Vertical transmission of PRRSV2 strain NVSL 97-7895 occurs during mid-gestation

Porcine reproductive and respiratory syndrome virus (PRRSV) has resulted in substantial economic losses in global swine production over the past thirty years. Historically, vertical transmission of PRRSV in pregnant pigs was believed to occur only after gestation day (GD) 72. Recent evidence, however, suggests that highly virulent strains may cross the placenta at an earlier gestational age. The objective of this study was to determine whether the now prototypical reproductive type 2 PRRSV (PRRSV-2) strain induces fetal infection during mid-gestation. Twenty-three pregnant gilts were challenged with a PRRSV-2 strain, NVSL 97-7895, at 10-day intervals from GD 50 to GD 80. At 21 days post inoculation, fetal preservation status, body weight, and organ weights were recorded, and fetal samples, including serum and tissues, were collected. PRRSV-inoculated (INOC) litters exhibited a higher incidence of meconium-stained fetuses and non-viable fetuses compared to control (CON) litters. All INOC litters, regardless of gestational age at inoculation, contained PRRSV-positive fetuses. Fetal serum and thymic viral load did not differ significantly by gestation age at inoculation. Virus recovery and immunohistofluorescence assays confirmed vertical transmission of PRRSV during mid-gestation. Furthermore, viable fetuses with high viral load were more frequently identified in mid-gestation inoculated groups, while all uninfected fetuses were found in late gestation inoculated groups. In conclusion, these findings clearly demonstrate that virulent PRRSV-2 strains can transplacentally infect the fetus during mid-gestation, and fetal outcomes are influenced by gestation day of infection.