By Mark Schwartz and Maria Pieters, Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota

Porcine reproductive and respiratory syndrome remains a disease of major economic consequence, with an annual cost to U.S. producers estimated to be $1.2 billion. While producers have made investments in additional measures in the attempt to reduce the incidence of PRRS outbreaks through enhancements to biosecurity, the Morrison Swine Health Monitoring Project data show that the annual incidence of PRRS in breeding herds remains at 25-35%. Meanwhile, the incidence of PRRSV infection in growing pig populations, based on previous research, ranges from 50 to 75%.

If producers and consulting veterinarians are to create effective strategies to address the prevention of PRRS virus entry into growing pig populations, an understanding of the modes of transmission and the quantification of risk factors in the exposure and infection of susceptible populations of growing pigs are critical to this endeavor. In regions of high pig density, both in the number of sites as well as number of pigs per site, the mechanisms of transmission and spread are many times clouded by the multiplicity of potential routes. An improved understanding of the role of PRRSV infection associated with the transport of pigs from the nursery to finishing phase is one step in quantifying the risks of PRRSV in growing pig populations.

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An on-going study, funded by the University of Minnesota Swine Disease Eradication Center, aims to quantify the frequency of PRRSV-positivity after transport from negative wean-finish split-down and the association with transport-related PRRSV exposure, utilizing oral fluid sampling.

While biosecurity practices and sanitation of transport vehicles are closely monitored for multiplication and breeding stock movement, the protocol and oversight can be less stringent for commercial phases of production. Health status (i.e. PRRS) is many times assumed, and transport vehicle sanitation is staged, with early and late week movements from nursery phase to finishing dependent on presumed PRRSV status. Thus, there seems to be a lack of sound understanding of the incidence of PRRSV infection as a result of PRRSV-contaminated transport vehicles subsequently hauling PRRSV-susceptible pigs, in particular during times of increased PRRS incidence.

Wean-to-finish production systems that utilize double-stocked nursery and the split-down of the over-stock into the finishing phase provide the unique ability to determine pre-movement PRRSV status of the population of origin and the post-movement PRRSV status of the destination population, while maintaining the population of origin as a reference. Sequencing of paired, origin and destination positive PCR (polymerase chain reaction) samples will provide insight into the frequency of PRRSV infection prior to transport from wean-finish to the finishing phase and that of transport-related exposure and infection.

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This set up is unique, in the sense that it provides the opportunity to determine the frequency of PRRSV infection of populations in the nursery phase, in wean-finish, prior to split down. The ability to ascertain post-movement PRRSV status and have a reference population is not possible in a conventional nursery and finishing model. In addition, the incidence of PRRSV infection as a function of pig density in the region surrounding the sites enrolled in this study will be evaluated, as well as an analysis of PRRSV variants in and among sites within a production system.

Preliminary observations from the first sites surveilled as described indicate a diversity of outcomes, including: PRRSV PCR-positive prior to split-down, sites in which the destination was detected PRRSV-positive by PCR and the origin on the second sampling, after having first tested PCR negative pre-split down, and sites which remained negative by PCR after transport at split down. Of the sites with PCR positive result at the time of destination surveillance, sequencing indicates high homology to the origin.

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The outcomes of this SDEC study will be shared with the swine production and veterinary community to support the continued improvement of practices and the reduction of disease prevalence in swine populations. The mission of the SDEC is to ensure that our findings advance scientific understanding and contribute to the sustainability and success of pig farming through the control and elimination of swine diseases.