I've lived in Italy where they had their outbreak 2009-10, came from contaminated blood plasma from Eastern Europe. All horses had to be tested, several came up positive. They'd give you the choice of putting down or quarantine for life. They have a herd of 20, I believe, living way out in the wilderness, some negative horses also, and all are healthy, nobody got newly infected.
It developed into a long term study, maybe it will eventually change the way of handling this disease. And not only mosquitoes are to blame, horse flies are much more efficient in transferring the bug. Testing states only that the horse is negative at the moment of drawing blood, it can turn around, get bitten and be positive, so the 12 month tests really prove nothing.
logic tells me keeping the biting bugs off is key. As simple as that.