I wish someone in the equine medical profession would come up with an answer.
IMHO, I think it starts with the horse's immune system because I personally know of two horses with recurrent abscesses. Their lifestyles and health don't begin to resemble each other. Horse #1, a registered APHA,
lived on ten acres of lush pasture with no rocks or stones. Reasonable amount of mud vs. dry because we're talking the OH/PA border.
The Owner changed his feed and farrier as many times as she had the options, took him to every highly qualified vet within 100 radius, even had her hay ANNND pasture soil tested.
Zip, zero, nada - nothing helped, nothing in the tests showed anything.
This was her only horse, she missed a year of riding trying to fix him, so sold him to someone that used him mostly in the arena environment and he was kept in stall much of the time. That helped but still not 100%. Horse #2, a registered Paso Fino,
lives on seven very rock acres in Tennessee, near the Kentucky border. He was formally diagnosed with Cushings & Insulin Resistance about four years ago.
Needless-to-say this horse is on a very strict diet that does NOT include grain. He is drylotted during the day, turned out with a grazing muzzle at night, so he can be with the other two horses, of whom he is the dominant one.
The Owner cannot keep him free from abscesses and he has Foundered once on her. Until the cushings hit, this horse was a rough and ready-never-had-an-issue trail horse. He is 22.
He now abscesses so bad that he is completely unridable and can't even carry her granddaughter around the yard a couple times.
I gave those two instances because I am well-acquainted with both of these horses who, once the abscesses started, never became completely sound again. The APHA is a lot younger than the Paso Fino, so my thought is his immune system is much stronger and that's why he doesn't YET have the issues the Paso does.
All I have done is put forth more questions than answers. Based on these two horses, who live completely different but well cared for lives, I'm not sure the OP will ever see her horse abscess-free
I sure hope so, and when that happens, please share what you did with the rest of the horse world because this seems to be an increasing issue.
It just has to be related to the immune system and somehow to diet. Maybe not the current diet, but enough of a bad diet in a previous environment to cause these particular horses permanent damage.