We had a mare with hypocalcemia several years ago. She was heavy in foal and initially I thought she was going into labor. She wasn't eating, appeared to be having mild contractions. I watched her some, & decided it wasn't labor, but didn't know what it was. Called the vet clinic--vet on call said to keep an eye on her. She stayed the same that night and by early the next morning she was obviously worse--she had thumps (called thumps I think because of the noise made when the diaphragm contracts--it's a very loud hiccup that sounds like a THUMP) and was having muscle twitches through her jaw and sides. She wouldn't/couldn't eat--her jaw was clenched tightly--she could walk but was reluctant to do so. I called the vet back & told him this was serious. He sent another vet out--she lived closer so could be here sooner. She came out, checked the mare, asked if she'd had a tetanus vaccine (she had) and so ruled out tetanus and said it had to be calcium deficiency. She'd never seen it in a horse but was very familiar with it in dairy cattle. She didn't even do a blood test, she was that sure of what the problem was. She had to go back to the clinic to get IV calcium but was back in less than an hour. She prepared two 20cc syringes of calcium & put the first one in, then started the second one. By the time half of that was run in the muscle tremors had stopped and the mare's eyes had brightened. she did not relapse and delivered a healthy filly 2 weeks later.
Hypocalcemia can kill in 48 hours or less--it is vital that the early signs not be missed. A horse out on pasture may well be sick for several hours before anyone notices, because she may stand or wander around with the other horses without anyone noticing that she isn't eating or moving right. Early signs maybe mistaken for colic. Some horses get sicker faster, so the early warning signs last for less time & are more likely to be missed-- A horse could be fine at evening feed and deathly ill by morning--which sounds like the case with your mare Theresa.
Muscles require calcium to work properly which will be why your mare is so splayed and stiff on her hind legs. As the heart is also a muscle, lack of calcium affects the heart--it will not be pumping properly, which accounts for the black blood and lack of oxygen--heart failure, according to my vet, is the cause of death in hypocalcemia. (Just an FYI for those not familiar with the condition and its effects)
If a horse was this sick from blister beetle poisoning I'm quite sure that there would be blisters/burns in the horse's mouth. As a side note to blister beetle poisoning...it isn't just hay from the 3 states listed above in an earlier post that can be a problem. I think that it's more common in hay from the southern states, but blister beetles have moved further north. We have them here but not in great numbers. There hasn't been a problem with alfalfa hay right around here (that I've heard of) but a couple years ago someone further north of here lost a couple horses--vet diagnosed cause of death as blister beetle poisoning, and the guy had been feeding his horses on locally grown alfalfa hay. There have also been occasional blister beetle poisonings in horses in various places in the northern US--on hay grown locally in those areas.
