What's strange Trevor is that we keep our mares in the same paddock every single year (and have done for probably 12 years) and have never had any problems. Last winter was a particularly dry winter too.
We sent the mares and foals up to the vet to be treated and they're home now. They said that it's easy to diagnose and actually pretty easy to treat these days so there's hope! Our foals seem good now.
I actually wonder if it came in the horse feed and hay rolls. The vet says no but it just seems too much of a coincidence.
What's another coincidence is that our vet sees a colic condition where the gut twists and gets stuck under the spleen once every 3 years or so and TWO of ours got this problem within 4 days of each other. A couple of expensive stays at Murdoch Uni Vet Hospital later and we hope it's another 20 years before we see that problem again