I am a firm believer in supplementing Vitamins A-D-E, particularly A. I have found that many horses get too little of it all year and in the winter and early spring, most horses that receive no supplements or highly fortified feeds get MUCH too little of it. Without it, horses have NO adequate immune response. It is why they get Rain Rot, goopy eyes, lice, etc. I know you cannot prove a negative, but I think it is much more than a coincidence that when WNV first came along, horses died all around us. Some people near us lost several. We did not have a single case.
People all around us have lost horses or had horrible cases of EPM. Opossums live around here by the thousands. We have not had a case and keep 50 - 60 horses running out on pasture with no shelters for any accept those in paddocks. Part of those have no shelters and only have trees or brush for wind-breaks.
Vitamin A (and D & E) can be supplemented very inexpensively. I use a mineral with a lot of Vitamin A in it and there are inexpensive supplements like Mare Plus that have high levels without the Vitamins that make horses get high and silly.
We do not have horses that colic and we just do not have sick horses. We don't have foals born with crooked or weak legs and we have nearly everything breed on one service. Like I said, you cannot prove a negative, but it sure cannot be a total coincidence with this many horses for this many years.