Its quite common, at least so my vet told me. As far as I know the frosted grass affects the gut. What I do now is the ponies do not get out first thing. I make sure they have hay first.Laminitis can also be brought on by hard ground, stress, foaling .I wonder if line breed animals are more prone to the problem. I have had Shetlands that get very fat no matter how little grass they get but never go down with it. One of my friends bought a pony years ago, it came off poor grazing so she put him in a small paddock with the same sort. he went down with laminits in the first week. did not show up in usual way. His breathing changed enough for her to notice so she got the vet. No heat in his feet, not lame as such but he died within 10 days. It had gone into his blood stream. Vets tried their best to save him. I never knew that this could happen. since then I check all the time that my lot have no signs. Have gone to a few talks on the subject. Learning more all the time.