My horse has had white line disease as well..
And it is not the same thing as thrush. Thrush attacks the frog, white line attacks the laminae.
Agreed that we need more photos of the hoof. We are looking for cracks and flare. I don't really see anything obvious on the hoof sole, except some black along the bottom (right) quarter in the photo.
My experience was that as I'd never seen it before, I didn't immediately recognize the problem, and neither did the first farrier. I was concerned about a crack that wouldn't grow out. I finally had my vet have a look and he diagnosed white line. He had me cleaning the hole out, soaking cotton in Betadine and stuffing that in the hole. The next farrier had me doing the same thing, It didn't work. The crack stayed the same height, and the black area remained, for over 4 months of treatment.
A different vet finally did the right thing, which was to pare the hoof wall back, clearing off all the diseased material, until only good hoof remained. She also put a decent roll on the toe to remove pressure from the crack (the idiot of a farrier I had at the time refused to do the same thing, even when asked). The dug-out part was about an inch high and two inches across. He was not lame or sensitive. There was still a bit of a hole that I had to inject some sort of special iodine solution into, but with most of the diseased area removed, I was able to get the iodine all the way into the disease that remained. The hoof grew out clean and the problem so far has not returned.
Good luck - if your horse truly has white line disease, it needs to be treated correctly and quickly, if it continues to spread up the laminae it can be much more difficult to treat and cause a lot of problems.
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