Secondary infection by other pathogens may occur because of various degrees of immune system dysfunction created by acute BVDV infections. The major role that BVDV plays in shipping fever is thought to be that of local immunosuppression in the lungs, allowing for secondary bacterial infections to occur. Finally, acute infections with BVDV are important in pregnant cattle because of the ability of the virus to cross the placenta and cause intrauterine infections of the fetus. Pathogenesis of intrauterine infections It is well documented that BVDV can cross the placenta and infect fetuses of all ages. The outcome of these infections are largely dependent on the stage of gestation when infection occurs.
Embryos appear to be resistant to infection until they hatch from the zona pellucida.
This occurs around day 10 of gestation. Susceptibility becomes greatest following implantation which occurs at day 20 of gestation. During this early phase of pregnancy, infection with BVDV often result in early embryonic death and resorption of the fetus. Fetal infection during the first trimester of gestation can result in abortion or the development of a mummified fetus. Persistent infections develop when BVDV is circulating around 100 days of gestation.