Porcine reproductive and respiratory syndrome (PRRS) is an infectious disease caused by PRRS virus (PRRSv). PRRSv is considered one of the most economically important infectious agents in the swine industry worldwide. This thesis evaluated immune management strategies to eliminate PRRSv from breeding herds, assessed the impact of those strategies in production losses, evaluated management risk factors associated with successful PRRSv elimination or failure, and lastly evaluated vaccination as means to reduce virus dissemination from infected growing pig populations.
Chapter 2 compared the use of two immunogens as part of load-close-expose programs in regards to the time it takes to reach PRRSv stability (TTS) from infected breeding herds. Results showed that despite the great variability of TTS between enrolled herds, there were some variables associated with shorter TTS. Specifically, herds that used LVI as the method of whole-herd exposure reached TTS earlier than herds that used MLV vaccines. Moreover, herds with history of PRRSv infection in the 3 years prior to our study reached TTS sooner than herds without history of PRRSv in those same 3 years. Furthermore, herds assisted by a specific veterinary clinic reached TTS sooner than all other herds suggesting that there might be management practices associated with shorter TTS. Our results also showed that PRRSv-monitoring must be done repeatedly over time to increase confidence of PRRSv-negative status of weaned piglets. Altogether, those findings represent prove of concept that herds can reach PRRSv stability as soon as 84 days from establishment of LCE. Further studies are needed to better understand specific factors associated with reaching short TTS, such as farm layout, pig flow and implementation of specific management practices during the period of herd closure.
Chapter 3 evaluated the effect of attenuated PRRSv vaccine inoculation compared to the use of live-virulent virus inoculation on production performance in breeding herds. It was shown that herds that used MLV vaccine as part of load-close-expose herd closure programs recovered production levels and had a less severe production impact than herds that used LVI. Also, herds that reported previous PRRSv infection reached time to baseline production (TTBP) sooner than herds with no history of PRRSv infection in the previous 3 years. Interestingly, herds assisted by a specific veterinary clinic recovered production faster than other herds, raising the hypothesis that specific management practices could be associated with herd closure effectiveness.
The chapter 3 addendum consisted of economic models to assist veterinarians to make informed decisions between LVI and MLV as part of LCE program to eliminate PRRSv. Taking in consideration the future prices for market hogs and pig feed of the next 12 months, and $13.52 loss per pig that is PRRSv-positive, the results suggested that MLV would be a better economical choice.
Chapter 4 was a follow-up from the previous 2 chapters and investigated herd-closure practices associated with successful PRRSv-elimination from breeding herds. It showed that the success rate of LCE programs was 76% and 92% for LVI and MLV herds, when failures associated with unrelated PRRSv were excluded from the analysis although these differences were not statistically significant. Moreover, one out of six herds that achieved 90 days of failure to detect PRRSv in due to wean piglets failed to achieve AASV category III, which indicates that more strict monitoring programs should be adopted in herds undergoing PRRSv elimination. Interestingly, herds assisted by different veterinary clinics had different set of recommendations of management practices to be followed during LCE, indicating that there was no agreement between veterinarians on what is the relative importance of each management practice evaluated. The variables associated with failure to reach AASV category III were a) being infected with a PRRSv of RFLP pattern 1-4-4 and b) holding back pigs at weaning for quality.
Chapter 5 focused on PRRSv-infected growing pig populations. Growing pig populations are a major source of PRRSv dissemination within regions. The effect of an attenuated PRRSv vaccine was evaluated on shedding and generation of aerosols of a wild-type virus from an infected growing pig population raised under Minnesota's swine industry field conditions. Our results showed that vaccinating acutely PRRSv infected growing pigs with MLV vaccine reduced duration and magnitude of PRRSv detection in aerosols exhausted from the pig barn.
Further studies are needed to characterize the effect of implementation (rather than recommendation) of key management practices on the success of PRRSv elimination programs. There is also a need to better understand the procedures to prepare live (virulent) virus inoculum to expose adult animals in load-close-expose programs. For instance, there is a need to document the effect of PRRSv infectivity dose, route of exposure and materials used to dilute the serum on TTS and TTBP. On the growing pig side, there is a need to validate other effective methods to decrease virus dissemination from infected pig sites that continue to pose a risk for neighboring pig populations, for example by inactivating PRRSv in aerosols coming out of infected pig barns.
Altogether, the work presented in this thesis provided science-based information on immune management strategies to control and eliminate PRRSv infection from pig populations. Information herein reviewed, discovered, described and discussed has direct implications to the swine industry and can assist veterinarians to choose appropriate strategies to reduce production losses and decrease PRRSv dissemination from infected pig populations.
University of Minnesota Ph.D. dissertation. February 2013. Major: Veterinary Medicine. Advisor: Dr. Montserrat Torremorell. 1 computer file (PDF); viii, 138 pages.
Linhares, Daniel Correia Lima.
Evaluation of immune management strategies to control and eliminate porcine reproductive and respiratory syndrome virus (PRRSv).
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.