Oh crap. I'd start with cervical rads and a neuro exam, grab some lab samples, and hire an equine ambulance to get him to the nearest VTH for in-depth studies. I wouldn't try hauling him in my personal rig unless I could get him slinged in there. I would not delay.
I'll put extra long jingly curb chains on everyone I ride today.
My older horse who has since passed on had a seizure in her early twenties. The vet said to give her some time off and she was back showing within about 6 months. I did not own her at the time and when the owner told me the story she was pretty upset. However, it was just one of those things.
No real answer but a positive outcome. Hope your boy is fine.
I just wanted to suggest checking your feed to be sure its free of any dust/mold smells right ect. A friend described a similar situation with her horse happening earlier this season---on checking the feed supply it was found to be contaminated with some type of mold fungal spore(confirmed by laboratory testing)--this was newly purchased feed from a major brand feed company.
My Cushings horse suffered a seizure that presented almost exactly like your horse's episode. The only difference was that I witnessed it, I didn't have to rely on second-hand knowledge. As I have also seen epileptic seizures in people, I knew instantly what I was looking at. My (now ex) vet was worthless, he said horses don't get seizures. So I had to do my own research. I found some studies that suggested a small percentage of Cushings horses have neurological deficits, some of which can be seizures. I think there was a thread some years ago on COTH regarding seizures in Cushings horses. In my case, I elected to put the horse down because he was a danger to me and DH, and also because his quality of life was declining.
I hope UT can find a problem that can be resolved. In my case, my vet refused to prescribe Pergolide, said he only prescribed it for broodmares. Had my horse been on it, he may not have been subjected to the seizure. Sending positive thoughts your way.
Update: I spent the entire day and $300, and learned nothing.
His blood work was normal, as was his neuro exam and all other physical findings. This effectively rules out viruses, including all the funky encephalitises, and it rules out toxins like poisonous plants and other liver problems, which can apparently cause seizures.
They took blood for an epm titre; if that comes back high (it'll take a few days) they'll check his spinal fluid. Otherwise, we just wait to see if it happens again.
Jingles. That's actually good, don't despair, you've ruled out quite a bit of stuff and that's good.
It's not like lameness where you can block this or that and come up with the definite thing. It takes a really good team effort to find the cause with something like this.