My trainer lost her best schoolhorse this summer. He seemed in perfect health, was being ridden by a student when the trainer noticed him stepping funny. She had student bring him back to see and he let out a very specific whinny. She told student to take tack off and student did just as he collapsed and died. There was kicking involved, but that was not struggling, that was the death throes.
She did have necropsy done and while he was young (11) and looked and acted very healthy (I competed him only about 2 weeks prior), it turned out he had more organ damage than every supposed. If I remember correctly, he died of a ruptured aorta, but the trouble with the organs was likely a contributing factor.
This was a horse that never had trouble keeping weight on, travelled well, was never in ill health, really just no signs.
She ended up putting down another horse (10yr old) who’s first sign of trouble was ‘weight loss’. He went from being something like 300 lbs overweight to 150lbs overweight. He was staying at another farm for the winter. The person that owned that farm had taken him as part of a board agreement to bring her horse to my trainer’s for the winter. Her horse lost of a lot of weight and looked ill when he first moved, but they thought it was the transition. When my trainer’s horse did not look well (something she would notice more than someone else due to his pasture puff figure), she had bloodwork done and he had picked up a bacteria at the other farm. By the time it was caught, it was to late to do anything about it.
The farm owner’s horse has likely picked up a bacteria as well, but came out of it better. A necropsy was done to determine what it was, but I never got around to asking the results. The farm owner felt very guilty and also very worried about her other horses.
Also, we had a horse die in a pasture of 4 other horses, several years ago, at another farm. A new pony was introduced. That pony and my pony never looked odd. My horse looked like she lost weight, but then picked it back up and perked up in about a week or so timeframe. The other horse, who was only 4, but had trouble keeping weight on anyway, died. We did not do a necropsy but thought back after the fact and surmised that the new pony brought a virus/bacteria into the field.