| Porcine reproductive and respiratory syndrome virus (PRRSV) was inoculated into multiple PRRSV-vaccinated and nonvaccinated late term pregnant sows for investigation of the effect of varied exposure dose on vaccine-induced protection, the effect of PRRSV infection on progesterone levels and ovary, and the relative suitability of virus isolation, immunohistochemistry, fetal serology, and reverse transcription polymerase chain reaction for the detection of transplacental fetal infection. In phase 1, dead and virus infected fetuses were identified at necropsy on postinoculation day 21 in 4 of 4, 3 of 3, and 3 of 4 litters from nonvaccinated sows and 0 of 4, 1 of 2, and 1 of 4 litters from PRRSV-vaccinated sows inoculated with 102, 104, or 10 6 CCID50 of PRRSV strain NADC-8 respectively. The rate of infection was significantly different (P < .001) between the vaccinated and nonvaccinated 102 CCID50 groups. Litter infection rates were lower in the higher dose vaccinated groups but not significantly different from nonvaccinates. No difference in the rate of infection of fetuses within transplacentally infected litters was identified regardless of dose or vaccination status. In phase 2, plasma progesterone levels were not different from controls regardless of vaccination status or dose group, no ovarian lesions were detected on light microscopy, and no PRRSV was detected in ovarian tissues by immunchistochernistry or by in situ hybridization. In phase 3, virus isolation, immunohistochemical staining, and fetal serology identified PRRSV infection in 48.6, 23.4, and 14.9% of 107 fetuses respectively, and identified at least one infected fetus in 10, 10, and 5 of 10 litters respectively. In-utero death with autolysis reduced the test efficacy of all three methods. Fetal thoracic fluid and tissues proved equally suitable for rtPCR detection of PRRSV. Pooling of fetal tissues or fluids from VI-positive animals with comparable material from negative controls had no detrimental effect on rtPCR results when evaluated at dilutions of 1:1, 1:2, 1:4, and 1:8. The results of rtPCR testing were positive in 100, 94.4, and 83.3% of VI-positive specimens allowed to autolyze at 4, 21 or 37°C respectively. Compared to the other testing modalities, rtPCR appeared to be impacted the least by autolysis. |
