I recently had to put down my barrel horse. She developed breathing problems two years ago. Very long story really shortened here but at first vets thought it was allergies. Multiple tests and medications and nebulizer  treatments she never responded. Scoped and ultrasound of lungs found fluid build up. Samples were taken and tested and we treated for what we thought was fungal only to again not respond. She has been through three vet hospitals with one being two states away. Never responded to any medication or supplement and continued to get worse. Would have multiple episodes of not being able to breath. Coughing so bad she would almost pass out. And breathing heavily like she sprinted a mile out of shape. Abdominal breathing last year of life even at rest and respiratory rate was ALWAYS increased, even in high doses of steroids. I am just wondering if anyone else has ever experienced this? She was 8 when this started died at 10. She is royal quick flash and Val's movin on bred. Any relations with breathing problems with these lines? Her registered name is just keep swimming. Ironic how her name is that and she indeed was swimming in fluid, at least her lungs were. One vet did suspect heart problems as in ultrasound he thought her one valve wasn't closing properly. Took video to ultrasound specialist and he didn't think there was anything wrong. She was sedated for this as she had multiple procedures done and needed to be sedated for one of those things. Also did traponin test that came back normal. So no heart damage detected. I can go more on everything that was done to her. And if I can take anything from this it is I should have gone to a big university at the first sign. I may have gotten an answer or possible treatment that could have worked instead of other vet clinics and spending over 10k. Between all those places. Yes she was also out 24/7 never stalled so out in fresh air. Soaked hay and pasture 24/7. Never had fever, never off of feed. 

Do you think its possible she breathed in some sort of toxin that caused damage? Years ago I knew of a paint mare that had very similar symptoms. She did not last as long as your horse - only maybe a month. No explanation for it and it was never determined the cause but her owner suspected some sort of toxin that she breathed in. The last ride before she became ill they rode through a section of forest where all vegitation was dead. 

So sorry you didn't get any answers.

I am thinking there is something going on that was missed! She was boarded at a place that had mulch and extremely fine stone dust down for an arena. Terrible dusty conditions and it was not maintained. Iam thinking it has something to deal with that. As my other horse develop breathing issues  shortly after I moved them. Now she has passed because of that as well but her issues were a little different and also weird. Vets are stumped with her case as well. She has stifle and coffin injections and was started strength training for her stifles when she started goose honking breathing and struggling really bad to breath just from walking up a slight hill. She went down shortly after it started and was suffocating. Right before she would have crossed over the bridge from suffocation a huge clot of blood came out her nose and she started breathing again. She was scoped and all her upper airways were clear no signs of bleeding from there even in her gluteal pouch. But the vets wanted to send her to a university for further scoping. They could only do upper not lower. But she couldn't make the jounney as she keep having those breathing episodes just not as bad and no blood those times. So I just think something was missed and this all had something to deal with that arena they were boarded at. But at the same time my horses were the only ones to have breathing issues from that place. 

I had a paint mare that started with similar symptoms.  She started coughing, which wasn't completely abnormal for her as she usually would cough off and on during the spring and summer and the vet told me it was allergies.  Her last summer in 2019, she started coughing quite a bit more so I took her to the vet again, did blood work, everything looked fine and he put her on a steroid injection to see if that would clear it up.  She rapidly declined from there and after taking her to a different vet who did x-rays and a scope, it was decided that the best option for her would be to put her down.  Something had damaged her lungs so severely that she had little quality of life.  The next day I had her put down.  The vet told me it was heaves, but I have my doubts still.