White Line Disease:
I've had two experiences with white line disease. My draft cross Freya was prone to it. My daughter's OTTB also had it, and for a while walked around with a large chunk of hoof wall missing. The area behind the hoof wall was separated from the laminae and to prevent further infection and problems the farrier removed the 'hollow' area. Although it's called white line disease, it's not really the white line that is affected. Rather it's the material between the wall and the white line that becomes infected.
Other Names for White Line Disease:
As the farrier trims the hoof and cleans up the sole, they may notice the change in the hoof and may find a hollow area under the hoof wall. Imaging may show the extent of the damage, and rule out other problems like laminitis.
After the hoof has been cleaned up it's important to keep the hoof clean and dry. This may mean keeping the horse stalled or in another pasture or paddock if conditions are muddy or wet. The horse may be able to return to work depending on how much of the hoof wall had to be removed. On our OTTB, the section removed was less than the width of my hand and extended about half way up the hoof. He was sound and was able to do flat work shortly after the white line disease was discovered. But in other cases, a lot more hoof wall may have to be removed. The open areas of the hoof may be packed with substances that prevent dirt and moisture entering and resist infection.
Because the exact causes of white line disease are so varied, it's difficult to do anything that will specifically prevent it from happening. Regular farrier visits, clean pastures and stalls and good nutrition are the most important elements of hoof health. Balanced trims will prevent heels from becoming under-run and toes that are too long that cause the hoof wall at the toe to separate from the underlying structures. In the case of my draft cross mare, our gritty soil was partially to blame and regular hoof trimming and frequent application of an antiseptic was required.