There is a new strategy out there that no longer relies on orphaning lambs. It is now thought that ewes don’t transfer the virus through milk or colostrum. The new approach is to maintain the ewe lambs separate from the ewe flock, tested prior to 12 months of age, they are permanently segregated and retested annually to confirm continuing test-negative status. This creates a base for a negative flock. These ewes never enter flock. The original flock is culled.
The significance I find in this that is the Midwest many production systems producers routinely maintain their ewe lambs as a separate unit. Unknowingly they have reduced test positive sheep.
In the end what difference does it make, the disease has no economic significance.