS control. More specifically, the thesis objectives are to: a) describe WT-PRRSV infection dynamics in growing pigs housed in wean-to-finish sites, b) estimate the incidence of WT-PRRSV infections in growing pigs and, c) identify risk factors related to production practices, herd characteristics and activities associated to the risk of new WT- PRRSV introduction into growing pig herds. Ultimately, our work will contribute to the current body of knowledge on PRRS to implement better control and elimination strategies at the farm and regional levels. This thesis is structured into four chapters: Chapter 1 includes a literature review on PRRS; Chapter 2 contains a study aiming at estimating WT-PRRSV incidence and dynamics of wild-type PRRSV infections in growing pigs; the second part of this study is presented in Chapter 3, where we analyzed risk factors for WT-PRRSV introductions in these wean to finish sites. Lastly, Chapter 4 offers a summary of main findings and overall conclusions of the thesis.