Final answer:
Exposure to BVD virus at 1-30 days gestation in a pregnant heifer typically leads to embryonic death due to the underdeveloped immune system of the embryo.
Step-by-step explanation:
When an immuno-naive pregnant heifer is exposed to a wild strain of Bovine Viral Diarrhea virus (BVD) at 1-30 days gestation, the most common outcome is embryonic death. The immune system of the developing embryo is not mature enough to mount a defense against the virus, leading to the failure of the embryo to continue developing. The body subsequently resorbs the embryonic tissues. Immunotolerance is not typically developed at this early stage because the immune system of the embryo is not sufficiently developed to recognize and become tolerant to the viral antigens. In cases of infection later in gestation, outcomes such as congenital abnormalities, immunotolerance, or even fetal resorption may occur, depending on the timing and specifics of the infection.