I have read here and in the Vet Forum about white line disease. We have a 4-year-old mare that has some white line missing in both front feet. One toe has an inch or more wide missing and part of it goes up under the hoof wall for 2 inches or more. The farrier found this a couple of weeks ago and we have been cleaning it daily and treating with a thrush and white line product. My questions are:
1) How fast does (can) white line spread?
2) How can we tell if we have killed the fungus?
3) I have been trying to do a good job cleaning out the holes, but was wondering how careful I need to be to not cause additional damage?

White line disease can spread pretty fast. You will know it is killed when there is no more separation or discoloration in the white line. You don't have to be too careful, but don't cause bleeding if possible. I would suggest a resection so that cleaning is easier, and there is good exposure to oxygen.