I thought he was having a heart attack

The other day I was talking to a member of the Wentworth Hunt and she mentioned that her draft cross had been very sick. When she described the symptoms I relayed the story of Cupcake and his mysterious illness. They had taken their draft to The New England Equine clinic and even they could not tell what was wrong with him. My vet ran extensive tests . I must confess, I have forgotten a lot of what she told me, but it seems both these drafts had the same problem. Symptoms were: And odd that they are identical....I took Cup for a ride, was not an exceedingly hot day in November, and he gets worked pretty regularly. He stopped on the trail and started to shake, he was sweating profusely, and I do mean profusely, dripping from his neck and off his chest. I thought my big white boy was having a heart attack and dropped my stirrups, preparing should I have to leap from his 17.3 height.
 My wonderful vet was not sure what caused the problem, but looked it up and found that the treatment for what she surmised was wrong was soaking his hay and changing his feed. He does not get a lot of grain as is, and he eats enormous amounts of hay in the winter, and shares with 3 other horses. If this was going to be his new regimen, I was in for a lot of work.  A while back one of the mares at Mountain Lane Farm tied up and got better as soon as she was given selenium and E. Mountain Lane's owner, Heather White, a wise horsewoman, thought perhaps we could try giving Cup selenium and see what happened. Within a few weeks there was a big change.
About a month later I was in Tracctor Supply and saw that they had selenium blocks. I had read that horses can actually get too much selenium, but that they will only eat what they need if they have the block free choice. My horses now have free access to not only an mineral salt block but also a selenium block and are doing great. They are not licking down the selenium as fast as the mineral which leads me to believe they are, now that they are back on pasture, in a good balance.
This leads me back to the Wentworth hunt Master's horse. I mentioned the selenium to her mother and I will be interested to see how her draft fares.