I would definately check out your pasture. Toxic plants can cause similar symptoms and often the horses won't touch it until the good grass and such are gone. Also with fall and such you get leaves blown in and some of them can be toxic if accidentally ingested. I had a friend who had nightshade all in her field and didn't know it for 4-5 years. Then the horses decided to get into it and she was able to save one of the two that got into it. To be on the safe side you might want to put the other horses up until you can sort it out. I would talk to the vet to see what you need to do until they can get there. I know that horses can make 48 hours laying down and when my horse impact colicked the vet said to let him lay as long as he wasn't rolling around. It gave him just enough energy to make the 6 hour trip to the vet school for surgery. Good luck and let us know how it turns out. On a side note, as your vet is so far away you might discuss with him what you might need to have a heavy duty first aide kit available for future emergencies. My vet keeps me well stocked (I am 45 min away) so that I can handle some things until he gets there.
We will keep hoping all goes well.
x2 heres a list of toxic plants to check for http://www.ansci.cornell.edu/plants/php/plants.php?action=display&ispecies=horses