Much of what you write of symptoms and conditions in the horses' environment remind me strongly of equine Insulin Resistance.
The condition has to do with the inability of insulin to handle carbs in the diet. A pretty simple test would show high insulin rates (It can't be used so is in higher concentration in the blood when the horse gets a high carb load - which triggers insulin production).
Looking into this I've found some strange things. One, ponies in research experiment had a 24% rate among them of IR. They are very prone to this problem, with fat, and pad like lumps of fat being a visual symptom. Anyone here know any fat ponies? Usually the pads of fat are over the eyes filling the socket, around the base of the tail on each side, sometimes just behind the shoulder and just in front of the girth line.
Highly prone to grass colic, and often then at high risk of founder (chronic laminitis), and lameness.
It too can make them easy prey for parasites and diseases since IR taxes and compromises the immune system just because it is so difficult for the horse's system to deal with. I do not know if there is a link to liver damage and or failure, but would not be surprised given the IR effects.
You might want to study up on IR and too, Cushings Disease, if your horse is older than 10. Cushings is a thyroid condition in horses. I believe humans can suffer from it as well. It's caused by a tumor, I believe rather slow growing, that presses against the thyroid. My view is as a lay person so you'd want to look at more authoritative resources.
Here is where I have found the most reliable data and information for how to diagnose properly with blood work, and how to treat to manage these conditions (they are not curable - but many times very manageable to the point of the horse being able to be put back in service - even Endurance horses are known to have it and still keep going - exercise helps in fact with IR).
Why do I care? Well, because I own an IR horse.
http://www.ecirhorse.com/At the above link is where I found what I needed, after nearly a six month struggle to find some way to help her. I was not helped by my veterinarian, though I think he did his best he may not have been up on the most current research and certainly not on a successful treatment regimen that I now follow. Difficult to learn, but very simple and doable once one practices it a bit.
She was a model of what I have described, two and half years ago in Idaho. Today she is you and others here have seen her in photos that Annaliese has taken of her and her Bonnie filly. A considerable change.
She was chronically effected by and untreated for laminitis before I received her, with untrimmed hooves. Today her hoof shape is nearly normal, and she is losing some of her tenderness, though I do boot her for rides, when I do ride, which isn't much. I prefer not stressing her hooves and mostly hand walk her on softer ground.
I have family in the hay business, as well as in academia as professors of animal nutrition would were kind enough to send me data on IR. An equine researchers at Rutgers university reported that incidentally (she wasn't doing IR research) as she tested for IR among other things for setting baseline standards for her various experimental groups, she found 10% of all the equines (inluding mules and donkeys) were in fact insulin resistant.
Founder, here in the U.S. at any rate, is the number one killer of horses. Spring and Fall the times when it's highest - just as the grasses are in growing cycle at their most high carb levels, starch and sugar.
The website whose link I've provided
http://www.ecirhorse.com/ will give you access to much more authoritative information than I've offered or can offer not having the science education in this ailment that I'd wish I did have.
One might assume this source, very science based, would be more traditional in the use of medications - not so. They use and recommend many botanicals and herbal remedies to deal with the various side effects of IR and Cushings Disease.
For instance I read about the uses of Turmeric there before I did here. Chasteberry, jiagolan Spirulina and similar herbals are recommended in some instances.
The biggest hurdle, should your horse test positive for IR, might well be the one I face. Altea, my IR mare, cannot be on grass, ever, none 0 - for the rest of her life.
She can have only tested low sugar hay, and that balanced with mineral supplementation. Oddly enough though, with the help of folks at the site I've listed, it's actually not costing me any more to feed than when I was using bagged feed and supplementation formulas.
Now I feed with only slightly more expensive hay, and a custom mineral and supplemental mix (ground flax, elemental minerals - not chemically created, vitamins, etc.) that is balanced to standards set by a group of national equine nutritionists for horses in various conditions. They even provide spread sheets to put the test numbers and characteristics into and out pops the correct mix of minerals, etc. that are needed.
The folks that helped me did all the work for me, but explained, as many times as I needed, why each element in the diet was needed, and how it worked for the horse, and how it dealt with the IR issue. Even how my hay, high in some minerals and low in others, high in protein, etc. could be offset by minerals and vitamins that would compensate so all elements would be taken up in the horse's gut as they should be.
Many of the health mysteries I've seen over the years with horses are now being answered as I continue to study and to use this group at
http://www.ecirhorse.com/ as both a resource and as road signs to good research on IR and Cushings.
I hope, if you explore it you find what you and your horse need to get well, or maintain more effectively. I may be too empathetic, as when I see a lame horse I can feel the pain in the limb effected. Arms for front legs, my own legs for hind. So getting Altea well has been a big thing for me, and of course for her.
Donald