White line disease (WLD) is the separation between the hoof wall and the sole at the white line in the horse's feet. It is caused by keratinolytic fungi. The white line connects the wall of the horse's hoof and the horn of the sole. Several factors can contribute to the development of WLD in the horse; these include poor hoof wall conformation which causes flaring of the hoof wall, imbalance of hoof moisture (such as when environment is too wet or to dry), hoof wall abscess which tracts upward inside the hoof wall, trauma to the hoof wall, laminitis
, and irregular shoeing or trimming intervals which allow the hoof wall to grow long.