Fetal Susceptibility to Porcine Reproductive and Respiratory Syndrome Virus

Abstract

Porcine Reproductive and Respiratory Syndrome (PRRS) is a swine disease caused by PRRS Virus (PRRSV), which besides respiratory disease, can also lead to reproductive failure. The extent of the reproductive disease can be highly variable within individual litters, demonstrating great variation in PRRSV susceptibility and resistance to the virus among litters, while intrauterine growth retarded (IUGR) fetuses have lower viral loads in both fetal thymus and in endometrium, indicating larger fetuses as more susceptible to transplacental PRRSV infection. Our hypothesis was that naturally occurring resistance/resilience to PRRSV infection and disease characteristic can be explained through the combination of techniques, such as fetal phenotype, metabolomics, and gene expression. We started by characterizing the temporal movement of PRRSV following maternal inoculation. It was observed that endometrium becomes infected as soon as 2 days post-inoculation (DPI), while fetal placenta and fetal serum presented viral RNA by 5 DPI. Demised fetuses due to PRRSV infection could be identified after 12 days of maternal infection, which established an ideal time point for future research. While this project was conducted, additional sampling of the endometrium and placenta was performed to determine the number of samples and analysis method required for an accurate, yet feasible viral load and histopathologic investigation. It was concluded that at least three random collected samples from each tissue are needed for both analyses, while these samples can be processed as a pool for viral RNA quantification. Moving forward on our objectives, a fetal metabolomics investigation was conducted and found differences in the metabolome of infected fetuses when compared to controls, as well as of IUGR fetuses related to normal developing fetuses. Most disturbances were related to amino acids pathways and lipid molecules, indicating differences in both nutritional transport ways and apoptosis pathways. To test the apoptosis pathway and the historically suggested hypoxia in PRRSV infected fetuses, two studies study between resilient and susceptible fetuses were conducted: 1) TUNEL staining of fetal tissues to detect apoptosis, and 2) expression of target genes in both apoptosis and hypoxia pathway. The results indicate that at 12 DPI both apoptosis and hypoxia were occurring in the fetal heart, while only apoptosis seems to affect the fetal brain; fetal thymus appears to be protected from these events. At 21 DPI, liver and thymus present with increased apoptosis in the highly infected animals, while only meconium-stained fetuses displayed apoptosis in the heart. In conclusion, fetal phenotype, metabolomics, and gene expression were able to identify naturally occurring PRRSV resilient or susceptible fetuses.