As SF says, it's transmitted by midges. So it can only transfer from cattle to sheep, or sheep to other sheep, when there are (actively biting) midges about to carry it.
Equally, when the cattle contracted it - probably around 6-8 months ago to be having birth defects now - then there must've been biting midges about at that time to infect the cattle, whereever they were. If they were near your sheep at
that time, then hopefully your sheep contracted it then and will now be immune.
The birth defects arise when the disease is contracted in the first 1/3 or so of the pregnancy, they say.