I know this is a couple of years old, but I had the same thing (seizure) happen to my mare this morning. I had just dispensed a tube full of dewormer in her mouth, turned around to get the next tube (she weighs 1350, so gets two), and when I turned back around her eyes were rolled back, her hind legs had quit working, she stumbled and fell straight back into the corner of her stall, then she fell over and almost upside down, thrashed her legs and looked to be spasming (sp?) a couple of times, then rolled back on her side, got back up, stood there for a minute, pooped, and then acted like nothing had happened. Except for the huge cut on the inside of her top lip which was bleeding profusely and took me a few frantic minutes to find! The whole thing happened so fast, I was RIGHT THERE, still holding the lead rope, and completely freaked out - I thought I had killed my horse when I saw her fall down so hard, that she had either had something rupture catastrophically inside or was having a heart attack. Or that I had poisoned her with the dewormer - but I was able to dismiss that pretty quickly, as it was still in her mouth, she hadn’t even swallowed it yet.
I called the vet immediately, of course, and when we talked he said that horses do have seizures (I have been around horses for nearly all my life and this is the first I had even heard or seen of this), that we may never know what triggered it, and it may happen again quickly or we may never see another again. He wasn’t too anxious to do a bunch of blood work or run a bunch of tests just yet, which I’m fine with. We’ll watch and see how she does the next few days. He too dismissed the idea of the dewormer being involved, agreeing with me that it was too quick, it hadn’t even entered her system yet.
But the more I think about it, I’m wondering about this - every time I deworm my mare, she does the flehmen thing - quite dramatically curls her upper lip and sticks her nose straight up in the air. I am quite sure she was about do that, or in the middle of it, when this happened. I need to call my vet back and revisit this with him, I told him she does that but later he asked specifically if I had lifted her head up or anything to get the dewormer in - I told him no and didn’t repeat the information about the flehman response - but I’m wondering now if there isn’t a connection. She does that a lot though (flehman), and this is the first time (obviously) she has had a seizure at the same time. If the two were even related?
I don’t know the lineage of this mare, although she is very Quarter Horse-ish. She’s a rescue, from a very reputable organization here in Texas, and I have had her nearly three years. She came to them with at least one other mare as part of an owner surrender, and it appears she was a broodmare at one time based of the size and flabbiness of her teats. Her age is estimated to be mid-teens. So the other idea I floated past the vet was HYPP - I have never seen an episode of that either, and not sure if it presents the same way. She had just finished her breakfast (3/4 lb of TC 30% and a handful of alfalfa pellets) and was hanging out with her best pal in her stall when I walked over to halter her for her deworming - which usually isn’t even necessary, she’s really good about that sort of thing. So she was just relaxed and not stressed, enjoying the breeze from the fan. He said if I was curious I could do a DNA test and have her checked for that, but he didn’t think that was it, that she doesn’t exactly look like an H/H horse. I don’t know.
So my question is really asking the OP for a follow-up - Inca, did your horse ever have another episode? My mare does not have a physical indicator of a possible issue, like yours did on her neck, the only thing I’ve noticed differently about her is she seems to have grown a new thick coat almost overnight. It’s not long or shaggy, just new and thicker - almost plush - than her summer coat. So of course, I’m thinking pituitary tumor… although I have read on here she’s not the only one to spring a new coat too soon!
