We are in northern Germany, and the sheep belonging to the farmers who rent some of our fields are just finishing lambing - started in December. The first lambing was not good, with quite a lot of lambs dying before birth, or born deformed. One is still bottle fed, and seemingly brain damaged. His leg is deformed so he cannot stand properly to feed from his mother. HIs eyes do not seem to open properly.
However, since about Feb. the lambs have all been fine and healthy. It seems to be so dependant on when the infected mosquitoes were about in relation to the age of the feotus. The farmers here have been lucky, if one can say that, in that their rams live with the ewes most of the year, and so the ewes don't all get pregnant at the same time, thereby spreading the risk period over a number of months. Maybe this is a strategy that could be used. It does mean an extended lambing period however.
I'm not yet clear if the immunity passes on genetically. If not that means the risk for first year mothers remains for next year, and each successive year I suppose.